NEW INSTRUMENTS AND METHODS OF MEASUREMENT

METHODOLOGY AND INSTRUMENTATION OF LONG-WAVELENGTH INFRARED SPECTROSCOPY

N. G. Yaroslavskii

I. INTRODUCTION

The entire infrared*) region of the optical spectrum, extending from the red boundary of the visible spectrum toward greater wavelengths and overlapping with the region of millimeter radio waves, is now customarily divided into three parts1, 4, 5: (a) the short-wavelength region, bounded by wavelengths of 0.75 and 2.5 μ (13,300–4000 cm\(^{-1}\)); (b) the mid-wavelength region, 2.5–50 μ (4000–200 cm\(^{-1}\)); and (c) the long-wavelength region, covering a large interval of wavelengths from 50 μ (200 cm\(^{-1}\)) to 1000 μ (10 cm\(^{-1}\)) and beyond.

Within the limits of the short-wavelength part of the IR spectrum, i.e., in the “near” IR region, lie bands corresponding to overtones and combination frequencies of the fundamental vibrations of molecules, as well as lines and bands of the electronic-vibrational spectrum of atoms and molecules.

In the mid-wavelength range (2.5–50 μ) are localized chiefly bands corresponding to the fundamental vibrational frequencies of light molecules and individual atomic groups. (This part of the spectrum is therefore sometimes called the region of fundamental vibrational frequencies.)

The methodology for measuring IR spectra in the wavelength region from 0.75 to 50 μ at present presents no difficulties and is fairly widely used in the practice of research and factory laboratories. Thus, for investigations in the short-wavelength part of the IR spectrum, which is most accessible experimentally, spectrometers with glass or quartz prisms are usually employed; in these instruments standard tungsten incandescent lamps in glass bulbs are used as sources, and photocells or photoresistors**) as detectors. In spectrometers intended for operation in the mid-wavelength IR region, prisms made from crystals of alkali-halide salts are used, including cesium iodide crystal, transparent up to 50–55 μ; the sources and indicators of IR radiation are Nernst and Globar glowers (IKR-1 glowers, silicon-carbide rods) heated by an electric current, and thermal detectors: metallic and semiconductor thermoelements and bolometers, as well as optoacoustic (pneumatic) detectors. At present, to obtain high resolution in these instruments, as in spectrometers for the short-wavelength region of the IR spectrum,

*) Hereafter, for brevity, the word “infrared,” “infrared,” etc., will be replaced by the designation “IR.”

**) For this reason the short-wavelength part of the IR spectrum is sometimes called the “photoelectric” region.

prisms are replaced by diffraction gratings—echelettes with a large number of rulings.

The methods and apparatus for obtaining and recording IR spectra in the region \(0.75\)—\(50\,\mu\) have been well developed and described in sufficient detail in Soviet and foreign review articles \(^{1-5}\).

In the long-wavelength region of the IR spectrum, i.e., in the wavelength interval from 50 to \(1000\,\mu\) and beyond, there are absorption and emission bands corresponding to changes in the rotational energy of gas and vapor molecules, as well as low-frequency vibrational bands of heavy molecules, radicals, and molecular complexes. In addition, various combination (difference) frequencies of the fundamental vibrations of different molecules, as well as frequencies of intermolecular vibrations, fall in this region.

The study of long-wavelength IR spectra of various gaseous substances opens up possibilities for the direct study of their molecular structure (determination of the moments of inertia of molecules, interatomic distances, and other molecular constants).

The study of the low frequencies of intra- and intermolecular vibrations of solids and liquids is necessary for investigating the structure of crystals, the nature of the liquid state, and intermolecular interactions. There are also other fields of application of long-wavelength infrared spectroscopy, whose methods are already being used at present in studying the optical and electrical properties of semiconductors and dielectrics in the region of large wavelengths \(^{40,42}\).

The application of methods of long-wavelength spectroscopy is apparently also important in the study of light scattering by loose bodies (powders) as a function of the size of the scattering particles, their shape, and their distribution in space. The aim of such a study may be to establish the laws of radiation scattering by artificially modeled disperse media with geometrical parameters precisely specified and comparable with the wavelength.

Finally, the existence of a recently established transparency band of the Earth’s atmosphere for electromagnetic oscillations with wavelengths of about \(1500\,\mu\) (\(1.5\) mm) gives grounds for believing that long-wavelength infrared spectroscopy will find application also in astronomical spectral investigations, for example in determining the temperature of planets \(^{6}\).

Although long-wavelength infrared (“thermal”) radiation was discovered exactly 60 years ago \(^{11}\), for a long time it found no application in molecular spectroscopy. This was explained by great experimental difficulties, connected chiefly with the extremely low energy in the long-wavelength IR spectrum, as well as by the absence of highly sensitive receiving-recording devices. Only recently, owing to the development of techniques for detecting weak radiation and methods for amplifying weak currents, and also owing to the development of new high-aperture diffraction gratings (echelettes) and aspherical reflecting optics of large dimensions, has the long-wavelength region of the IR spectrum begun to be used ever more widely for solving various problems of physics and chemistry. Nevertheless, the number of works devoted to investigations in this region of the spectrum is still very small.

In the present review, which by no means claims to be exhaustive, an attempt has been made to systematize information concerning the methods of long-wavelength infrared spectroscopy and the apparatus developed in recent years abroad and in our Union for obtaining and recording spectra in the wavelength region up to \(1600\,\mu\).

II. METHODS FOR ISOLATING

LONG-WAVELENGTH INFRARED RADIATION

To obtain long-wavelength infrared monochromatic radiation, the following methods may be suitable: 1) the method of focal isolation with quartz lenses, 2) a method based on total internal reflection, 3) selective reflection from crystals (the “residual rays” method), and 4) monochromatization by means of diffraction gratings.

