NEW INSTRUMENTS AND METHODS OF MEASUREMENT

EQUIPMENT AND METHODS FOR STUDYING RAMAN SCATTERING SPECTRA IN GASES

Ya. S. Bobovich and V. M. Pivovarov

INTRODUCTION

Studies of Raman scattering spectra in substances that are in the gaseous or vapor state are of great interest. This is due, on the one hand, to the fact that the existing theory of the phenomenon is, strictly speaking, applicable to isolated molecules. Therefore, a number of its conclusions can be verified only by studying the spectra of gases and vapors at relatively low elasticities. On the other hand, the possibility of studying purely rotational and rotational-vibrational Raman scattering spectra is also extremely important. Although rotational and rotational-vibrational spectra, as is known, can also be obtained by methods of infrared absorption and radiospectroscopy, the Raman scattering method has, in comparison with them, a number of fundamental and technical advantages. First of all, let us note that, for the first two methods, the study of purely rotational spectra is accessible only for dipolar molecules. The indirect data that can thereby be obtained from rotational-vibrational spectra are not always amenable to simple interpretation. At the same time, in Raman scattering, rotational transitions are active for all molecules, with the exception of molecules of the spherical-top type. This circumstance substantially expands the range of possible objects of investigation.

However, even in the case when information on the parameters of a molecule that characterize its rotation (rotational constants, moments of inertia, distances between atoms) can be obtained both by the infrared absorption method and by the Raman scattering method, the latter of them still deserves a certain preference. This is connected with the fact that, according to the selection rules,

the distance between the lines of the $S$- and $O$-branches in combination scattering is twice as large as in infrared absorption. Consequently, with the same accuracy of frequency measurement, in the first case the molecular parameters listed above can be determined more accurately. Another point is also very important here. It turns out that in rotational-vibrational spectra of combination scattering, generally speaking, a larger number of rotational branches is observed than in infrared spectra. This circumstance favors a more detailed study of the interaction between rotational and vibrational degrees of freedom.

From the experimental point of view, spectroscopy of combination scattering in gases can at present successfully compete in its capabilities with infrared absorption spectra. As for obtaining pure rotational spectra of combination scattering, the technique here is much simpler than in the study of the far infrared region.

Thus, spectroscopy of combination scattering in gases has great prospects for application and further development.

1. DESCRIPTION OF INSTALLATIONS FOR OBTAINING SPECTRA OF COMBINATION SCATTERING IN GASES

In comparison with the study of liquids and even solids, the recording of combination-scattering spectra in gases and vapors represents a difficult experimental problem. The reason for this lies chiefly in the extremely low intensity of the radiation.

The specific features of working with gases are connected primarily with the light source for exciting the spectrum and with the cuvette into which the gaseous substance under study is introduced. It is therefore advisable first of all to dwell in detail on these two elements of a gas installation.

A. Light sources

In the spectroscopy of combination scattering of liquids, as is known, the high-pressure mercury arc has found wide application as a light source. Such light sources include domestic lamps of the IGAR, PRK, ARK types, American lamps of the H-1 type, and others. The principal merit of these lamps is their high radiation brightness. They are, however, of little use, and in most cases entirely unsuitable, for the purpose of exciting spectra in gases, chiefly because of the strong continuous background in the frequency region of the combination-scattering lines. The combination of this continuous background with the extremely low intensity of the scattered radiation makes it impossible to record the spectrum.

The background intensity of various high-pressure lamps depends on many factors, but above all on the current density, the dosage of mercury, and the inert gas. Consequently, lamps of different types will differ somewhat in background intensity. Thus, for example, lamps of the PRK type are apparently, for these reasons, noticeably worse than their prototype—the ARK lamp, which was formerly produced by our industry. Nevertheless, the choice of a high-pressure lamp of one type or another as a source for exciting spectra does not fundamentally solve the question of a continuous background.

