Emission Electron Microscope

V. I. Miliutin

Introduction

The electron microscope that gives images of objects emitting electrons, and is usually called an emission microscope, was the first to appear at the dawn of the development of electron optics.

Already at the first stage of its development, many interesting data were obtained on the physics of the processes occurring during the operation of various types of cathodes, on the crystalline structure of metallic surfaces, and a number of others. However, as soon became clear, the designs of the emission microscope used in the initial period suffered from a major shortcoming: they had insufficient resolving power, considerably less than that of the light microscope. At the same time, the transmission-type microscope, owing to its smaller aberrations, in 1936 reached and surpassed the resolution limits of the light microscope. Therefore the main attention of researchers was directed toward the development, improvement, and application of the transmission-type electron microscope. In recent years this type of microscope has undergone considerable development. In our country, in the Soviet Union, several types of such microscopes with high resolution have been created.

In the field of the development of emission microscopy, however, there was a certain lag. Until 1942 nothing was known of experiments connected with increasing the resolving power of the emission microscope. Only in 1942 was it possible to achieve, for the emission microscope, a resolving power equal to and exceeding the resolving power of the light microscope[^1],[^2],[^3]. It turned out that the basic condition for increasing the resolving power is an increase in the electric field at the surface of the object being imaged. The increased resolving power makes it possible to obtain higher magnifications of the images of the objects under investigation.

These successes in the development of the emission microscope again attracted to it the attention of a wide circle of researchers. With great—

with success, investigations of various types of cathodes were continued. Especially thoroughly studied were the processes associated with the activation and operation of oxide, barium, and thoriated cathodes, migration phenomena, and the adsorption properties of the active substance on the cathode surface. The emission microscope made it possible to obtain a cinematographic record of the processes taking place during the activation and operation of cathodes. Further, the use of the emission microscope for studying the crystalline structure and recrystallization processes in metals proved extremely successful, giving metallographers a powerful tool for investigating metals under various thermal conditions. The possibility of observation at different temperatures gives the emission microscope significant advantages over the light microscope.

In their design, emission microscopes can be divided into many different types, depending on the kind of emission of the object under investigation, the aims of the investigation, and the type of electron lenses used.

Among them it is necessary to single out microscopes without lenses, i.e., projection microscopes or, as they are sometimes called, projectors. In these, the image of an object in the form of a wire or a point is projected by radially moving electrons onto a cylindrical or, respectively, spherical screen surrounding the object. The magnification depends on the ratio of the radius of the cylindrical (spherical) screen to the radius of the wire (point). Projectors have been described sufficiently fully recently by B. M. Tsarev^4 on the pages of this journal.

The purpose of the present article is to describe briefly the basic layouts of the emission electron microscope with electrostatic and magnetic lenses and to give a review of investigations carried out with its aid mainly after 1937.*)

I. BASIC PRINCIPLES OF OPERATION OF THE EMISSION MICROSCOPE

1. Features of the emission microscope

The mechanism of image formation in the emission microscope is quite different from that in the microscope of the transmission type. In the latter, the image is formed by an almost monochromatic beam of fast electrons passing through the object, whereas in the emission microscope the image is formed by slow electrons emitted by the object. In the emission microscope the object is at the same time also the source of electrons. This feature imposes limitations on the resolving power of the emission microscope. In Fig. 1 are shown diagrams illustrating the mechanism of image formation

*) Electron-optical investigations of cathodes carried out before 1937 are described in the review by N. D. Morgulis^5.

in a transmission-type electron microscope (a) and an emission microscope (b).

In the first case, fast electrons, passing through the object, are scattered owing to their interaction with the atoms of the substance of the object. The loss of electron velocity for thin objects is insignificant. The scattering is proportional to the thickness and density of the object. For example,

Fig. 1. Mechanism of image formation in an electron microscope operating in transmission (a), and in an emission microscope (b).

at point \(A\) of the object (Fig. 1,a), by virtue of the greater thickness of the object (or, analogously, by virtue of greater density), the scattering is greater than at point \(B\). Owing to the different scattering of the electron beams and the action of the aperture diaphragm, located near the object, a different number of electrons from different regions of the specimen will pass through the objective lens. Denser, and also thicker, regions of the specimen will correspond to darker regions in the image. As a result, a contrast image of the specimen is produced.

The mechanism of image formation is different in the emission microscope. Here the electrons emitted by the object have insignificant and, at the same time, different velocities. Therefore they must first be accelerated by an electric field closely adjacent to the cathode, and then focused by one lens or another in order to obtain an image. In Fig. 1,b, by way of example, the path is shown of electron beams emitted by the object in the accelerating field and in the lens. Different emission of electrons by different regions of the object (for example, at points \(A\) and \(B\)) creates contrasts in the image plane; regions with greater emission correspond to brighter

places in the image. Owing to the directed action of the accelerating field, all electrons emerging from a point of the cathode in all directions of the hemisphere adjacent to the cathode pass through the electron lens and, consequently, onto the image plane. As for the aperture of the beam at the object, here, in contrast to a microscope of the transmission type, it is very large.

2. Immersion objective

The separation of the objective of an emission microscope into an accelerating field and a lens, shown in Fig. 1,b, is, however, artificial and was done for clarity; in reality the field formed at the cathode by a certain system of electrodes arranged before the cathode is simultaneously both accelerating and focusing. A system of electrodes forming such a field is called an immersion objective. The most widespread type of immersion objective consists of a cathode and two plane diaphragms with circular openings, situated before the cathode and having corresponding potentials relative to the cathode (Fig. 2). The equipotential surfaces formed by them act, in general, as a collecting lens. With its aid one can obtain a magnified image of the cathode.

Fig. 2. Diagram of an immersion objective.

The name “immersion” was given to such an objective by analogy with light optics, owing to the difference in refractive index on the two sides of the objective.

The properties of the immersion objective shown in Fig. 2 were investigated by Johannson[^6] and later by F. A. Savchenko[^7].

Experimentally, dependences were established of the optical parameters of the objective on the geometry of its electrodes and their potentials.

For example, it has been shown that the sharpest image of the cathode \(K\) is obtained if the distance between it and the diaphragm \(D_1\), and also between the diaphragms \(D_1\) and \(D_2\), is, approximately, \(1\ \mathrm{mm}\), the diameters of the apertures of the first and second diaphragms being \(1\ \mathrm{mm}\) and \(1.2\ \mathrm{mm}\), respectively.

The dependences of the magnification \((M)\) and of the potential of the first diaphragm \((U_1)\), ensuring a sharp image of the cathode, on the distance \(a\) between the latter and the first diaphragm (at a constant distance from the cathode to the screen \(L=240\ \mathrm{mm}\) and a constant potential of the second—anode—diaphragm \(U_2=750\) volts) are shown in Fig. 3. From the curves it is seen that, as \(a\) increases, the magnification decreases, and the potential of the first diaphragm \((U_1)\) must be increased in order to preserve a sharp image. The magnification of the objective also depends strongly on its other geometrical and electrical parameters; for example, as was established experimentally by F. A. Savchenko, when \(a\), \(d\), \(c\), and \(\dfrac{U_1}{U_2}\) are increased, it decreases.

Fig. 3. Dependence of the magnification \(M\) of the immersion objective and of the potential of the first diaphragm \(U_1\) on the distance between the cathode and the first diaphragm.

The upper curve of Fig. 3 makes it possible to determine the focal length \((f)\) of the immersion objective by means of the relations

\[ \frac{1}{l_1}+\frac{1}{l_2}=\frac{1}{f} \quad \text{and} \quad M=\frac{l_2}{l_1}, \]

where \(l_1\) and \(l_2\) are the distances from the center of the objective to the cathode and to its image, respectively.

Taking into account that \(l_2 \gg l_1\), we find

\[ f \simeq l_1=\frac{l_2}{M}. \]

For the system under consideration \((l_2=L=240\ \mathrm{mm})\),

\[ f=\frac{240}{M}\ \mathrm{mm}. \]

The smallest focal length that can be achieved lies within the magnitude of the diameters of the diaphragm apertures.