The first three methods have low resolving power; however, they have certain advantages over the fourth, owing to their simplicity and to the considerable energy of the long-wavelength radiation obtained. These methods, on the other hand, are unsuitable in cases where high resolution is required, for example in the study of rotational spectra of molecules. In such cases it is necessary to carry out monochromatization with the aid of diffraction gratings.

1. The method of quartz lenses

The method of focal isolation of long-wavelength IR radiation with the aid of quartz lenses was first implemented by Rubens and Wood^7 in 1910.*) The method is based on the fact that, being transparent in the long-wavelength

Fig. 1. The method of quartz lenses^7.

region, quartz has a large refractive index (of the order of 2.5) in the wavelength region greater than 50 μ, whereas in the region of short waves and the visible part of the spectrum its refractive index is approximately equal to 1.5.

Figure 1 shows a schematic diagram of an arrangement for isolating long-wavelength radiation by the method of quartz lenses. The quartz lens \(L_1\) refracts the long-wavelength radiation of the radiation source \(A\) more strongly than the short-wavelength radiation. Owing to the diaphragms made of black paper \(D_1\) and the metal screen \(D_2\) with an aperture, only the long-wavelength radiation enters the space beyond \(D_2\). To further increase the homogeneity of the radiation, a second lens \(L_2\) is used, which repeats the action of the first lens. It is needed in order to remove the short-wavelength radiation scattered at the surfaces of lens \(L_1\), and to focus the long-wavelength radiation on the receiver \(M\).

The dimensions characterizing the Rubens and Wood arrangement were as follows: the diameters of the working apertures of both quartz lenses were \(7.5\ \text{cm}\), their thickness at the edges was \(0.3\ \text{cm}\), and in the middle \(0.8\ \text{cm}\); the focal lengths of the lenses for visible rays were \(27.3\ \text{cm}\), and for long-wavelength

*) Before Rubens and Wood, Lenard^8 used this method for monochromatization of ultraviolet radiation; he should be considered the inventor of the quartz-lens method. This method lies at the basis of those still used at the present time, the so-called focal monochromators for the ultraviolet region^9.

IR radiation—approximately 12 cm. The diameters of diaphragms \(B\) and \(D_2\) were 15 mm, and those of diaphragm \(D_1\), 5 mm.

With the total thickness of the quartz layer used in the setup, the quartz is completely opaque approximately up to \(80\,\mu\), while at \(95\,\mu\) it transmits about 20% of the incident long-wave radiation, and its transparency increases with increasing wavelength. In this connection the curve of the energy distribution over the spectrum falls rather steeply toward the short waves, and much more slowly toward the long waves.

To measure the wavelengths selected by the setup, which depend on the relative arrangement of the lenses, the source, and the receiver, an interferometer consisting of two quartz plates was placed in the path of the rays at \(D_2\). During the measurements a glass plate \(C\), opaque to long-wave radiation, served as the shutter.

The quartz-lens method, along with the “residual-rays” method described below, was widely used in early work in studying the transmission and reflectivity of various materials in the region of large wavelengths\(^{14,15}\). However, because of its very low resolution it could not be used for investigations of molecular rotational spectra.

2. Method of total internal reflection

In 1928 Ench and Laski\(^{10}\) proposed a simple method for isolating any spectral interval in the wavelength region of the order of \(80\)—\(100\,\mu\). As is known, total internal reflection of radiation occurs not strictly at the boundary between two media, but in a certain region of the second medium whose dimensions are of the order of the wavelength. If, in the second medium near the reflecting surface, one places another plate made of the material of the first medium, then part of the radiation penetrating into the second medium enters this plate and no longer returns to the reflected beam. The limiting width of the air gap at which radiation of a given wavelength still reaches the second plate depends on the wavelength and increases with it. Taking advantage of this circumstance, one can construct an apparatus whose scheme is shown in Fig. 2. Cube \(I\), made of a material transparent to long-wave radiation, retains the short-wave radiation, deflecting it in direction 1; the remaining, longer-wave radiation is divided in cube \(II\), with a thicker air interlayer, again into two parts, the long waves passing in direction 3, while the shorter waves are deflected in direction 2, where an approximately monochromatic beam is obtained, the degree of monochromaticity and the mean wavelength of which depend on the thickness of the air layers of cubes \(I\) and \(II\).

Fig. 2. Method of total internal reflection\(^{10}\).

The described method of monochromatizing long-wave radiation, however, did not find wide application because of the small aperture, the absence of substances sufficiently transparent in the long-wave IR region, and the need to carry out mechanical displacement of the prisms of both cubes with precise fixation of the small thickness of the air gap between them.

3. The “residual rays” method

The most widespread method for isolating separate, sufficiently narrow regions of long-wavelength radiation is the “residual rays” method, based on the property of crystals to selectively reflect (and absorb) radiation in regions of anomalous dispersion, i.e., near the natural vibration frequencies of crystal lattices, where so-called “metallic” reflection is observed. This method, proposed as early as 1897 by Rubens and Nichols ^11, is used in various investigations even at the present time ^12.