The most acceptable light source for work with gases is a low-pressure mercury lamp. The principal merit of such lamps, which has determined their wide use in the study of gases, is their very weak background and narrow lines.

As is known, two forms of gas discharge are characteristic of low-pressure lamps: a glow discharge (current density \(10^{-2}\)—\(10^{-4}\ \mathrm{a}/\mathrm{cm}^{2}\)) and an arc discharge (current density up to \(10\ \mathrm{a}/\mathrm{cm}^{2}\)). Glow-discharge lamps are supplied with an alternating voltage from 0.5 to \(10\ \mathrm{kV}\), while arc-discharge lamps are supplied with either direct or alternating voltage of \(110\)—\(220\ \mathrm{V}\).

When working with glow-discharge lamps, the resonance line \(\lambda = 2537\ \text{Å}\) is usually chosen as the exciting line. The choice is determined by the energy distribution in the spectrum of these lamps. It should be noted, however, that many organic compounds absorb in this region. In addition, they may fluoresce and undergo photochemical decomposition when illuminated with short-wavelength radiation. Therefore the method of exciting the spectrum with the line \(\lambda = 2537\ \text{Å}\) can by no means be considered universal.

When working with arc-discharge lamps, usually made of Pyrex or ordinary glass, the line \(\lambda = 4358\ \text{Å}\) is, as a rule, used for excitation of the spectrum.

We shall describe several types of lamps, approximately following chronological order.

One of the first to be used in the spectroscopy of combination scattering of gases was the Küpper-Juitt arc-discharge lamp\(^{1-4}\). Its length was \(1.5\ \mathrm{m}\), and the diameter of the discharge tube \(20\)—\(35\ \mathrm{mm}\). Depending on the diameter of the discharge tube, the operating current varied within the limits \(2\)—\(9\ \mathrm{a}\). The cathode was liquid mercury, the anode metallic. The temperature of the coldest part of the lamp—the liquid cathode—was \(+80\)—\(+100^\circ\ \mathrm{C}\), corresponding to a mercury-vapor pressure reaching \(0.1\)—\(0.3\ \mathrm{mm}\) Hg. In some cases air cooling of the lamp was used\(^{5}\). A typical construction of a lamp of this type is shown in Fig. 1.

Among the operational shortcomings of such sources are the difficulty of ignition and the need for supply by direct voltage. Most important, however, is their comparatively low efficiency.

This is associated with the low current density; increasing the current density would inevitably increase the vapor pressure of mercury and, consequently, the intensity of the continuous background.

In the spectroscopy of combination scattering in gases, lamps of the Cooper-Hewitt type were used in 1929–1935.

Beginning in 1934, low-pressure lamps with a glow discharge came into use. Initially they were filled only with mercury (the Hanovia Sc-2537 quartz lamp\(^{6—10}\)). Subsequently, to facilitate ignition, a metered quantity of neon was added to the lamp (Ne—Hg lamps). Their power was brought up to 500 W. With such a power it is still not difficult to transform the voltage from 120–220 to 500–10000 V, required to supply this source. The lamp operates at normal temperature, which corresponds to a mercury vapor pressure of approximately 0.001 mm Hg. In design, lamps of this type, specially manufactured for investigations

Fig. 1. Low-pressure mercury lamp of the Cooper-Hewitt type.

Fig. 2. Mercury spiral glow-discharge lamp.

in the field of combination scattering in gases, were a spiral with an extended length of about 3 m. A typical design of the lamp is shown in Fig. 2.

Owing to the comparatively low efficiency of these lamps, photographing spectra usually required long exposures, sometimes reaching 200 hours even when working with instruments of not very great dispersion.

A further, moreover radical, improvement of low-pressure lamps with an arc discharge is connected with the use of local water cooling of the liquid cathode and anode (or of one cathode). Thanks to this it was possible sharply to increase the electrical and luminous power of the lamp without increasing the vapor pressure of mercury. The power of modern lamps of this type reaches 2–3 kW.