Fig. 4. Dependence of the focal length of the immersion objective on \(\dfrac{U_1}{U_2}\).

In Fig. 4 is presented the dependence of the focal length of the immersion objective on the ratio of the potentials of the first and second

diaphragms \(\left(\dfrac{U_1}{U_2}\right)\) for two values of the aperture diameter of the first diaphragm: 1 mm (curve 1) and 2 mm (curve 2).

The curves of the dependence of the distance \(a\) between the cathode and the first diaphragm on \(\dfrac{U_1}{U_2}\), under the condition that a sharp image of the cathode is maintained (Fig. 5), have an analogous form. It is evident from the figure that the character of the dependence between \(a\) and \(\dfrac{U_1}{U_2}\) remains the same for all values of the diameters of the diaphragm apertures; moreover, the curves corresponding to larger values of the aperture diameter are shifted to the left, toward smaller values of \(\dfrac{U_1}{U_2}\). In addition, it is evident that, with a gradual decrease of \(a\), the curves approach the abscissa axis asymptotically, i.e., for small \(a\) and given \(d\), \(c\), and \(L\) (Fig. 3), the focusing of the image on \(\dfrac{U_1}{U_2}\) is practically independent.

Fig. 5. Dependence between the distance from the first diaphragm to the cathode \((a)\) and the ratio of the potentials on the first and second diaphragms \(\left(\dfrac{U_1}{U_2}\right)\), for various diameters of the diaphragm apertures (the same for both diaphragms): 1) 1 mm, 2) 2 mm, 3) 3 mm, 4) 3.95 mm, 5) 6.05 mm.

The properties described above pertain to the simplest type of immersion objective shown in Fig. 3.

A substantial improvement in the optical properties of an immersion objective can be obtained by changing the profile of the first diaphragm, bringing it closer to the shape of an equipotential surface of the field. For example, with the aid of the profiled diaphragm shown in Fig. 6,a, the field of view of the objective can be increased by 70% in comparison with a flat diaphragm, and the quality of the image obtained can also be improved.

Some other types of immersion objective\(^{8,9}\), which have found the widest application in cathode-ray tubes, are shown in Fig. 6 (b and c).

3. Resolving Power of the Emission Microscope

One of the most important characteristics of the emission microscope, as of any microscope, is its resolving power, which determines the sharpness of the image and the useful magnification of the instrument.

The resolution of a microscope is usually understood as the smallest distance between points of an object that are still clearly visible separately in the image. Experimentally it is determined by measuring the distance between two closest-lying, but separately visible, points of the image and dividing it by the magnification of the microscope. As an object for these purposes one chooses an item with a large number of separate small points, for example, the surface of nickel with barium atoms deposited on it ^10.

Fig. 6. Types of immersion objective.

What, then, determines the resolution limit of the emission microscope? It is determined chiefly by the following four factors: a) the phenomenon of diffraction of electron waves (the fundamental limit); b) imperfections of the optical system (lens aberrations); c) distortions caused by the object itself; d) space charge of the electrons at the cathode and in the beam.

The first factor, as in the case of the transmission electron microscope, plays an insignificant role and may be neglected. Cases in which diffraction phenomena must be taken into account will be discussed below. As for the remaining factors, the emission microscope has its own special features, which were partly noted above.

Many works ^11, ^12, ^13, ^14 have been devoted to the theoretical study of aberrations and of the resolving power of the emission microscope. Aberrations. As a result of aberrations, each point of the object is imaged in the image plane as a circle of finite size. The resolvable distance is determined by the distance between the centers of two tangent circles or, consequently, by their diameter.

In order to estimate the magnitude of the circle of confusion, and hence the value of the resolving power^13, the emission microscope may be replaced, as we did above, by a model consisting of an accelerating field and a lens adjacent to it (electrostatic or magnetic). In the accelerating field, electrons emerging from points of the plane cathode with energy \(e\mathscr{E}\) form, in the first approximation,

Fig. 7. Electron trajectories in an accelerating field.

a bundle of parabolas. In this case the outermost parabola is formed by an electron which has left tangentially to the surface of the cathode (Fig. 7, a). Tangents to the branches of this parabola, drawn at points at a definite distance from the cathode, intersect behind the cathode (on the left in Fig. 7, a and b). The point of intersection \((A')\) gives the virtual source of the electrons. The angle \(\alpha_1\) of the tangent with the \(z\)-axis is determined by the ratio of the radial and longitudinal components of the velocities at the point of tangency:

\[ \operatorname{tg}\alpha_1=\frac{v_r}{v_z}=\sqrt{\frac{\mathscr{E}}{U}}, \]

where \(\mathscr{E}\) is the voltage equivalent to the initial velocity of the electron, and \(U\) is the accelerating voltage.

For an electron that has left the cathode at an angle to the normal \(\beta < 90^\circ\), assuming that its energy is the same \((e\mathscr{E})\), the radial velocity is determined by the relation

\[ \sqrt{\mathscr{E}_r}=\sqrt{\mathscr{E}}\sin\beta . \]

The angle \(\alpha_2\), formed by the tangent with the \(z\)-axis, is in this case determined from

\[ \operatorname{tg}\alpha_2= \frac{\sqrt{\mathscr{E}}\sin\beta} {\sqrt{U}+\sqrt{\mathscr{E}}\cos\beta} \simeq \sqrt{\frac{\mathscr{E}}{U}}\sin\beta, \]

if one takes into account that \(\mathscr{E}\ll U\).

The point of intersection of the tangent with the axis no longer coincides with the point \(A'\) determined above. The new point \((B')\) can, obviously,

determine if one continues the parabolic trajectory into the depth of the cathode to its vertex and then, as before, intersects the axis of the parabola with the tangent drawn from the point of the trajectory under consideration \((B)\).

It is easy to show that the distance between the axes of the two parabolas under consideration is equal to

\[ r = l \frac{\mathcal{E}}{U}\sin 2\beta . \]

For \(\beta = 45^\circ\) this distance will be maximal:

\[ r_{\max}=l\frac{\mathcal{E}}{U}. \]

As \(\beta\) varies from \(0^\circ\) to \(90^\circ\), the point of intersection of the tangent with the corresponding axis of the parabola will describe a circle of diameter

\[ d=2r_{\max}=2l\frac{\mathcal{E}}{U}. \]

This circle, from which the electrons appear to emerge, thus replaces the point of the cathode. Near this circle the bundle of rays has the smallest cross-section, whose diameter \((CD\) in Fig. 7, b), according to an approximate estimate, is two times smaller than the diameter of the circle found and is therefore

\[ l\frac{\mathcal{E}}{U}. \]

If now, in the plane passing through the points of the parabolas at which we drew the tangents, an aberration-free lens is placed, then in the image plane, instead of a point, there also arises a circle of confusion of diameter \(l \frac{\mathcal{E}}{U}\) (for a system magnification equal to 1).

Hence the resolution will be equal to

\[ \delta = l\frac{\mathcal{E}}{U}=\frac{\mathcal{E}}{E}, \]

where \(E\) is the field strength at the cathode.

This expression is correct to within a constant factor of the order of several units.

Taking into account the width of the energy spectrum of the emitted electrons, the following value is obtained for the resolution\({}^{12}\):

\[ \delta = 4\frac{\bar{\mathcal{E}}}{E}, \]

where \(\bar{\mathcal{E}}\) is the most probable initial velocity of the electron, expressed in volts.

For heated cathodes, under the assumption of a Maxwellian velocity distribution, the most probable electron energy is proportional to the temperature:

\[ e\bar{\mathcal{E}} = kT,\ \text{i.e.}\ \bar{\mathcal{E}}=\frac{kT}{e} \]

and, consequently, for the resolution we obtain:

\[ \delta=\frac{4kT}{eE}. \]

Thus the resolution is proportional to the absolute temperature and inversely proportional to the field strength at the cathode.

In order, for example, to image a heated cathode at a temperature of \(1160^\circ K\) (\(kT \simeq 0.1\) eV) with a resolution better than that of a light microscope (\(0.2\,\mu\)), it is necessary to have a field strength at the cathode equal to \(20\,000\) V/cm.