The idea of the “residual rays” method is as follows. Let \(r\) be the reflecting power of the given substance in the region of “metallic” reflection. Then

\[ r=\frac{(n-1)^2+n^2\chi^2}{(n+1)^2+n^2\chi^2}, \]

where \(r\) differs little from unity, since in the region of anomalous dispersion \(2n\) is small in comparison with \(n^2\chi^2+n^2+1\). Let, further, \(\rho\) be the reflecting power of the substance in the region of normal dispersion, in which the Fresnel formulas remain valid, i.e.

\[ \rho=\frac{(n-1)^2}{(n+1)^2}. \]

Let \(i\) denote the ratio of the intensities in two regions of the continuous spectrum, one of which lies in the region of anomalous dispersion and the other in the region of normal dispersion. After \(k\)-fold reflection from the surface of the given substance this ratio becomes equal to \(i_k=i(r/\rho)^k\). If, for example, \(i=1\), \(r=0.9\), and \(\rho=0.45\), then after fourfold reflection \(i_k=2^4=16\), whereas after a single reflection \(i_k\) is equal to only two. Thus, by means of multiple reflections one can isolate from the spectrum a definite interval of wavelengths, namely the interval adjacent to the natural vibration frequency of the crystal lattice of the substance. This explains the name “residual rays,” given to the radiation isolated by this method.

To obtain “residual rays,” only the most significant reflection maxima of crystals can be used, since otherwise the intensity of the long-wavelength radiation, whose measure is \(r^k\), will prove too small. The degree of homogeneity of the residual rays depends not only on the number of reflecting surfaces, but also on the distribution of intensity in the spectrum of the incident radiation. The wavelength corresponding to the maximum of the isolated radiation also depends on these circumstances, since \(i\) is different from unity.

In Fig. 3, borrowed from work ^13, the course of the reflecting power is compared for a number of crystals commonly used at present as reflectors in carrying out the method described here *).

To the data of the figure, which gives an idea of the choice of crystals suitable for carrying out the “residual rays” method, one should add information on the reflecting power of the crystals InSb ^17 and KRS-5 ^44 (a mixed crystal TlJ+TlBr). The first of these has a sharp maximum with 90% reflection at \(54.6\,\mu\), the second—a more

*) Information on the reflecting power of other crystals, as well as data on the properties of various materials in the region of large wavelengths, may be found in the review of early works on long-wavelength IR spectroscopy, carried out in 1923 by Weniger ^14, and also in the monographs of Schäfer and Matossi ^15 and Parodi ^16.

broad maximum (75%) at 200 μ. For the region of still longer waves, suitable crystals have not yet been found.

Fig. 3. Reflection from crystal surfaces¹³.

In installations intended for isolating long-wave radiation by the “residual-ray” method (their schemes are shown in Fig. 4), three or four reflections from plane crystal surfaces are usually used. In this case the half-width of the isolated spectral regions, for example, in the case of NaCl crystals (reflection maximum at 52 μ), KCl (62 μ), and KBr (83 μ), is 43, 39, and 24 cm\(^{-1}\), respectively¹².

Fig. 4. Residual-ray method¹⁶; \(M_1, M_2, M_3, M_4\) are crystals; \(A, B, C, D, E\) are spherical mirrors; \(R\) and \(S\) are the receiver and the source of radiation.

It is possible to construct an installation that would make it possible to obtain a significantly larger number of reflections from the surface of one and the same crystal and thereby increase severalfold the degree of monochromatization (purity) of the isolated radiation. Such an installation may be based on the multiple-reflection methods proposed by White¹⁸, and also by Bernstein and Herzberg¹⁹, for obtaining large optical paths in gas cells having relatively small dimensions.

The scheme of an installation constructed according to White’s method is shown in Fig. 5. Two identical concave spherical mirrors \(A_1\) and \(A_2\), as well as a rectangular crystalline plate with a concave spherical surface \(B\), having one and the same radius of curvature, are arranged so that the center of curvature of the plate lies between the mirrors \(A_1\) and \(A_2\), whose centers of curvature \(a_1\) and \(a_2\), in turn, must lie symmetrically, at some distance from one another, on the spherical surface of the plate \(B\). Then the radiation entering the instrument through the entrance aperture \(S_1\) will be reflected and focused several times on the surface of the crystalline plate \(B\) before it leaves the exit aperture \(S_2\) of the instrument.

Fig. 5. Obtaining multiple reflections from a crystal.

From Fig. 5 it is easy to see that the number of reflections from the crystal (or the number of intermediate images of the entrance aperture \(S_1\) on its surface) is always odd and is equal to

\[ n=\frac{d}{r}-1, \]

where \(d\) is the distance between the entrance and exit apertures, and \(r\) is the distance between the centers of curvature of the mirrors \(A_1\) and \(A_2\).

Thus, for fixed \(d\), the number of reflections—and consequently also the degree of monochromaticity of the isolated radiation—will depend only on the angle of rotation of the mirrors \(A_1\) and \(A_2\) about their common center \(b\). With sufficiently small \(r\) (\(<5\ \mathrm{mm}\)) and realistic dimensions of the crystalline plate \((55 \times 10 \times 8\ \mathrm{mm})\), this scheme apparently makes it possible to obtain more than 10 reflections. Transition to other wavelengths can easily be accomplished by changing plates made from different crystals (according to the data of Fig. 3) and mounted on a common movable table.*)

The advantage of the “residual rays” method is that, by comparatively simple means, it is possible to obtain long-wavelength radiation of high intensity with relatively high homogeneity. A drawback of the method is the limitation of its region of application to a wavelength of \(200\,\mu\), as well as the circumstance that it is impossible to isolate any desired wavelength from a continuous spectrum. Nevertheless, at the present time this method is used not only for obtaining information on the transparency and reflectivity of various

*) We note that when crystalline plates are used in the region of longer wavelengths, there is no need to make them with an accurately polished spherical surface, since a matte surface, while scattering short-wavelength radiation, reflects long-wavelength radiation well; moreover, such plates can evidently be made by pressing or by depositing crystalline layers on a glass spherical surface by evaporation of substances in vacuum or by precipitation from solutions.

materials in the long-wavelength IR region ^15, but also in the study of the fundamental vibration frequencies of certain organic and inorganic substances, for example, cis- and trans-dichloroethylene, 1,2-dichloroethane, methyl and propyl alcohols, acetaldehyde, n-hexane, n-pentane, and carbon suboxide \( \mathrm{C_3O_2} \) ^12.