One variant of the new source that has become widespread is the spiral low-pressure lamp, known as the “Toronto” lamp\(^{11,12}\). It is made in the form of a spiral approximately 2 m long from a Pyrex tube about 30 mm in diameter. Both electrodes are liquid, mercury, with water cooling. Owing to the cooling of both electrodes, a more favorable distribution of intensities among the components of the blue mercury spectral ...

triplet than when only one cathode is cooled. To increase the lamp power still further, a thin additional tube with water cooling is sometimes introduced inside the spiral[^13]. A diagram of such a lamp is shown in Fig. 3.

Fig. 3. Spiral low-pressure mercury lamp of the “Toronto” type with internal cooling of the working part (according to White et al.[^13]).

A design disadvantage of lamps of the “Toronto” type is that they are intended for operation in a vertical position. In the study of gases, however, a horizontal arrangement proves to be much more convenient.

In our laboratory, horizontal spiral low-pressure lamps have been used for several years[^14]. They are made of molybdenum glass. The tube diameter is about 25 mm. The extended length of the spiral is about 1.5 m. The cathode is liquid, mercury, with water cooling; the anode is molybdenum, hollow. To facilitate ignition, as is usually done[^12],[^15],[^16], our lamps include, near the cathode, a “standby” anode (also mercury). The construction of the electrode parts is shown in Fig. 4. The lamps are supplied with direct voltage and are designed for an operating current of up to 25 A. At this current the voltage drop across them is approximately 100 V. The lamp is ignited by means of a powerful high-voltage pulse applied to the working electrodes while the arc is burning in the circuit of the “standby” anode. Before being switched on, the lamp is preheated by an electric furnace.

Further improvement of powerful low-pressure mercury lamps consisted in applying external cooling of their working part to a temperature somewhat exceeding the temperature of the electrodes[^17].

To understand the meaning of this improvement, it is necessary to consider the energy balance in mercury lamps at various densities

Fig. 4. Typical construction of the electrode parts of a low-pressure mercury lamp.

Fig. 5. Power balance in the positive column in mercury vapor at a current density of about tens of $\mathrm{mA/cm^2}$. $\eta_{\mathrm{rez}}$ and $\eta_{\mathrm{nerez}}$ are, respectively, the powers of resonant and nonresonant radiation; $\eta_{\mathrm{st}}$ and $\eta_v$ are, respectively, the heat losses at the walls and in the volume.

current and the different elasticity of mercury vapor. In Fig. 5 the energy balance at a current density of the order of \(ma/cm^2\) is shown schematically; the graph in Fig. 6 refers to a current density of several \(a/cm^2\). If one takes the point \(p = 0.001\) mm Hg (which approximately corresponds to \(20^\circ\)C), then the energy balance in Fig. 5 corresponds to a glow discharge, while in Fig. 6 it corresponds to an arc discharge. From a comparison of the ordinates it is seen that, in the case of the higher current density, losses at the walls increase noticeably. They are expressed in an increase in the temperature of the walls. It follows from this that the state of the walls—their temperature—plays an essential role in the discharge process. Since at low vapor pressure the mean free path of the particles is large, for this reason they practically do not collide with one another, but only with the walls of the discharge tube. If, however, the wall temperature is lowered so that it nevertheless remains higher than the temperature of the electrodes, then the kinetic energy of the particles decreases, although the vapor elasticity remains the same. A decrease in kinetic energy will naturally reduce the Doppler width of the line, and consequently the intensity at the line maximum will increase.

Fig. 6. Power balance in the positive column in mercury vapor at a current density of several \(a/cm^2\). The designations are the same as in Fig. 5.

It was precisely such measures—some cooling of the working part of the lamp—that were undertaken in one of the latest versions of the direct low-pressure lamp\(^{17}\).

Fig. 7. High-power low-pressure mercury lamp with an additional cooling jacket (according to Stoichev \(^{17}\)).