Fig. 8. Resolution of the immersion objective (Fig. 2) as a function of the anode voltage \((U_2)\) for various \(a\).

However, the expression obtained determines the lower limit of the resolvable distance. Aberrations of the lens adjacent to the accelerating field, especially chromatic aberration, worsen the resolving power of the microscope. Nevertheless, this expression is fundamental in estimating the resolution limits of an emission microscope.

Figure 8 shows the dependence of the resolution \(\delta\) on the anode voltage \(U_2\) (the voltage of the second diaphragm of the immersion objective) for four positions of the first diaphragm. In this case, in the formula

\[ \delta=4\frac{\bar{\varepsilon}}{E} \]

for the most probable energy of the electrons emitted from the cathode, \(\bar{\varepsilon}=0.1\) V is taken.

It is seen from the figure that for \(a=0.5\) mm (at which the sharply imaged region of the cathode is maximal), at a voltage \(U_2=10^4\) V, the resolution limit of the light microscope is reached. Thus, the principal means of improving the resolving power of the emission electron microscope as compared with the light microscope is to increase the field strength at the cathode, which not only reduces the dimensions of the scattering circle considered above that arises in the accelerating field, but also reduces the chromatic aberration of the lens.

A second way to improve the resolving power of the emission microscope consists in stopping down the marginal rays, i.e., reducing the aperture of the imaged electron beam. In the presence of diaphragming, the resolution is determined by the following expression\({}^{13}\):

\[ \delta=\delta_0\sin\beta(1-\cos\beta), \]

where \(\delta_0\) is the resolution in the absence of diaphragming, \(\beta\) is the angle between the limiting ray of the diaphragmed beam at the cathode and the axis of symmetry.

The intensity of the transmitted beam \(I\) turns out to be equal to

\[ I=\frac{I_0}{2}(1-2\cos\beta), \]

where \(I_0\) is the total intensity of the undiaphragmed beam emerging from one point of the cathode.

The result of the calculations is presented in Fig. 9. When the diameter of the beam in the focal plane of the objective is reduced by 70%, the intensity of the beam that has passed through the diaphragm is reduced by 50%. At the same time the resolving power increases by approximately a factor of 5. The position of the diaphragm is determined by the position of the focal plane in which the beam cross-section is smallest. This focal plane usually lies near the anode diaphragm. Therefore, first of all it is necessary to reduce the aperture of this diaphragm. The beam width, for example, for \(\delta=0.1\,\text{V}\), \(U=40\,000\,\text{V}\) and a distance from the cathode to the diaphragm of \(1\,\text{cm}\), is equal to \(\sqrt{\frac{\delta}{U}}\cdot 1\,\text{cm}=0.015\,\text{mm}\). Consequently, the diaphragm aperture in this case must have a value of \(0.01\,\text{mm}\) or less.

Fig. 9. Dependence of intensity and resolution on the aperture of the beam.

However, when such small diaphragms are used, it is already necessary to take into account electron diffraction, which worsens the resolution.

The optimal value for the diameter of the aperture diaphragm is estimated\(^{15}\) at \(4\,\mu\), at which it is possible to obtain a resolution of the order of \(10\,m\mu\) and less.

Already the first attempts to construct an emission supermicroscope led to a resolution of \(\sim 20\,m\mu\).

Influence of irregularities of the object surface. The fact that the field strength at the cathode is decisive for the resolution of the emission microscope also means that any distortion of the field at the cathode will very strongly affect the quality of the image. Since the surface of the object is the first refracting surface of the objective, in the case of irregularities and differences of potentials it will deform the lens itself. Since at the beginning of their path the electrons have an insignificant velocity, changes of the field at the surface

may lead to significant distortions of the image. The resolution of the microscope, owing to surface irregularities, may deteriorate severalfold. To reduce these distortions, the surface of the cathode is made as smooth as possible by careful grinding and polishing.

Other factors. In addition to the principal factors described above, it is also necessary to mention others that likewise affect the resolution and image quality of an emission microscope. These include the space charge near the cathode, and also along the entire length of the beam, which may be considerable when oxide cathodes are used or if there is an appreciable residual gas in the microscope. The image of an oxide cathode, for example, in the presence of a large space charge near it, as will be shown below, acquires a “cloudy” structure. An increase in the field at the cathode leads to a reduction of the space charge. In the case of a high current density in the beam, the field of the space charge increases the circle of confusion in the image. However, the influence of the space charge on image quality has so far been little investigated.

The image quality and resolution of an emission microscope naturally depend on the accuracy of the positioning and centering of the lens electrodes, the mechanical stability of the microscope during operation, the grain size of the screen (photographic plate), the influence of external fields (for example, the earth’s magnetic field, the magnetic field of the cathode heating current, etc.). All these factors must be taken into account in designing microscopes.

II. DESIGNS OF THE EMISSION MICROSCOPE

Emission microscopes, depending on the type of lenses employed, may be classified as follows: a) electrostatic systems, the principal part of which is the immersion objective; b) magnetic systems, using magnetic lenses; c) combined systems, using combinations of an immersion objective with magnetic lenses.

The use of one or another system in practice is determined, on the one hand, by the nature of the object under consideration and, on the other, by the magnifications that it is desired to obtain.

In the case of two-stage magnification, the lenses of the second stage for an emission microscope do not differ in any way from the lenses of a transmission-type microscope.

1. Electrostatic Systems

The design of an electrostatic microscope is based on various types of immersion objective considered above. Figure 10 shows a simple typical electrostatic microscope.

with one stage of magnification, which was used for the investigation of oxide cathodes[^17].

The construction of the microscope is clear from the figure. An immersion objective with a profile diaphragm is mounted in a glass vessel. The cathode is a nickel plate heated by a tungsten wire and coated with oxide. With the aid of the bellows \(F\)

Fig. 10. Diagram of an electrostatic emission microscope. \(C\)—cathode, \(A\) and \(B\)—diaphragms separated by a steatite insulator 1 mm thick (the aperture of the first diaphragm: a cone of 1–1.5 mm), \(S_1\) and \(S_2\)—leads from them, \(P\)—aquadag, \(K\)—screen, \(E\)—ground joint, \(F\)—bellows for moving the cathode along the axis, \(G\)—screw, \(L\)—leads of the heater and cathode.

the cathode can be moved along the axis in order to change the distance between it and the first diaphragm.

The image is projected onto a willemite screen deposited on the glass of the vessel, which is located at a distance of 30 cm from the second diaphragm. To eliminate the effects of charging of the glass by electrons, the inner part of the tube is coated with conducting graphite (aquadag). The magnification given by this system, depending on the ratio of the potentials on the diaphragms and on the distance from them to the cathode, can vary. The maximum magnification is of the order of 100 times. The anode voltage used is up to 4000 volts. The microscope described is the earliest design and does not provide high resolving power.

Electrostatic microscope with high resolution. To increase the resolving power, according to the conclusions of the theory, it is necessary to raise the accelerating voltage to 20–50 kV. This is associated with considerable difficulties that have to be overcome in designing the microscope. They coincide with the difficulties that occur in ordinary electrostatic microscopes operating in transmission, namely: it is necessary to ensure high breakdown voltages of the lenses and great precision in the manufacture of the parts. The decisive point is the design of the immersion objective, which gives the first stage of magnification. On

Fig. 11 shows examples of immersion-objective designs by various authors, and Table 1 gives their principal parameters.

Fig. 12 shows the arrangement of the Mecklenburg² immersion objective, designed for high voltages. It was made from

Fig. 11. Various types of immersion objective:
a) according to Johansson, b) according to Bersch,
c) according to Mahl, d) according to Mecklenburg.

an objective single lens of an electrostatic microscope of the transilluminating type by replacing the upper external diaphragm with a cathode. Below the cathode, at a distance of 0.1 to 0.3 mm, there is an intermediate diaphragm mounted on a special insulator. The diameter of the aperture of this diaphragm is 2 mm and its thickness is 0.7 mm. The anode diaphragm, with rounded edges, is located at a distance of 2 mm from the intermediate diaphragm and has an aperture of 2.4 mm.