4. Monochromatization by means of diffraction gratings

Great successes in the decomposition of infrared radiation have been achieved with the aid of diffraction gratings, the use of which is the only possible means of obtaining high resolution in the long-wavelength region of the spectrum. The use of prisms is excluded, since at present there are no materials possessing sufficient transparency in thick layers in the wavelength region \(>50\,\mu\) and suitable for the manufacture of prisms *).

Fig. 6. Profiles of diffraction gratings:
a) wire, b) laminar, c) echelette.

In early investigations of long-wavelength IR spectra, transmitting wire ^21 and reflecting laminar (plate) ^22 gratings were used (Fig. 6, a and b). In recent years, and at the present time, exclusively reflecting stepped echelette gratings ^23 have been used; owing to a specially chosen groove profile (Fig. 6, c) they have the ability to concentrate a large part of the incident energy in a narrow range of diffraction angles, for example, in the spectral region of one of the first orders. Under the most favorable conditions, the intensities of long-wavelength radiation isolated with the aid of wire, laminar, and stepped (echelette) gratings are in the ratio \(1:4:10\) ^13.

The distribution of wavelengths in the spectrum given by reflecting gratings is described by the well-known equation

\[ m\lambda = d(\sin \psi \pm \sin \varphi), \]

where \(m\) is the order of the spectrum, \(d\) is the grating constant, and \(\varphi\) and \(\psi\) are the angles of incidence and diffraction **).

If the spectrum is scanned by rotating the grating at fixed angles of incidence and reflection, as is usually the case in spectrometers, then the wavelength distribution will be

\[ m\lambda = 2d \cos \frac{\theta}{2} \sin \beta \]

*) As early as 1898, Rubens and Aschkinass ^20 attempted to isolate long-wavelength radiation using acute-angled quartz prisms. However, the result was unsatisfactory, since they could not sufficiently eliminate short-wavelength radiation.

**) In this formula the sign “\(+\)” corresponds to the case in which the incident and diffracted rays lie on the same side of the normal to the grating, and the sign “\(-\)” to the case in which these rays are situated on opposite sides of it.

or \(m\lambda = K\sin\beta\), where \(\theta\) is the angle between the directions of incidence and diffraction, \(\beta\) is the angle of rotation of the grating (Fig. 7), and \(K\) is a constant of the instrument.

The relative distribution of intensities, for example in the first-order spectrum of an echelette, depending on the wavelength and on the form of the groove on its surface, is determined by the expression

\[ J_{\mathrm{rel}}=\left(\frac{\sin \frac{\pi\Delta}{\lambda}}{\frac{\pi\Delta}{\lambda}}\right)^2, \]

in which, in the case of small values of the angle between the directions of incidence and diffraction \((\theta = 0)\), the quantity \(\Delta\) has the value

\[ \Delta = 2d\sin(\beta-\alpha), \]

where \(d\) is the echelette constant, \(\beta\) is the angle of its rotation, and \(\alpha\) is the angle of inclination of the step (see Fig. 6, e).

Fig. 7. Reflection from an echelette.

It is easy to see that the maximum intensity will be possessed by radiation diffracted in the direction of specular (“geometrical”) reflection from the working surfaces of the echelette steps. This direction is sometimes called the “blaze” of the echelette, and the angle between this direction and the normal to the echelette surface is termed the “blaze” angle.

The use of diffraction gratings requires the elimination of spectra of higher orders, which are superimposed on the principal spectrum in the form of an undesirable background. Usually, in spectrometers with diffraction gratings intended for operation in the near and middle IR regions, preliminary monochromatization with the aid of prisms of low dispersion is used for separating the orders. In spectrometers for the long-wavelength region such a method is inapplicable, and the elimination of the energy-rich spectra of higher orders, as well as the suppression of short-wavelength scattered radiation, is carried out by means of the described methods of quartz lenses and “residual rays,” and also by using various transmitting and reflecting filters and by applying the method of selective modulation, which will be considered below.

The ratio of the energies of the interfering (short-wavelength) and selected (long-wavelength) radiation is especially large when working at the “blaze” angles of echelettes. Outside the “blaze” angles this ratio is greatly reduced. Nevertheless, the elimination of interfering radiation, which in energy many times exceeds the useful radiation, presents considerable difficulties.

Let us consider one more method of selecting long-wavelength radiation, first proposed in 1952 by Walsh \(^{24}\).

The method is based on the use of reflecting zone plates whose profile is similar to the profile of an echelette. The action of such plates is analogous to the focusing action of transmitting Fresnel zone plates; however, owing to the specially chosen profile of the reflecting annular zones, there is a considerable gain in the energy of the IR radiation selected in a definite direction.

Let us consider a parallel beam of radiation incident on an echelette zone plate, the radial section of which is shown in Fig. 8. Then the radiation of wavelength \(\lambda\) diffracted by the plate will be

focus at the point \(O\), located at a distance \(f\) from the center of the plate, according to the condition

\[ r_n=[n\lambda(2f+n\lambda)]^{1/2}, \]

where \(n\) is the number of zones, \(r_n\) their radius.