The structural design of such a lamp is shown in Fig. 7. As can be seen, the lamp has three independent external jackets. In two of them, connected with the electrodes, water circulates at a tempera-

temperature \(\sim 20^\circ\) C; in the third, enclosing the working part of the lamp, water heated to \(\sim 50^\circ\) C. Comparative tests show that, in efficiency, this lamp surpasses the “Toronto” lamp by approximately a factor of four, if they are reduced to the same consumed power. At the same time, according to the estimate made, solely on account of the Doppler effect the intensity of the lines should increase by approximately a factor of two. Apparently, other factors must also be taken into account.

In service life, powerful low-pressure mercury lamps are inferior to high-pressure lamps. Usually it is about 300 hours and is limited by blackening of the lamp glass. The causes of this phenomenon are unclear. It is possible that traces of organic matter on the glass itself and in the mercury play a certain role. Therefore, in manufacturing lamps, thorough cleaning of them is required. A successful choice of the type of glass is also important.

In conclusion we present a table illustrating the intensity of the background near the line \(\lambda = 4358\) Å, as well as the distribution of intensity among the components of the blue triplet in some mercury lamps.

Background intensity in the spectrum and distribution of intensities among the components of the blue triplet in some mercury lamps
(according to Rank and McCartney \(^{18}\))

B. Gas Cells

First of all, in the design of gas cells it is advisable to note the general point that determines, in general, their suitability for obtaining a spectrum from a gas.

The spectrum of combination scattering from a gas (all the more so, a purely rotational one) can be recorded only if the light entering the spectral instrument from the walls of the cell (i.e., mainly stray light) is eliminated. This is achieved, on the one hand, by the method of illuminating the slit of the spectrograph, and on the other—by the design of the gas cell.

Figure 8 presents various methods of eliminating stray light, achieved by the construction of the cell.

Labels in the figure: light trap; illuminated part; diaphragmed part.

Fig. 8. Various methods of eliminating stray light in gas cells.

The next important point in the construction of gas cells is their reliable sealing. It can be achieved by sealing the cell by soldering, or by using gaskets. The constructive solution of the second method is possible in two variants (if one restricts consideration to the most interesting question—the joining

metal frame with the glass). In the first of them (Fig. 9, a) an end gasket is used, in the second (Fig. 9, b) a surface gasket. Both of these variants are based on the principle of a seal with a compensated area, the first of them being suitable for operation with a pressure drop of several tens, and the second of only several atmospheres[^19].

Rubber of various grades is usually used as the material for the gaskets.

If the cuvette is intended for operation at different temperatures, then the use of the first sealing variant during heating is inevitably associated with weakening of the gasket owing to the different expansion coefficients of the glass and metal of the tightening rods. This circumstance limits the upper temperature limit and the length of the cuvette. The second version of the joint is, evidently, free from this drawback.

Fig. 9. Methods of sealing with gaskets:
a — end gasket; b — surface gasket.

Usually cuvettes are made of quartz or Plexiglas tubes and, more rarely, of glass ones. The choice of fused quartz is determined by its transparency in the ultraviolet (which in some cases is essential) and the high thermal strength of the material. Unfortunately, because of technological difficulties it is not possible to obtain quartz (or Plexiglas) tubes of large diameter (50–100 mm) and great length (1.5–2 m) with a considerable wall thickness (5–10 mm).

In mechanical strength, glass is little inferior to quartz and Plexiglas. If the ratio of the outer diameter of a glass tube to the inner diameter is not less than 1.1–1.2, then at ordinary temperature it is capable of withstanding pressures of up to 50 atm. However, the thermal coefficients of glass and quartz, as is known, differ between

with one another significantly. Therefore, the use of glass for thermostated cuvettes limits both axial and transverse temperature differences.

A very significant step forward in the design of gas cuvettes was made in connection with the use of mirror systems^20,17, which made it possible to use the volume nonabsorbing emitter—namely, the gas filling the cuvette—much more effectively.