The accelerating voltage (the potential of the anode diaphragm) is from 20 to 30 kV, so that the field strength at the cathode at

Table 1

	Johansson	Bersch	Mahl	Mecklenburg
Diameter of the 1st diaphragm, in mm	1	1.3	1.8	2
Diameter of the 2nd diaphragm, in mm	1.2	1.4	5	2.4
Distance a, in mm	variable	1	0.2	0.2
Anode voltage, in kV	1	30	20	30

20 kV is 40 kV/cm. The potential of the intermediate diaphragm varies from −700 V to +700 V relative to the cathode, depending on the distance to the cathode. By changing this latter distance, the magnification varies from 60 to 120 times.

As the projection lens (the second stage of magnification) a double single lens is used, which makes it possible, by switching the high voltage, to pass from a magnification of 1800 times to higher magnifications.

On Fig. 13 are shown the general appearance and the diagram of the ray path of Mekenburg’s electrostatic emission microscope. In Fig. 14 are pre-

Fig. 12. Immersion objective: K—cathode, Z—intermediate diaphragm, A—anode diaphragm.

Fig. 13. General view of a high-resolution electrostatic emission microscope (left) and diagram of the path of the electron rays (right).

sented images of various portions of the barium-strontium oxide cathode, obtained with the aid of this microscope at a total magnification of 50,000 times (electron-optical magnification 1800 times).

It is evident from the figure that points at a distance of \(40\,m\mu\) are well resolved. Thus, the resolving power of this microscope is almost an order of magnitude higher than that of a light microscope. The use of a more suitable cathode (a smoother one) and a further increase in the field strength at the cathode can still further increase the resolving power of the emission microscope.

Fig. 14. Emission image of various regions of a barium-strontium oxide cathode (electron-optical magnification 1800, total magnification 50,000).

2. Magnetic systems

Designs of the magnetic emission microscope appeared almost simultaneously with electrostatic microscopes\(^{18}\). The basic elements of a magnetic microscope are the cathode, the accelerating system, one or two magnetic lenses, and a screen (photographic plate).

As an example one may cite the microscope used by N. G. Sushkin\(^{19}\) for the study of oxide cathodes. It consisted of an evacuated glass tube, into one side of which, on a ground joint, were introduced the cathode and a cylindrical anode mounted on a single stem; on the other side, likewise on a ground joint, a bulb with a fluorescent screen was fixed (Fig. 15). Shielded magnetic coils (\(\Phi K\)-1 and \(\Phi K\)-2) were used as the objective and projection lenses. The magnifications obtained in this microscope were approximately 35–40 for one lens and 150–200 for both. Thanks to a mirror, optical images of the cathode could be obtained under the same conditions as the electron-optical ones.

Figure 16 also shows a magnetic emission microscope of simple design together with the power-supply circuit\(^{20}\). The arrangement of the microscope is clear from the figure. In addition to the lens \(L\) described, which is an ordinary shielded magnetic coil, there is a pair of coils \(K_1\) and \(K_2\), necessary for shifting the image on the screen.

The cathode, shown separately in the right-hand part of the figure, has an original design. The emitting surface is

Fig. 15. Schematic of an emission microscope with magnetic lenses.
\(K\) — cathode, \(A\) — anode, \(Ш_1, Ш_2, Ш_3\) — ground sections, \(C\) — metallic layer, \(Э\) — screen, \(ФК\text{-}1, ФК\text{-}2\) — focusing coils (lenses), \(З\) — mirror, \(A_1, A_2\) — cameras, \(O\) — objective, \(M\) — microscope (light).

an oxide deposited on the surface of a T-shaped platinum cap, the upper part of which is separated from the tungsten wire heating it by a metallic plate, in order to prevent the light from the incandescent wire from falling on the fluorescing

Fig. 16. Schematic of a magnetic emission microscope with power-supply circuit.
\(A\) — cathode part, \(L\) — lens, \(K_1, K_2\) — coils for shifting the electron beam, \(B\) — metallic coating, serving simultaneously as the anode.

screen. The brass shell surrounding the cathode has the potential of the cathode. The microscope operates at an anode voltage equal to 2000 V and gives a magnification of the order of 60 times.

Fig. 17. Magnetic emission microscope for studying photocathodes: a) with one stage of magnification, b) with two stages of magnification; \(K\)—cathode, \(A\)—anode, \(D_1, D_2\)—diaphragms, \(L_1\)—glass lens, \(L_2\)—magnetic lenses, \(P\)—quartz lamp, \(E\)—screen.

Figure 17 shows two microscope layouts with magnetic lenses that were used to obtain images of photocathodes with single-stage \((a)^{21}\) and two-stage \((b)^{22}\) magnification.

Analogous designs of the magnetic emission microscope were also proposed by a number of other researchers.\(^{23,24}\)

The use of a combined microscope proved very fruitful; this is usually understood to mean a design in which, in order to obtain the image, an immersion

objective and one or two magnetic lenses, i.e., both electrostatic and magnetic focusing are used simultaneously. Strictly speaking, the magnetic systems described above, in essence, also represent a combination of a peculiar immersion objective with magnetic lenses, since the accelerating system, consisting of the cathode and the accelerating electrode, is also a two-electrode immersion objective. But for definiteness we shall adhere

Fig. 18. Combined emission electron microscope:
\(K\)—cathode, \(1\) and \(2\)—diaphragms of the immersion objective,
\(3, 4, 5\)—cylindrical anodes, \(F\)—window, \(R\)—bellows.

to the classification given above, and shall reserve the term “immersion objective” for a system of electrodes consisting of a cathode and two or three diaphragms located in front of it.

A typical combined microscope, consisting of an immersion objective and two magnetic lenses, is shown in Fig. 18. This microscope was also adapted for obtaining optical images of the cathode. Its construction is clear from the drawing. It makes it possible to obtain an image of the cathode either with the aid of the immersion objective, or with the aid of the magnetic lenses alone, or, finally, with the aid of both.

In the constructions of the magnetic microscope described above, long-focus lenses are used (of the order of several centimeters). The resolving power and magnifications that they give are too low.

The development of the magnetic electron microscope operating in transmission proceeded along the line of using short-focus magnetic lenses. For this purpose pole pieces introduced inside the microscope are used, as a result of which the resolving power and magnification of the microscope are considerably increased.

An attempt to use a short-focus magnetic lens for constructing an emission microscope with high resolution was made by Kinder\(^ {16}\). Fig. 19 shows a diagram of the objective proposed by him. It consists of a cathode, in the form of a molybdenum ribbon,

cylinder with an aperture diameter of 2 mm, and a separate objective diaphragm with an aperture of 1 mm, placed on the upper pole shoe. The lens is a magnetic coil with pole pieces. The focal length of the lens is on the order of several millimeters. The magnification obtained with such a microscope many times exceeds that for all the magnetic systems described above. It can reach 1800-fold and more (with two stages of magnification).

Fig. 19. Diagram of the objective of a magnetic emission supermicroscope. The distance between the cathode and the Wehnelt cylinder is 0.5 mm; the distance between the Wehnelt cylinder and the objective diaphragm is 10 mm.

When checking photographs obtained with this microscope, it turned out that the resolution is 90–140 mμ, i.e. better than the resolution of a light microscope.

Thus, the emission supermicroscope has now been realized both in electrostatic and in magnetic form.

III. APPLICATION OF THE EMISSION ELECTRON MICROSCOPE

The emission microscope has found its greatest use in the study of:

1) all types of thermionic cathodes (of pure metals, thoriated, barium, oxide, etc.),
2) the crystalline structure of metals,
3) photocathodes and secondary-emission cathodes,
4) autoelectronic emission.

A fairly detailed description of electron-microscopic studies of cathodes, especially thermionic cathodes, was given by N. D. Morgulis⁵ in his 1937 review, in which the principal works carried out before 1937 are presented.

Therefore we shall deal mainly with works published after 1937.