The profile of the reflecting annular zones can be chosen so that the direction of the diffracted beam of the given wavelength coincides

Fig. 8. Radial section of a zonal echelette plate.

with the direction of specular reflection from the surface of each zone, as is the case for an echelette. Then the angles of the zone steps, depending on their radius, in the first approximation must be equal to:

\[ \alpha_n=\frac{1}{2}\operatorname{arctg}\left(\frac{r_n}{f}\right). \]

If, on a special machine, echelettes were ruled whose annular grooves would satisfy the two conditions written above, then these echelettes, in Walsh’s opinion, could be successfully used in a high-aperture (with a relative aperture less than \(1/2\)) spectrometer for isolating long-wavelength radiation in the region from 50 to 500 \(\mu\). One possible scheme of such a spectrometer is shown in Fig. 9. Radiation of a given wavelength from the source \(S\) is collimated by the zonal plate \(Z_1\) and then, with the aid of a similar plate \(Z_2\), is focused on the detector \(D\). Radiation of other wavelengths will be focused at other points lying on the focal curve \(FC\). It can be focused on the receiver by changing the distance between the plates \(Z_1\) and \(Z_2\).

Fig. 9. Schematic diagram of a long-wavelength IR spectrometer with zonal echelette plates\({}^{24}\).

To cover the entire spectral region from 50 to 500 \(\mu\), several pairs of such zonal plates are required, since one pair of plates isolates only a limited range of wavelengths.

The author of the method believes that, with the aid of the proposed procedure and using the principle of multiple monochromatization described by him earlier \(^{25}\), it is possible to attain high resolving power in the long-wavelength region of the IR spectrum.

III. SPECTROMETERS FOR THE LONG-WAVELENGTH IR REGION AND THEIR APPLICATIONS

1. Brief historical survey

One of the first spectrometers with a wire diffraction grating was the instrument built in Germany by Czerny in 1925 \(^{26}\). Despite the fact that, owing to its small aperture ratio, the instrument had low resolving power, Czerny succeeded in resolving the rotational structure in the spectra of vapors of halogen-hydrogen compounds in the region from 40 to 100 \(\mu\) \(^{27}\).

In 1929, Badger and Cartwright \(^{28}\) built in America an instrument with a reflecting laminar grating and recorded the long-wavelength IR spectrum of ammonia.

The best of the instruments with wire diffraction gratings was Barnes’s spectrometer \(^{29}\), built in 1934\(*\). With this instrument, long-wavelength IR spectra were obtained for water \(^{30}\), ammonia and heavy ammonia \(^{31}\), as well as benzene in the vapor and liquid states \(^{32}\). On a similar instrument, Cartwright investigated the spectra of water vapor \(^{33}\) and the spectra of liquid (ordinary and heavy) water \(^{34}\).

However, spectrometers with wire and laminar gratings had insufficient resolving power and were not suitable for studying the fine structure of molecular rotational spectra. For example, with Barnes’s instrument, whose slit widths were \(4.42\ \mathrm{cm}^{-1}\) at \(50\,\mu\), \(1.67\ \mathrm{cm}^{-1}\) at \(100\,\mu\), and \(1.14\ \mathrm{cm}^{-1}\) at \(150\,\mu\), it was impossible to resolve the doublets in the ammonia spectrum, the distances between whose components are \(1.33\ \mathrm{cm}^{-1}\).

The first long-wavelength IR spectrometer of large aperture and high resolving power was developed by Randall \(^{35}\) in 1932 and built at the University of Michigan. Thanks to the use in this instrument of large echelette gratings \((250 \times 550\ \mathrm{mm})\), high-quality parabolic and elliptical mirrors, and also sensitive thermoelements, the author was able, in the spectral region from 18 to 140 \(\mu\), to achieve a resolution of \(0.5\text{--}1.0\ \mathrm{cm}^{-1}\) in obtaining the spectra of gaseous \(\mathrm{NH_3}\) and \(\mathrm{PH_3}\) \(^{36}\), as well as the spectrum of water vapor \(^{37}\), which is still used for testing and calibrating instruments intended for work in the long-wavelength region of the IR spectrum.

In 1938, Randall’s instrument was improved \(^{38}\) by enclosing it in vacuum chambers and introducing continuous recording (photographic recording) of spectra with the use of a Firestone resonant galvanometric amplifier. On this instrument, at a resolving power of \(0.5\ \mathrm{cm}^{-1}\), the absorption spectra of heavy water at various vapor pressures were recorded \(^{39}\).

\(*\) Built by Barnes in 1932, after the type of Czerny’s instrument \(^{23}\), the spectrometer with wire diffraction gratings \(^{29a}\) had poorer characteristics. Nevertheless, the data obtained with it in the wavelength range from 20 to 135 \(\mu\) on the transparency of fused and crystalline quartz of various thicknesses, as well as of rhombic sulfur, mica, cellophane, celluloid, black paper, and various soot coatings, are of definite interest.

In 1939 in Germany, Maer ^40, using a laminar diffraction grating made by him with a constant of 4 mm, constructed a spectrometer operating in the region from 200 to 500 μ. A high-pressure quartz mercury lamp was used as the source of long-wavelength radiation, and the receiver was a radiometer specially designed for the long-wavelength region. With the aid of this instrument the spectra of water vapor ^41 were investigated, as well as the transmission of some solid dielectrics in order to determine their dielectric constants in the wavelength region 0.2–0.5 mm *).

Further investigations in the long-wavelength IR region of the spectrum were resumed only in the postwar period.

In 1950 McCubbin and Sinton ^43, using an instrument they had built with a small-size echelette (73×100 mm) and quartz optics, a pneumatic receiver, and a high-pressure quartz mercury lamp, investigated the transparency of the atmosphere in the region 100–600 μ at a resolution of 3–5 cm\(^{-1}\), and also obtained transmission curves in this spectral region for some gaseous (NCl, NH\(_3\)) and solid substances (crystalline and fused quartz, glass, magnesium oxide, polyethylene, polystyrene, paraffin, Teflon, mica, and crystals: LiF, NaCl, KBr).