Let us consider in detail the possible cases of application of mirror systems^20.

The simplest device of this type is a plane mirror at the end of the cuvette. By doubling the length of the cuvette (if the reflection coefficient is conventionally taken as unity), such a system therefore gives a twofold increase in intensity.

Fig. 10. Diagram of a single-mirror system (horizontal section).

Another example of a mirror system is the use of a single concave spherical mirror. Let us explain the action of such a system by the drawing (Fig. 10). Let \(KL\) be the image of the collimator lens and \(MN\) the image of the slit (horizontal section) produced by the condenser lens. The rays \(KN\) and \(LM\) determine the effective volume of the radiating substance. If a concave mirror is placed so that its center of curvature is at point \(a\) of the image of the slit, then each point \(P\) of the effective volume, in addition to the direct beam (Fig. 10, a), sends to the slit also a return beam reflected from the mirror (Fig. 10, b). Thus, if absorption is not taken into account, the intensity of the spectrum increases by \(1+R\) times, where \(R\) is the reflection coefficient of the mirror.

A more effective two-mirror system is shown in Fig. 11. The concave mirrors \(A\) and \(B\) have equal radii of curvature. The centers of their curvature lie at points \(a\) and \(b\), respectively. If \(S_1\) is the image of the slit on mirror \(B\), obtained from mirror \(A\), then from any point \(P\) of the effective volume four beams enter the slit of the spectrograph along the following paths:

1. A direct beam from \(P\) (Fig. 10, \(a\)).
2. A reverse beam from \(P\) with one reflection (Fig. 11, \(a\)).
3. A direct beam from \(P\) through \(S_1\) and \(P_1\) with two reflections (Fig. 11, \(b\)).
4. A reverse beam from \(P\) through \(S_1\) and \(P_1\) with three reflections (Fig. 11, \(c\)).

Thus, the intensity of the spectrum in this case should increase by a factor of \(1 + R + R^2 + R^3\).

Fig. 11. Diagram of a two-mirror system.

A schematic representation of a four-mirror system is shown in Fig. 12. Mirrors \(C\) and \(D\) may be parts of a single concave mirror with a transparent longitudinal slit \(S_1\), or its halves. Mirror \(A\) (with the center of curvature at point \(a\)) images \(S_1\) as \(S_2\) on mirror \(D\). Then the beam returns to mirror \(A\), and the cycle is repeated again.

Thus, in a four-mirror system the slit \(S\) is imaged by segments \(S_1, S_3, S_5,\ldots\) from mirror \(A\) and by segments \(S_2, S_4, S_6,\ldots\) from mirror \(B\). It is easy to imagine that point \(P\) has two series of images \(P_2, P_4, P_6,\ldots\) and \(P_1, P_3, P_5,\ldots\), located on opposite sides of the cuvette axis. Reasoning analogously to the preceding case, one can show that the intensity then increases by a factor of \(1 + R + R^2 + R^3 + \cdots + R^n\). Here \(n\) is the number of images of slit \(S\) on the front mirrors \(C\) and \(D\).

The construction of a gas cuvette with a mirror system must be such that the possibility of direct light from the lamps falling on the mirrors is excluded. This condition is satisfied in the design shown in Fig. 8, \(d\). Here the mirrors are mounted at the ends of the cuvette.

The effectiveness of a mirror system, all other conditions being equal, obviously depends on the reflection coefficient of the mirror coating. Recently, multilayer mirrors with a reflection coefficient in the working region (4000—5500 Å) reaching 98—99% have become available[^21]. Thus a four-mirror system constructed on the basis of such mirrors, depending on their dimensions, can increase the intensity of the spectrum by a factor of 20—30.

Fig. 12. Diagram of a four-mirror system.

When working with mirrors, one should take into account the possibility of small changes in the reflection coefficient associated with damage to the mirror layer under the influence of heated gases or vapors and other causes. A decrease of the reflection coefficient by only 1% will already weaken the spectrum by \(n\)% (where \(n\) is the number of images of the slit). Therefore quantitative measurements of line intensities in this case must be carried out with caution.