1. Heated cathodes

Electron-microscopic studies of cathodes made of pure metals, i.e. cathodes without a special emitting coating, were first carried out with the aid of an electrostatic electron microscope. Electron-optical images showed crystalli-

...the structure of the metal. The main difficulty in electron-microscopic studies of pure metals without coating with an active substance is that, in order to obtain sufficient emission, they must be heated to a very high temperature, which is possible only for refractory metals. On the other hand, at high temperature such cathodes are intense emitters of light, which hampers electron-microscopic observations. Therefore, electron-microscopic investigation of cathodes without an active coating has not found wide application. As we shall see below, coating the surface of a metal with a monoatomic layer of an active substance (for example, barium) makes it possible to obtain images of the crystalline structure of most metals at a comparatively low temperature.

Thoriated cathodes. Thoriated cathodes, which include first of all thoriated tungsten and thoriated molybdenum, have been the subject of a number of investigations, since they are among the first active cathodes, very valuable for the technology of vacuum devices. The first electron-optical observations of thoriated tungsten26 led to the conclusion that the active thorium film on the surface of tungsten is formed by diffusion of thorium from within to the surface of the core at separate places on it and by subsequent migration of thorium over the surface (with the appropriate heat treatment of the thoriated tungsten). Electron-optical observations did not confirm Langmuir’s theory, according to which thorium diffusion takes place uniformly over the whole surface. The emission activity of a thoriated cathode, as shown by a number of authors, is determined by the concentration of thorium atoms on the cathode surface, which depends on the rate of two processes occurring during heating of the cathode: the diffusion of thorium to the surface of the core and its evaporation. The emission microscope makes it possible to study the course of these processes and to determine the places of diffusion and evaporation of thorium on the cathode surface.

The most complete investigation of thoriated tungsten with the aid of an electrostatic emission microscope was carried out by Ahearn and Becker.27 They studied the processes of diffusion of thorium, its migration over the surface and evaporation, and also the influence of the grain structure of tungsten on these processes. The electron images show that emission occurs only from separate regions of the surface (spots), whose appearance and dimensions coincide with the appearance and dimensions of the grains visible in ordinary optical images.

The electron images show that thorium diffuses to the surface at a relatively small number of chaotically arranged points, around which a thorium film is formed, migrating over the surface upon further heating. As a result of continuing migration, the surface density of thorium decreases,

which is seen from the decrease in the brightness of a spot when it increases in size. The process of change in the size and brightness of spots on the surface with a change in temperature is clearly seen in Fig. 20, where electron images are presented of polycrystalline thoriated tungsten, first annealed at a temperature of \(2600^\circ\)K and then treated for a short time from \(1500^\circ\) to \(1800^\circ\)K. Photograph a shows that the greater part of the surface does not emit, and almost all the emission comes from several small bright spots. The remaining photographs in Fig. 20 show the “life history” of these spots during the process of heat treatment (temperature increase). If one follows, for example, the spot in the right-hand part of the image, then we see that its size and brightness increase (a, б, в); then the size continues to increase, but the brightness decreases slightly because of a decrease in the surface density (г, д, е); and, finally, ж and з show that the spot spreads over the surface, greatly decreasing in brightness.

Since thorium spots resemble volcanic eruptions, the authors call them thorium “eruptions,” changing in space and in time. Concerning the number and frequency of these “eruptions,” the authors give interesting data. For coarse-grained surfaces, of the total number of observed eruptions, \(1/3\) occur at grain boundaries; moreover, the frequency of their appearance is different for different grains, and on some grains they do not appear at all. Annealing the surface at a high temperature temporarily increases the frequency of “eruptions.” For example, if the surface of a tungsten filament is annealed at a temperature of \(2900^\circ\)K for 1 minute and the number of “eruptions” per minute is counted at a temperature of \(1925^\circ\)K, then this number rapidly decreases with time, soon reaching a constant value equal to approximately \(1/3\) of its initial value. If the filament is rapidly cooled and annealed again, the frequency of eruptions temporarily increases.

Electron images of a single crystal of thoriated tungsten filament reveal alternating active and inactive bands parallel to the axis of the filament. These bands are probably associated with the planes of the single crystal. Thorium “eruptions” take place only on the active bands. The latter indicates that thorium diffuses more readily in certain directions of the crystal than in others. At the same temperatures at which thorium “eruptions” occur (i.e., about \(1500^\circ\)K), migration of thorium is also observed over the polycrystalline surface of thoriated tungsten. The fact that, in the process of growth of the spots, they remain round shows that thorium migrates with equal velocity in all directions.

In contrast to this, on the surface of a single crystal thorium migrates anisotropically. X-ray analysis showed that the emitting surface of the single crystal coincides with the plane (211)

and the predominant direction of thorium migration lies in this plane in the \((111)\) direction.

Electron images of thoriated tungsten made it possible to determine the differences in work functions for different crystals. To a first approximation, the reciprocal ratio of two exposure times (at constant temperature and voltage) for different crystals with the same blackening density on the photographic plate gives the ratio of their emissions, and by means of the equation

\[ i = AT^2 e^{-\varphi/kT} \]

the difference in the work functions of two grains, \(\varphi_2-\varphi_1\), is determined. For different grains it has values from \(0\) to \(0.6\ \text{V}\).

For the study of the structure and activation processes of thoriated molybdenum, a high-resolution emission microscope was successfully used. Two photographs of thoriated molybdenum are shown in Fig. 21. Photograph \(a\) was obtained with an electrostatic microscope\(^1\) at a magnification of 2000 times. It clearly shows the discrete distribution of thorium on the surface of molybdenum (bright spots). Photograph \(b\) was obtained with a magnetic microscope\(^16\) at a magnification of 3000 times. It is clearly seen that thorium emerges predominantly at the grain boundaries. However, the photograph also shows that separate sites of thorium emergence may also lie in the middle of a grain.

Oxide cathodes. Oxide cathodes have especially wide application in electrovacuum technology. Therefore, the greatest number of electron-microscopic investigations has been devoted to them.

The results of these investigations, in the main, reduce to the following. First of all, the emission pattern of all oxide-paste cathodes has a clearly expressed heterogeneous character, with the presence of a large number of separate islands, the shape and dimensions of which depend on the method of application and on the thickness and structure of the layer.

In the work of N. G. Sushkin\(^19\), results of investigations of thick oxide cathodes are presented. From a comparison of light and electron images, he showed that cracks emit considerably more strongly than the rest of the surface. The author explains this by three factors: 1) the increased emitting surface of the cracks; 2) the higher temperature of the oxide layers lying near the core; 3) the increased amount of active barium at the bottom of the crack.

The method of activation of the cathode is of substantial importance: in the presence of an electric field (electrical activation) or without it. The first method leads to the formation of emission islands, whereas with the second method a relatively uniform pattern of emission distribution is obtained. Under all conditions, a thin-film cathode gives a more uniform pattern than a thick-film one. During prolonged operation of the cathode, deactivation usually occurs.

When an oxide cathode is operating, an interesting phenomenon is often observed, usually called the phenomenon of “reversal”\(^{28,30}\), which consists in the fact that the dark places in the image of the cathode after some time become bright, while the bright places, conversely, become dark (alternation of the “negative” and “positive” of the image). This is observed especially distinctly when a mesh has been scratched on the surface of the cathode. The phenomenon of “reversal” is explained by a surface displacement of the active substance caused by a nonuniform temperature distribution along the surface of the cathode. Readers wishing to become more thoroughly acquainted with this phenomenon are referred to \(^{5}\).

Simultaneous observation of the emission pattern of oxide cathodes with the aid of an electron microscope and measurement of their electron emission\(^{29}\) leads to the conclusion that the surface of the cathode, already as a result of the first step of activation (both thermal and electrolytic), when its emission is still very small, is practically completely covered with emission centers, the character of whose distribution remains practically unchanged as the emission grows. This makes it possible to conclude that the increase of emission during activation of oxide cathodes is very little connected with an increase in the effective emitting surface of the cathode, but depends, apparently, mainly on a decrease in the work function for electrons in the emission centers.