In 1952 the same authors ^44 created a high-aperture instrument with a relative aperture of 1:1, built according to Pfund’s autocollimation scheme (an echelette with an aperture, a parabolic mirror), on which, at a resolution of 1–2 cm\(^{-1}\), spectra of water vapor were obtained in the region from 100 to 700 μ. In addition, the spectral reflection of crystalline TlCl, TlBr, TlJ, PbS, PbCl\(_2\), ZnS, CsBr, as well as KRS-5 and KRS-6 ^45, was measured. In the same year, Oetjen and co-workers ^46 built a vacuum recording spectrometer with an echelette of 7 lines per 1 mm, operating in the wavelength region from 40 to 150 μ. The radiation source was a platinum strip heated by an electric current and coated with thorium oxide, and the detector was a Golay pneumatic receiver with an electronic amplifier tuned to a light-flux modulation frequency of 10 c/s. Testing the instrument by obtaining spectra of water vapor and ammonia at various pressures of the latter showed that its maximum resolving power is 0.5 cm\(^{-1}\). Subsequently, on this instrument in the region 40–150 μ, the rotational spectra of gaseous molecules were measured: HCl, DCl, HBr, and NH\(_3\) ^47, ND\(_3\) ^48, PH\(_2\)D and PHD\(_2\) ^49, as well as the spectra of PH\(_3\), PD\(_3\), AsH\(_3\), and AsD\(_3\) ^50, from which the rotational constants of the molecules of the compounds studied were determined. Recently, with the aid of this instrument in the region 20–200 μ, the optical constants of crystalline InSb ^17, the transmission of ZnS powder suspended in paraffin and polyethylene ^51, and also the rotational spectra of the molecules HJ, DJ, DBr, H\(_2\)Se, NH\(_2\)D and NHD\(_2\) ^52, and the spectra of the molecules CO, NO, N\(_2\)O in the region from 100 to 600 μ ^65, have been investigated.

In 1952 in Japan, Yoshinaga and Yamada ^53 also built a long-wavelength IR spectrometer with a grating—an echelette—made by them, intended for operation in the spectral region from 25 μ and beyond. The radiation source in their instrument was a silicon-carbide rod (“Globar”), and the detector was a compensated thermocouple directly connected to a galvanometer. However, because of the absence of an amplifying

*) A similar instrument with laminar gratings and an echelette in a double Pfund arrangement was built later by Meyer ^42. On this instrument, in the spectral region from 150 to 500 μ, spectra of atmospheric water vapor were obtained and the dielectric constants of some insulators were measured from their reflectivity in the long-wavelength IR region.

systems and the imperfections of the echelette, the authors did not succeed in recording radiation with wavelengths greater than 35 μ.

In 1953, Bohn and coworkers54 in America and Adney55 in France constructed recording vacuum instruments for the region from 20 to 80 μ. In Bohn’s spectrometer, two echelettes were used in first order: one (for the 22–40 μ region) had 24 lines per 1 mm, the other (for the interval 40–77 μ) had 15 lines per 1 mm; the resolution was 1–2 cm−1. In Adney’s instrument, having a relative aperture of 1 : 2.5, one echelette of 14 lines per 1 mm was used, which in the second order, in the region from 20 to 60 μ, made it possible to obtain a resolution of 0.74 cm−1. In both instruments, low-inertia thermoelements with alternating-current amplifiers were used as receivers. With the aid of these instruments, for the purpose of refining thermodynamic functions, the spectra of cis- and trans-dichloroethanes56, ammonia57, and methyl alcohol58 were studied.

In 1956, Adney reported on a small spectrometer he had developed for the long-wavelength region59, in which the collimating mirror, with a focal length of 10 cm, had dimensions of 4 × 4 cm, and the width of the slits, having a height of 16 mm, varied within the limits from 0.5 to 1 mm. In this spectrometer two echelettes with 8 and 4 lines per 1 mm and a Schwartz thermoelement with modulation were used. Interfering radiation was eliminated by Christiansen filters, as well as by the use of methods of selective reflection and selective modulation. The results obtained with this instrument, however, have not yet been published.

An instrument for investigation of the most long-wavelength region of the IR spectrum was the vacuum spectrometer recently built in West Germany by Genzel and Eckhardt60.

Using in the Ebert-Fastie arrangement a set of echelettes with constants 315.5; 625.0; 833.3 and 1250.0 μ, a quartz mercury lamp as the source, and metallic low-inertia bolometers with an electronic amplifier and self-recorder, the authors were able, with sufficiently good resolution, to record the rotational spectra of water vapor, as well as the spectra of gaseous HCN, NH3, and H2S in the region from 300 to 1600 μ (1.6 mm). At the same time, in the spectrum of ammonia the components of the fine structure of the rotational lines (inversion doubling) were clearly resolved, and their dependence on pressure was studied.

Thus, at the present time the gap between the optical and electrical (microwave) ranges of the electromagnetic spectrum* has been bridged, and the optical spectrum in this interval is recorded with a resolution not inferior to the resolution usually achieved in work in the short-wavelength and medium-wavelength regions of the infrared spectrum.

In conclusion to this brief review of the literature, we note that, along with the development of new instruments intended exclusively for studies in the long-wavelength region of the infrared spectrum, attempts are now being made to use ordinary standard prism spectrometers for these investigations. Thus, for example, Plyler and Acquista62, removing the prism in a standard Perkin-Elmer spectrometer and replacing Littrow’s plane mirror by echelettes of size 57 × 76 mm with 13 or 7 lines per 1 mm, and also replacing the window on the thermoelement by a cesium iodide window (when working in the region up to 56 μ) or quartz, obtained the possibility, with the use of appropriate reflec—

* It should be noted that, with the use of purely radio-engineering methods, it is now possible to obtain spectra in the region of wavelengths shorter than 1 mm. For example, the rotational spectrum of the OCS molecule has been recorded in the region 0.7 mm (700 μ)61.