A prism system[^22] is largely free of this drawback. The principle of its operation is analogous to that of the mirror system. Instead of reflection from a metallic surface, however, total internal reflection is realized here. The efficiency of a prism system is approximately half that of the corresponding mirror system. Prism systems have not yet been used by anyone in cuvettes for combination scattering. They were developed specially for the investigation of weak volume sources of gas discharge.

Let us now consider the method of thermostating cuvettes. Heating of cuvettes is usually carried out by electric heaters placed on the unilluminated parts of the cuvettes (on the diaphragm parts and on the light trap[^16,^17,^23,^24]). Sometimes cuvettes with a thermostating jacket on the illuminated part are also used[^5,^20]. Both types of cuvettes are shown in Fig. 13. It is important that the mounts with mirror syste-

heated with coils to a somewhat higher temperature than the working part of the cuvette. Otherwise vapors will inevitably condense on the mirrors, which is completely inadmissible. The first type of cuvette, generally speaking, is more constructive than the second; however, here one has to tolerate a certain temperature gradient in the longitudinal direction.

Fig. 13. Types of thermostated cuvettes: a—thermostating on the unilluminated parts; b—thermostating on the illuminated parts.

Filling the cuvette with gas or vapor is carried out in various ways. Cuvettes in which sealing is accomplished by soldering are filled with a metered quantity of liquefied gas or frozen liquid. The required gas density is then attained after evaporation of the liquefied gas at room temperature, and the vapor pressure after additional heating of the cuvette. Into cuvettes sealed with gaskets, the gas is introduced either from a cylinder or from some other source. As for vapors, in this case the filling operation reduces to introducing into the cuvette a metered quantity of liquid and heating it. Before filling, the cuvette is, of course, evacuated of air by the gas or vapor under investigation.

In conclusion we shall give some data on the gas cuvette used in our work.[^14] The cuvette is a thick-walled tube of molybdenum glass with an internal diameter of 46 and a wall thickness of 9 mm. The length of the illuminated part of the cuvette is 190 mm. A tube similar to the one used was tested for rupture by hydraulic pressure. It turned out that destruction occurs at a pressure of 115 atm. Thermal tests showed that the tube withstands a temperature difference along the wall of not less than 80°C.

No rupture tests of the tube under the combined action of pressure and temperature were carried out. In any case, it withstands heating to 90°C at a gas pressure of 8 atm. The tube is secured by means of three tie bolts between two end-face

sheaths with branch pipes each 150 mm long. White vacuum-rubber gaskets are used for sealing.

The mirror system consists of four half-mirrors with a radius of curvature of 500 mm, mounted in frames with all the necessary adjustments. The frames are installed inside the branch pipes. The halves of the front mirror form a 2 mm slit. To increase the reflection coefficient of the mirrors, a three-layer coating was applied. According to measurements, the reflection coefficient in white light was 98%.

The layout of the cuvette is shown in Fig. 14.

B. Spectral instruments and some applications of the method

In describing spectral instruments, it is important to distinguish between two periods: before and after the appearance of powerful low-pressure mercury lamps and multiple-mirror systems. The first period is characterized by the use of high-aperture prism instruments. They are of no special interest, and we shall not discuss them at all. The sharp increase in the efficiency of gas cuvettes and excitation sources has made it possible in recent years to proceed to the use of instruments of large dispersion and resolving power.

We shall briefly describe two typical instruments of this kind.