The emission microscope described above (Fig. 10) made it possible to study in detail the influence of the size of particles of oxides of various carbonates (barium, strontium, calcium) and of the methods of applying the oxide layer on the magnitude of electron emission\(^{17}\). It was shown that the method of preparing carbonates strongly affects the size of the particles of the final double carbonate. By changing the nature of the alkaline agent or of the precipitated carbonate, one can obtain a large number of carbonates with different particle sizes; the latter were determined with the aid of an optical microscope.

Table II shows the influence of methods of preparing carbonates on the particle sizes and of the latter on oxide emission.

It is evident from the table that, with a decrease in the particle size (of one type of carbonate), the emission of the oxide cathode increases.

In Fig. 22 electron microphotographs of certain carbonates are presented, showing, in comparison with light-optical images, that the size of the emitting spots representing oxide particles on the surface of the layer is of the same order as the initial size of the carbonate particles. The emission patterns also show that the smaller the particle size, the more uniformly the emission is distributed over the surface of the cathode.

The emission pattern, moreover, proves to depend strongly on the method of applying carbonates to the metal substrate. Fig. 23 shows electron images of cathodes obtained by wetting

by applying to the metallic substrate a suspension strongly diluted with a nonvolatile liquid. Photographs a and b are similar to those shown in Fig. 22, but image c (triple carbonate \(\mathrm{Ba}_{56}\mathrm{Sr}_{31}\mathrm{Ca}_{13}\)) differs greatly from Fig. 22,c. This image appears almost homogeneous, on the basis of which the conclusion is drawn that the layer consists

Table II

Nomenclature	Composition (parts by weight)	Preparation method	Particle size (in \(\mu\))	Type of particles	Emission at \(1\ \mathrm{v}/\mathrm{cm}\) in \(\mu a\)
\(\mathrm{K}_1\)	\(\mathrm{Ba}, \mathrm{Sr}\) (equimolecular)	\((\mathrm{NH}_4)_2\mathrm{CO}_3\) is added to the nitrate solution. Alkalized with \(\mathrm{NH}_4\mathrm{OH}\)	50	Mainly spherical	70
\(\mathrm{K}_2\)	\(\mathrm{Ba}, \mathrm{Sr}\) (equimolecular)	\(\mathrm{Na}_2\mathrm{CO}_3\) is added to the nitrate solution	3	Large number of prismatic needles	130
\(\mathrm{K}_3\)	\(\mathrm{Ba}, \mathrm{Sr}\) (equimolecular)	\(\mathrm{CO}_2\) is passed into the nitrate solution. Alkalized with \(\mathrm{NH}_4\mathrm{OH}\)	15	Mainly spherical
\(\mathrm{K}_4\)	\(\mathrm{Ba}, \mathrm{Sr}\) (equimolecular)	\(\mathrm{CO}_2\) is passed into the nitrate solution. Alkalized with \(\mathrm{NaOH}\)	\(<\tfrac{1}{2}\)	Large number of needles	40
\(\mathrm{S}_1\)	\(\mathrm{Ba}_{56}\mathrm{Sr}_{31}\mathrm{Ca}_{13}\)	\((\mathrm{NH}_4)_2\mathrm{CO}_3\) is added to the nitrate solution. Alkalized with \(\mathrm{NH}_4\mathrm{OH}\)	100	Large crystalline aggregates	50
\(\mathrm{S}_3\)	\(\mathrm{Ba}_{56}\mathrm{Sr}_{31}\mathrm{Ca}_{13}\)	\(\mathrm{Na}_2\mathrm{CO}_3\) is added to the nitrate solution. Alkalized with \(\mathrm{NH}_4\mathrm{OH}\)	7	Large number of needles	140
\(\mathrm{R}_1\)	\(\mathrm{Ca}, \mathrm{Ba}\) (equimolecular)	\((\mathrm{NH}_4)_2\mathrm{CO}_3\) is added to the nitrate solution. Alkalized with \(\mathrm{NH}_4\mathrm{OH}\)	3	Mainly spherical	35
\(\mathrm{R}_2\)	\(\mathrm{Ca}, \mathrm{Ba}\) (equimolecular)	\(\mathrm{Na}_2\mathrm{CO}_3\) is added to the nitrate solution. Alkalized with \(\mathrm{NaOH}\)	—	Spherical	35

of small particles. The cathodes shown in Fig. 22 were obtained by spraying the suspension onto the substrate, whereas the cathodes shown in Fig. 23 were obtained by wetting. The difference in the character of the image for different methods of applying the suspension (Fig. 22,c and 23,c) is explained by the fact that the triple carbonate consists of a large number of particles of different sizes (from \(<\tfrac{1}{2}\ \mu\) to \(120\ \mu\)). Therefore, during spraying, mainly large particles are deposited on the surface of the substrate, whereas during wetting, on the contrary,—

smaller, owing to the slow drying of the surface and the flow of larger particles.

Let us turn to a consideration of the influence of space charge on the quality of the electron-optical image of oxide cathodes. The space charge at the surface of the cathode, especially an oxide cathode, which gives a considerable electron emission, plays an essential role in the formation of the cathode image and, most often, worsens the image quality. In order to reduce the influence of the space charge, on the one hand the cathode temperature is lowered, and on the other hand the anode voltage is greatly increased.

Mecklenburg², using the two-stage electrostatic microscope described above, carried out some investigations of oxide-paste cathodes and of the influence of space charge on their quality.

Figure 24 shows the image of an oxide cathode. The cathode was obtained by applying a layer of barium-strontium carbonate \(0.1\ \mathrm{mm}\) thick to a nickel substrate, followed by its activation. In the photograph, large, strongly emitting regions of the cathode surface are visible. Owing to the influence of the strong space charge at the cathode, these regions are not imaged sharply, and resemble a “cloudy” structure. Only certain emission centers, which “show through” the cloud of space charges, can be imaged sharply.

Calcination of the cathode greatly changes its emission pattern. Figure 25 shows the cathode at different stages of calcination. In this case the barium-strontium paste was applied in a very thin layer. After slight calcination, large and small emission centers with a strong space charge are visible in image \((a)\). After further calcination with an applied anode voltage, the emission centers became considerably smaller (\(\sim 70\ \mathrm{m}\mu\)), and the space charge is less noticeable \((b)\). At the same time, individual cracks on the nickel surface can be observed, as light and dark strokes. With intensified calcination, the emission pattern changes completely in a short time \((c)\). The fine-grained structure of the paste disappears and a patterned picture strikes the eye. The individual emission centers, indicated by light points, are evidently small barium globules which have not yet spread over the cathode surface or evaporated.

After this, the cathode was again carefully heated. The barium then evaporates strongly, a fairly homogeneous thin emitting layer appears, and the crystalline structure of the substrate becomes visible \((d)\).

As can be seen from the figure, emission from different crystalline surfaces is different. Particularly conspicuous are dark spots of various sizes, which change their form when the cathode is brought closer to the diaphragm (strengthening of the field at the cathode).

2. Electron-microscopic investigations of the crystalline structure of metals

With the aid of the electron microscope, the crystalline structure of certain metals had already long ago been discovered. It was observed at first for such metals as pure tungsten, a platinum-rhodium alloy, and nickel. As it subsequently turned out, the deposition of a thin layer of an active substance (barium, strontium, cesium, etc.) on the surface of the metal under investigation considerably facilitates observation of its crystalline structure.

The nature of the image of the structure of metallic surfaces has not yet been studied sufficiently fully. However, it is a fairly plausible supposition that the work function and, consequently, the emission of different crystalline surfaces are different, which produces different intensities of the corresponding places in the electron image. By the photoelectric method[^31] it was established for most metals that the difference in work function for different planes of a crystal amounts to several tenths of an electron-volt. For activated metallic surfaces one must also allow for the influence of different filling of the metal surface by the active substance.

The strong emission of activated metals makes it possible to obtain a magnified image of the surface structure even at comparatively low temperatures. This makes it possible to study separate crystalline modifications (phases) of a metal, and also directly to observe phase transformations and the course of recrystallization[^32],[^33]. It is possible, for example, easily to observe phenomena occurring in the transition from the cubic body-centered ($\alpha$-phase) to the cubic face-centered ($\gamma$-phase) modification of iron at a temperature of about $900^\circ$, and even to obtain a cinematographic record of the transition process.