...filters instead of mirrors, to record the absorption spectra of water vapor and of certain substituted ethylenes and ethanes in the region from 50 to 125 \(\mu\) \(^{63}\).

Below we shall consider the characteristics of long-wavelength IR spectrometers and of their individual elements (radiation sources and receivers, filters, etc.), and shall also describe the layouts and designs of several laboratory installations, including a model of a long-wavelength infrared spectrometer (DIKS-1), built by us in the Soviet Union in 1956 \(^{64}\). On this instrument, after its improvement by enclosing the entire optical system in vacuum and by using more sensitive receiving-recording devices, rotational spectra of water vapor at various pressures were obtained in the region from 20 to 500 \(\mu\), with a maximum resolution up to \(0.3\ \mathrm{cm}^{-1}\) and an accuracy up to \(0.05\ \mathrm{cm}^{-1}\).

2. Characteristics of spectrometers and their elements

As noted above, the first and principal difficulty in carrying out spectral investigations in the long-wavelength IR region is the extremely small amount of energy in the long-wavelength spectrum of thermal radiation sources. Calculation shows that, when an absolutely black body at a temperature of \(2000^\circ\ \mathrm{K}\) is used as the source, the energy value at a wavelength, for example, of \(100\ \mu\) amounts to one millionth of the energy at the maximum (at \(1.44\ \mu\)), while the energy value at \(200\ \mu\), according to the data of Table I, decreases by more than a factor of 10. For real thermal sources this ratio increases still further.

Table I

Relative distribution of energy in the spectrum of an absolutely black body at a temperature of \(2000^\circ\ \mathrm{K}\)

Therefore a spectral instrument intended for isolating long-wavelength monochromatic radiation should, as far as possible, have greater light-gathering power and at the same time possess high resolving power.

These requirements can be met by a spectrometer having a large echelle, a short-focus collimating mirror, and slits of large angular dimensions.

The use of an echelle in such a high-aperture instrument, especially when high-temperature thermal radiation sources are used, requires overcoming a second major difficulty in carrying out spectral investigations in the long-wavelength region, which consists in the necessity of eliminating the energy-rich spectra of higher orders superimposed on the principal spectrum, as well as the short-wavelength radiation scattered in the instrument.

In order to obtain an idea of the ratio of the energies of useful and interfering (short-wavelength) radiation, let us consider Table II, which gives the relative intensities of various spectral regions diffracted in the direction of the “blaze” of an ideally reflecting echelle having, for example, 7.5 lines per \(1\ \mathrm{mm}\) and concentrating radiation with \(\lambda = 100\ \mu\) into the first-order spectrum. The data in the third column of the table were calculated for a radiation source having a temperature of \(1300^\circ\ \mathrm{K}\) and obeying Planck’s law.

It is seen from the table that if, for example, the investigations are carried out in the spectral region near \(100\mu\), i.e., in the “blaze” region of the given echellette, then the ratio of the total interfering (short-wave) radiation to the useful radiation is approximately \(1.75\cdot 10^5\)*). Outside the blaze angles this ratio decreases greatly; nevertheless, rejection of the short-wave radiation remains a serious problem.

Consequently, in long-wave IR spectrometers effective methods must be provided for suppressing short-wave radiation and, on the other hand, radiation sources must be used which possess as large as possible a ratio of the energies of long-wave and short-wave radiation. Naturally, the receiving-recording systems of such spectrometers must have maximum sensitivity.

Table II

Relative intensities of different spectral regions obtained when using an ideal echellette at the blaze angle and an absolutely black body at \(1300^\circ\) K \(^{46}\)

The presence of strong absorption of long-wave radiation by atmospheric water vapor, extending up to a wavelength of \(1503\mu\), imposes one more condition on the design of long-wave IR spectrometers: their optical system must be isolated from room air and must permit evacuation to pressures of \(0.1\)—\(0.2\) mm Hg, at which absorption bands of water vapor in air of normal relative humidity (50%) no longer appear in the long-wave spectrum. Let us note that by no other means (drying the instrument, purging it and filling it with dry nitrogen, etc.) is it possible to avoid the harmful absorption by water vapor; therefore all modern long-wave IR spectrometers are constructed as vacuum instruments.

A. Radiation sources

For work in the region of wavelengths up to \(100\)—\(150\mu\), ordinary thermal emitters are suitable. These include: a silicon carbide rod (“globar”), the Auer—Welsbach mantle widely used in early spectroscopic work, and also the positive crater of a carbon arc. However, rejection of short-wave radiation is more difficult when the latter is used.

The silicon carbide rod, used in works \(^{54}\) and \(^{64}\), is suitable only in the region up to \(100\mu\), since in the longer-wave region its radiation is small.

As a result of testing various thermal sources in work \(^{46}\), it was established that the most effective emitter in the spectral region from 40 to \(150\mu\) is a platinum strip coated with thorium oxide and [[unclear: continuation cut off on page]].

* In actuality this ratio is somewhat smaller, since the surface of the echellette scatters short-wave radiation. In addition, in the calculations the entrance and exit slits were considered infinitely narrow; with the use of wide slits the ratio should decrease still further.

heated by an electric current. The strip had dimensions \(100\times 7.6\times 0.13\) mm and was heated to a temperature of \(1300^\circ\) K by a current of about 100 amperes. It is assumed that this source has a better ratio of the intensities of long-wave and short-wave radiation than a black body at the given temperature.