One of them\(^{25}\) is a two-prism glass spectrograph, built according to Littrow’s autocollimation scheme, with two interchangeable collimators, the larger of which has a relative aperture of \(1 : 42\). The linear dispersion of the instrument in the region of the blue mercury line \(\lambda = 4358\) Å is \(2\) Å/mm. The prisms of the spectrograph are enclosed in an airtight chamber with a window of thick glass of optical quality and finish. A constant pressure is maintained in the chamber. Owing to this, the dispersion of the instrument does not depend on

Fig. 14. Diagram of a gas cuvette with mirrors (according to Pivovarov and Bobovich).

fluctuations of atmospheric pressure. All optical surfaces (including the window of the ground-level chamber) are antireflection coated. The spectrograph is placed in a thermostatted room. The accuracy of thermostating is \(\pm 0.2^\circ\) C. With this spectrograph it is possible to resolve \(1\text{--}2\ \mathrm{cm}^{-1}\).

An instrument of still greater resolving power was described in \(^{17}\). It is built on the basis of a concave 21-foot grating operating in an Eagle mounting. The grating has a ruled area of \(7 \times 3\) square inches, 1500 lines per inch, and is capable of directing energy into the region \(\sim 5000\) Å of the second order of the spectrum. The linear dispersion of the instrument is about \(1.3\ \text{Å}/\mathrm{mm}\), and its resolving power is \(0.3\text{--}0.4\ \mathrm{cm}^{-1}\). More detailed information on this instrument is not contained in work \(^{17}\).

Despite the high efficiency of gas apparatus in certain cases, for example in the study of the rotational-vibrational structure of bands, it is desirable to increase it still further. For this purpose some authors \(^{25}\) place, immediately in front of the photographic plate and parallel to the direction of dispersion of the instrument, a short-focus cylindrical lens, which strongly compresses the image of the spectral line in height. The luminosity of the instrument is thereby sharply increased.

With a slit height of 15 mm the increase in efficiency can be brought up to 15 times. In this case, however, owing to the curvature of spectral lines, the resolution in the spectra is somewhat worsened. It can be partially corrected by using curved slits. Instruments with gratings are evidently to a considerable extent free of this drawback.

To illustrate the possibilities of the new apparatus, we shall give two examples.

In order to verify certain conclusions of the theory of additivity of derivatives of polarizability, the spectra of vapors of chloro-substituted methanes (carbon tetrachloride, chloroform, methyl chloride, etc.) were studied. To excite the spectra, five powerful straight low-pressure mercury lamps were used. The cuvette had no mirror system, with a working length of 600 mm. Vapor pressure was up to 7 atm. A two-prism spectrograph with two cameras, 1:4 and 1:10. It proved possible to resolve the rotational-vibrational structure of the bands, and also to obtain bands corresponding to the first overtones \(^{16}\). Figure 15 shows photometric curves of one of the spectra obtained.

With the aid of a high-resolution instrument with a concave diffraction grating and a gas cuvette with a four-mirror system, equipped with two straight low-pressure mercury lamps, briefly described above \(^{17}\), purely rotational spectra of various molecules were obtained, including heavy ones such as \(C_6H_6\) and \(C_6D_6\). In the latter case the distance between components is \(0.63\ \mathrm{cm}^{-1}\). The pressure of the vapor or gas was \(1\text{--}2\) atm. Average exposures

24 hours. With an instrument having a relative aperture of 1:10, purely rotational spectra are obtained in several minutes. Some spectrograms are shown in Fig. 16.

Until now we have considered only the photographic method of recording spectra of combination scattering of gases. However, it is quite obvious that in a number of cases, for example when measuring the integral intensities of lines on instruments of moderate dispersion, the photoelectric method undoubtedly deserves preference. The successes achieved in the technique of exciting spectra from gases made it possible for us to carry out experiments to establish the possibility of their direct recording^14. As far as can be judged from the literature, no one had previously undertaken such attempts.

As is known, spectral instruments intended for photoelectric recording are distinguished by a number of specific features^26. In particular, it is advantageous to equip them with high slits.

Fig. 15. Photometric curves of the spectrum of vaporous CCl$_4$ (according to Welsh and coauthors^16). $sw$ is the slit width (in cm$^{-1}$).