In Fig. 26 a series of photographs is shown illustrating this process for iron activated with the double carbonate $(\mathrm{SrCO_3+BaCO_3})$. As is seen in the figure, starting from the $\alpha$-state (photograph $a$), as the temperature is raised the $\gamma$-crystals grow from right to left (photographs $b$—$z$). After the $\gamma$-state is reached ($z$), the temperature was lowered ($i$—$o$), the $\alpha$-crystals displace the $\gamma$-crystals and the $\alpha$-state is restored again. The fact that the crystallites grow from one side of the specimen to the other is a consequence of the existence of a temperature gradient in the iron strip. This dual transformation process is repeated in the photographs from $p$ to $u$ ($\alpha \to \gamma$) and from $ф$ to $ц$ ($\gamma \to \alpha$).

From the fact that the transition processes are visible in the electron emission picture, one may conclude that in the region of crystal growth the quantity or arrangement of the active atoms changes from the initial state, corresponding to the $\alpha$- (or $\gamma$-) crystals,

to its final state, corresponding to $\gamma$ (or $\alpha$)-crystals, by means of migration or evaporation.

When iron is activated with strontium, in view of the fact that the mobility of strontium on iron at the transition-point temperature is high, this “adjustment” of the strontium atoms to the new state occurs practically instantaneously.

Let us point out here one more very interesting phenomenon observed in the course of investigations of emission patterns of metallic surfaces activated by electropositive atoms[^33][^34]. This phenomenon consists in the fact that, under certain conditions, a system of parallel lines of high intensity appears on individual crystals (Fig. 27). Such a system of lines appears, for example, if a normally activated iron–nickel cathode is again coated with strontium (together with strontium oxide) and heated at a temperature of about $900^\circ\mathrm{C}$. From the fact that the lines run, for the most part, from boundary to boundary of the crystals, it may be concluded that they are connected with the orientation of the crystal lattice; the authors regard them as corresponding to traces of slip planes on the surface of the crystal. On the irregularities of the surface (ridges and depressions) formed by the slip planes, the adsorption capacity for atoms of electropositive metals is higher than on the rest of the crystal surface.

The fact that the lines disappear after prolonged annealing of the cathode and do not appear again after repeated annealing of the activator is regarded as an indication that the surface irregularities are destroyed by heating.

Of great interest are electron-optical investigations of the emission crystal patterns of zirconium. At a temperature of about $865^\circ\mathrm{C}$ it undergoes allotropic transformations from a hexagonal ($\alpha$) to a cubic ($\beta$) structure.

With the aid of the electron microscope it proved possible to observe directly, in emission patterns, the $\alpha$ and $\beta$ modifications of the structure of zirconium. However, observation of the actual process of the $\alpha \to \beta$ transition was made difficult by insufficient emission near the transition temperature ($865^\circ\mathrm{C}$). To obtain an electron image of zirconium, a temperature of at least $1050$–$1100^\circ\mathrm{C}$ is necessary. In this case the image shows more or less homogeneous crystals ($\beta$ modification), rapidly changing in size from a few tenths to $1$ mm and more (Fig. 28, a). At temperatures below $1000^\circ$ the emission becomes very weak and the electron image practically disappears. But if the surface of zirconium is activated with barium by evaporating it from an external evaporator, appreciable emission is observed at a temperature of $600$–$650^\circ$, at which the $\alpha$ modification occurs (Fig. 28, b). This emission again disappears on heating to $700^\circ\mathrm{C}$ (owing to the disappearance of barium atoms from the surface).

Fig. 20. Thorium eruptions on a fine-grained tungsten ribbon. The photographing temperature is 1490° K. For each photograph, the temperature and time of heat treatment, the average degree of thoriation \((f)\), and the exposure are, respectively:
a) 2550° K, 30 sec, 0.03, 60 sec; b) 1530° K, 1 min, 0.06, 1 min; c) 1570° K, 1 min, 0.06, 1 min; d) 1660° K, 1 min, 0.06, 1 min; e) 1680° K, 1 min, 0.1, 40 sec; f) 1720° K, 1 min, 0.01, 1 min; g) 1775° K, 1 min, 0.11, 30 sec; h) 1830° K, 2 min, 0.11, 30 sec.

Fig. 21. Electron micrographs of thoriated molybdenum: a) according to Bersh (magnification 2000), b) according to Kinder (magnification 3000).

Fig. 22. Electron micrographs of carbonates deposited on the core by spraying (magnification 100): a) \(K_1\) (see Table 1), b) \(K_2\), c) \(S_1\).

Fig. 23. Electron micrographs of carbonates deposited on the core by wetting (magnification 100): a) \(K_1\), b) \(K_2\), c) \(S_1\).

Fig. 24. Electron image of a barium-strontium oxide layer 0.1 mm thick on a nickel core.

Fig. 25. Electron images of a cathode with a thin oxide layer at various stages of annealing:
a) after heating for \( \frac{1}{2} \) hour without an external field,
b) after heating for \( \frac{1}{2} \) hour with an external voltage,
c) after heating at a higher temperature,
d) after evaporation of the main mass of the active substance.

Fig. 26. Electron-optical observations of four transformations following one another in iron at \(900^\circ\):
\(a\)—\(z\): \(\alpha \to \gamma\); \(i\)—\(o\): \(\gamma \to \alpha\);
\(p\)—\(u\): \(\alpha \to \gamma\);
\(\phi\)—\(\psi\): \(\gamma \to \alpha\).

Fig. 27. Lines of high intensity on the surface of strontium-activated nickel iron: a) after the first application of the activator, b) after evaporation and a second application of the activator, c) after annealing and a third application of the activator.

Fig. 28. Electron-optical observation of allotropic transformations of zirconium: a) emission pattern at 1100° C, b) emission pattern at 600° C, c) emission pattern upon repeated raising of the temperature to 1150° C.

Fig. 29. “Reversal” of the relative emission intensity of different crystals during exploratory heating of activated zirconium from 600° C to 700° C.

Fig. 30. Images of pure zinc (magnification 7): a) photoelectron, b) light.

Fig. 31. Photoelectron images of platinum foil (magnification 25):
a) \(t = 20^\circ\mathrm{C}\), b) \(t = 900^\circ\mathrm{C}\), c) \(t = 1640^\circ\mathrm{C}\), d) \(t = 20^\circ\mathrm{C}\) (after heating to \(1640^\circ\mathrm{C}\)).

Fig. 32. Photoelectron images (magnification 30×): a) of a nickel cathode, b) of a silver-plated copper cathode.

Fig. 33. Photoelectron images of an oxygen–cesium photocathode: a) under illumination with white light, b) under illumination with light of wavelength \(3000\text{–}4000\ \text{Å}\), c) under illumination with light of wavelength \(6000\ \text{Å}\).

Fig. 34. Light (a) and electron-ion (b) images of a zinc plate with a mesh and scratches applied to it.

Thus, one can observe only the $\alpha$- and $\beta$-modifications, without observing the transition itself. Using other activators, apparently, it will be possible to observe the transition process itself as well.

In the electron image corresponding to the $\alpha$-modification (Fig. 28, б), it is seen that the regions formerly occupied by $\beta$-crystals are still noticeable, but in each of these regions one or more systems of parallel or intersecting layers are visible, giving overall an acicular pattern reminiscent of the structure encountered in martensitic steels. If the temperature is again raised above the transition point, the $\beta$-structure is restored again (Fig. 28, в), with a slight change in the left part caused by crystal growth. Thus, the $\alpha \rightleftarrows \beta$ transformation is, essentially, a reversible process.

When barium-activated zirconium is heated from $600^\circ$ to $700^\circ$, the phenomenon of “reversal” of the intensity of different crystals takes place (Fig. 29), which is especially noticeable for crystals 1 and 2. This can be explained by the fact that the temperature at which the decrease in emission due to barium evaporation exceeds the increase in emission due to the rise in core temperature is different for different planes of the crystal.