We used a high-aperture monochromator developed earlier in our laboratory and constructed on the basis of a large plane diffraction grating^27.

Since, however, the relative aperture of the collimator in this instrument is 1:6.5, and the aperture of the mirrors of our cuvette is 1:12, in order to fill the collimator with light it was necessary to use an additional condenser. This circumstance, as well as the need to vignette the portions of the image onto which parasitic light from the walls of the cuvette falls, led to the fact that the illuminated

Fig. 16. Pure rotational spectra of the molecules \(C_6H_6\), \(C_6D_6\), and \(N_2\). (According to Stoichev\(^ {17}\).)

Visible annotations in the figure:

• \(C_6H_6\)
• Anti-Stokes
• \(Hg\,4347\,\text{\AA}\)
• Exciting line \(Hg\,4358\,\text{\AA}\)
• Stokes

• \(S(7a)\), \(S(6a)\), \(S(5a)\), \(S(4a)\), \(S(3a)\), \(S(2a)\), \(S(1a)\), \(S(0)\), \(S(20)\), \(S(30)\), \(S(40)\), \(S(60)\), \(S(70)\), \(S(80)\)

• \(C_6D_6\)

• Anti-Stokes
• \(C_6H_6\)
• Stokes
• Exciting line \(Hg\,4358\,\text{\AA}\)
• Doublet resonance

• \(S(7a)\), \(S(6a)\), \(S(5a)\), \(S(4a)\), \(S(3a)\), \(S(2a)\), \(S(0)\), \(S(20)\), \(S(30)\)

• \(N_2\)

• Anti-Stokes
• \(Hg\,4339\,\text{\AA}\), \(Hg\,4347\,\text{\AA}\), \(Hg\,4544\,\text{\AA}\)
• Exciting line \(Hg\,4358\,\text{\AA}\)
• Stokes
• Doublet resonance
• \(S(10)\), \(S(5)\), \(S(0)\), \(S(5)\), \(S(10)\), \(S(15)\)

part of the slit was only 8 mm (with the full slit height 50 mm).

Nevertheless, we succeeded in recording photoelectric spectra of combination scattering of two gases: CO₂ and N₂. The work was initially carried out at a pressure of 10–13 atm. Later it proved possible to reduce the pressure to 5 atm. The spectra were excited by the blue line \(\lambda = 4358\) Å. The spectral width of the monochromator entrance slit was \(\sim 25\ \text{cm}^{-1}\).

Fig. 17. Series of three successive recordings of a section of the spectrum of CO₂ molecules in the region \(1100\text{—}1500\ \text{cm}^{-1}\) (according to Pivovarov and Bobovich¹⁴). Gas pressure 5 atm.

For illustration, Fig. 17 shows a series of three successive recordings of a section of the CO₂ spectrum in the region \(1100\text{—}1500\ \text{cm}^{-1}\). From these recordings one can see the good reproducibility of the intensities of the lines with frequencies 1285 and \(1388\ \text{cm}^{-1}\)—components of the Fermi resonance splitting for the line of the totally symmetric vibration of the molecules. The mean value of the ratio of the intensities of these lines, obtained from eight measurements, is \(0.70 \pm 0.02\). Similar reproducibility was also obtained in recording the \(2330\ \text{cm}^{-1}\) line of the N₂ spectrum.

The ways of further increasing the efficiency of the photoelectric method are obvious: it can be achieved by increasing the diameter of the gas cuvette and, consequently, of the mirrors, thanks to which it will be possible not only to fill the collimator of the spectral instrument properly with light, but also to make fuller use of its slit in height.

Thus, the experiments described above have shown the possibility of direct recording of combination-scattering spectra from gases, opening prospects for precise quantitative measurements in this field of spectroscopy.

CITED LITERATURE

1. R. W. Wood, Phil. Mag. 7, 744 (1929).
2. R. W. Wood, G. H. Dieke, Phys. Rev. 35, 1355 (1930).
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