3. Photocathodes

For the investigation of the emission properties and structure of photocathodes, the emission microscope has been used comparatively little. Most of the studies, relating mainly to the early period in the development of electron microscopy (1932–1936), concern photoemission from pure metals. The study of complex photocathodes has not received proper development even in recent years.

Photocathodes, unlike thermocathodes, have certain features that make it difficult to obtain good enlarged images. The chief one is that they give small emission currents, insufficient for obtaining a high brightness of the screen glow, even at high accelerating voltages. Furthermore, the need to illuminate the photocathode makes it impossible to place the accelerating electrode very close to the cathode and, consequently, to obtain a strong electric field at the cathode (if semitransparent cathodes are not considered). For complex photocathodes, the investigations are, in addition, complicated by their sensitivity to deterioration of the vacuum and, as a result, by the impossibility of using inside the microscope massive metallic parts, in particular the pole pieces of magnetic lenses, with whose aid large magnifications can be obtained.

The first electron-optical images of a photocathode consisting of a zinc plate, illuminated by a quartz arc, were obtained with the aid of a magnetic shielding coil.

The accelerating voltage was equal to 30 kilovolts. The magnification was only 2.5 times.

Further investigations of photocathodes \(^{22,35}\) with the aid of the electron microscope shown in Fig. 17 made it possible to reveal their crystalline structure and its change upon heating. In Fig. 30, for example, electron (a) and optical (b) images of a zinc cast cathode are given with a magnification of \(\sim 7\); the identity of these two images is beyond doubt.

Studies of platinum foil show an increase in nonuniformity with increasing temperature (Fig. 31). Upon cooling of the cathode this nonuniformity remains. Images в and г indicate the similarity of the distribution of thermo- and photoemission.

In Fig. 32 images are given of nickel (a) and copper (b) cathodes magnified 30 times. The crystalline structure of nickel is clearly visible. The copper cathode was etched in vacuum; the light bands correspond to clean, unoxidized areas of the metal, possessing high photosensitivity.

With the aid of photoelectron images it proved possible to observe the processes of melting and crystallization of metals (for example, zinc).

It was found possible, further, to obtain images of minerals (for example, galena and copper pyrites).

Of considerable interest is the study, by means of the electron microscope, of complex photocathodes that have become widespread in the technology of electronic devices. However, electron-microscopic investigations of them are limited, so far as is known from the literature, to the works of Moriva \(^{36}\) and Sushkin \(^{21}\).

The first of them, with the aid of electron images with a magnification of 25 times, studied the distribution of emission over the surface of a silver-oxide–cesium photocathode \((\mathrm{Ag{-}Cs_2O{-}Cs})\), as a function of the wavelength and the plane of polarization of the incident light.

When the cathode surface was illuminated with white light (Fig. 33, a) and with light of wavelength \(\lambda = 3000—4000\,\text{\AA}\) (Fig. 33, b), the electron-optical images show a comparatively uniform distribution of photoemission over the cathode surface. The sensitivity of the cathode was only \(1.5\ \mu\mathrm{a}/\mathrm{lm}\). Illumination of the cathode with light of wavelength \(\lambda = 6000\,\text{\AA}\), however, gives a strongly nonuniform picture of the distribution of photoemission; the emitting regions are arranged in separate groups (Fig. 33, c). The author explains this nonuniformity of photoemission by a nonuniform distribution of the values of the work function over the surface of the photocathode.

When the cathode was illuminated with white light polarized in a plane perpendicular and then parallel to the plane of the photocathode, no difference was found in the character of the emission patterns; the distribution of emission over the cathode surface was homogeneous in both cases; at any rate, the nonuniformity is not noticeable at a 25-fold magnification.

Such magnification is clearly insufficient for studying the structure and emission properties of complex photocathodes.

In the work of N. G. Sushkin, results are given of certain investigations of the structure of an antimony-cesium photocathode at magnifications of 40–50 times. The diagram of his microscope is shown in Fig. 17, a; the photocathode had a sensitivity of the order of 40 μA/lm. Onto the photocathode was projected the light image of the filament of an incandescent lamp. The surface of the cathode, by means of a magnetic lens, was imaged on the screen of the microscope. In the electron-optical image of the filament, dark points were visible, which were sharp images of individual cathode particles protruding on the surface of the photosensitive layer. The distance between them in the electron image was equal to 0.1 mm. These points enabled the author to estimate the resolving power of the electron-optical system he used; the resolved distance proved to be 0.002 mm.

The last two works provide only preliminary data and indications of the possibility of further investigations of complex photocathodes. A necessary condition for the success of further studies of photocathodes by methods of emission microscopy is an increase in magnification and, at the same time, in the resolving power of electron-optical systems.

Along with photoelectrons, the possibility was established37,38,39 of using secondary electrons to obtain images. For this purpose one may use both electrostatic and magnetic systems. However, obtaining a high resolving power of the microscope in the case of secondary electrons presents considerable difficulties owing to large chromatic aberrations. Up to the present, this type of electron-microscopic investigation has not found wide application.

IV. OBTAINING ELECTRON-OPTICAL IMAGES AT ATMOSPHERIC PRESSURE

In conclusion we shall point to the original experiments on obtaining electron-optical images of metallic surfaces at atmospheric pressure, carried out by G. V. Spivak and R. A. Lukatskaya4.

One of the experiments amounted to the following. A zinc plate measuring \(3 \times 3\ \mathrm{cm}^2\) was placed at some distance (\(\sim 15\ \mathrm{cm}\)) from an ebonite plate, on the reverse side of which there was a tinfoil coating. If the zinc plate is connected to the negative pole, and the tinfoil coating to the positive pole, of a high-voltage source (\(\sim 10\text{–}15\ \mathrm{kV}\)), then, when the zinc plate is illuminated with ultraviolet light, an image of the structure of the surface of the zinc plate is observed on the surface of the ebonite plate (provided that it is dusted with talc).

Depending on the shape of the electrodes and the distance between them, various magnifications were obtained (up to 100 times). In Fig. 34 are shown: a) an ordinary light photograph of a zinc plate with a mesh applied to it by means of lacquer and with horizontal scratches around it; b) its image by the method described above.

The mechanism of image formation proposed by the authors is, in the main, as follows.

Photoelectrons emitted from the surface of the zinc are transformed in considerable numbers into negative ions immediately upon leaving the cathode. In collisions with air molecules, the slow negative ions, giving up almost all their energy, in fact come to a stop and then again begin to move along the field. The value of the force must be such that the thermal motion does not prevail over the directed drift of the ions. Under these conditions the negative ions, moving along the lines of force, carry the image of the cathode to the anode. The number of photoelectrons leaving a given place on the cathode determines the number of negative ions reaching the ebonite plate. The scratched and smooth places of the zinc plate emit different numbers of electrons, which in the final result gives the image of the mesh.

According to the authors, the resolution that it has been possible to obtain in the latest experiments is several microns.

Despite their preliminary character, the experiments of Spivak and Lukatskaya open up an entirely new, very interesting direction in the optics of charged particles.

CONCLUSION

We have briefly described the features and designs of the emission microscope, as well as the variety of phenomena that can be investigated with its help. The emission microscope has found especially successful application in the study of incandescent cathodes, which are widely used in the technology of electronic devices. It is also fruitfully used for studying the crystalline structure of various metals, thereby opening up a new field in metallography.

Electron-microscopic studies of photocathodes and secondary-emission emitters are still limited to a small number of works and do not give reliable results. Of particular interest should be the investigation of complex photocathodes, which up to now have scarcely been studied at all with the aid of the microscope. The use of photoelectrons together with thermoelectrons will make it possible to investigate the surfaces of various objects at any temperatures. However, in the field of photocathodes and secondary-electron emitters one must expect new difficulties because of the large spread of the initial velocities of the electrons.

The central problem in the field of emission microscopy is the increase of the resolving power of the microscope. The first

experiments and theoretical studies in this field give grounds for believing that this problem will be successfully solved. In resolving power, the electron microscope has already now left the light microscope far behind. There can be no doubt that it will find the broadest application in various fields of science and technology.

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