It is not impossible that other refractory oxides also possess the same properties; however, the corresponding investigations have not yet been carried out.

In the wavelength region from 100 to \(1000\,\mu\) and beyond, the only radiation source at present is the high-pressure quartz mercury lamp, which, as Rubens showed as early as 1911,^66 has weak continuous emission in the long-wave part of the spectrum.

The continuous long-wave radiation of mercury vapor excited in a discharge, and its dependence on voltage, current density, vapor pressure, lamp-bulb dimensions, etc., have been investigated many times, beginning in 1938.^67 However, up to the present time there is no unified point of view on the nature of this radiation. On the one hand, it is interpreted as a manifestation of vibrational-rotational transitions in excited quasi-molecules of mercury. According to another hypothesis, it arises as a result of the braking of electrons in the field of positive ions. In favor of the first point of view is the presence of long-wave radiation from zinc and cadmium discharges (which are situated in the same column of Mendeleev’s table and have molecular potential curves analogous to those of mercury), and the absence of such radiation in sulfur and thallium discharges, which is inexplicable from the standpoint of the second hypothesis.^68

Whatever the nature of the long-wave emission of the mercury lamp may be—its radiation requiring further investigation—it is suitable for carrying out spectral measurements in the wavelength region from 100 to \(1600\,\mu\).^60

Fig. 10. Transmission of quartz, paraffin, soot, and paper in the region \(50\text{--}250\,\mu\).^69,13

B. Methods of eliminating short-wave radiation

The elimination of scattered radiation of shorter wavelengths in spectrometers intended for investigations in the long-wave IR region is carried out with the aid of various selectively transmitting and selectively reflecting filters, and also by applying the methods discussed above for isolating long-wave radiation (the quartz-lens method and the “residual” rays method) and by using the method of selective modulation of the light flux.

1. Transmitting filters. As filters that block short-wave radiation and transmit long-wave radiation,

may serve: crystalline quartz, paraffin, lampblack, black (photographic) paper, matted polyethylene, and certain other materials. The transmission of these materials in the regions from 100 to 240 μ and up to 600 μ is shown in Figs. 10 and 11.

A plate of crystalline quartz 4–5 mm thick absorbs all short-wave infrared radiation, beginning at a wavelength of 5 μ; in the long-wave region its transmission increases, reaching 80% at 400 μ. With a layer thickness of 35–40 mm, crystalline quartz

Fig. 11. Transmission of some materials in the region 100–600 μ^44.

completely absorbs in the region from 4 to 100 μ and transmits up to 75% at 500 μ. Fused quartz transmits considerably worse.

A plate 1 mm thick, made of paraffin with a high boiling point (68–72° C) or of ceresin, strongly absorbs and scatters short-wave radiation and transmits long-wave radiation well. A thick paraffin plate (5–6 mm) at 100 μ transmits 50%, and at 600 μ—more than 75%.

The entire visible and near region of the IR spectrum is completely absorbed by lampblack; its transmission increases strongly in the long-wave region and depends to a considerable extent on the thickness and density of the layers. Thus, a layer obtained by uniformly depositing 0.6 mg of soot on 1 cm² of the surface of a plate of paraffin, quartz, or polyethylene transmits more than 95% of radiation of wavelength 250 μ, whereas a layer with density 5 mg/cm² transmits in this region only 50%

(Fig. 10). Black tracing paper and photographic paper also have good transmission.

Organic plastics: polyethylene, polychlorotrifluoroethylene (Teflon), polystyrene, polyvinyl chloride (Vinidur), and polyurethane, with a layer thickness of 1 mm and more, strongly absorbing medium-wave radiation, become “transparent” in the long-wave region (Figs. 11, 12). Sometimes the surfaces of plates made from these materials are matted with emery paper, which leads to an additional reduction of short-wave radiation through scattering. Usually polyethylene, along with quartz, is used for making spectrometer windows and cuvettes, while Vinidur and polyurethane are used as filters when working in the longest-wave region of the spectrum.

2. Reflecting filters. Besides crystals (“residual rays”), echelette diffraction gratings used in the zero order, as well as matted metal surfaces, possess the property of selectively reflecting long-wave IR radiation.

According to White^70, an echelette grating having a constant \(d\), somewhat smaller than the wavelength \(\lambda\) of the radiation being isolated, acts as a plane mirror for this and longer-wave radiation, i.e., such an echelette reflects radiation with \(\lambda > d\) and \(\lambda \gg d\) into the zero order. Radiation of shorter wavelengths (\(\lambda < d\) and \(\lambda \ll d\)), however, is reflected by the echelette into other orders, which are diffracted to the sides and are not captured by the optical system of the instrument. Consequently, in this case the echelette plays the role of a one-sided filter, specularly reflecting long-wave radiation and scattering short-wave radiation. When working in a broad long-wave region of the spectrum, it is necessary to use several such echelettes. Thus, in investigating the region from 100 to 1600 \(\mu\), in work^60, along with transmitting filters (quartz, paraffin, soot, Vinidur, polyurethane), two echelette filters were used with constants: 0.211 mm for the region 100–700 \(\mu\) and 0.625 mm for the region 700–1600 \(\mu\).

Fig. 12. Transmission of Vinidur and polyurethane in the region 300–1100 \(\mu\)^60.

Reflection of radiation from a plane aluminum or silver mirror matted with emery powder also leads to scattering of short-wave radiation with comparatively high reflection of long-wave radiation. When working in different regions of the spectrum, the mirror should be matted so that the structure of its surface contains grains differing little in size from the wavelength of the radiation to be selected. In practice the matter reduces to choosing the corresponding grade according to grain size^64.

3. Method of selective modulation. Elimination of scattered short-wave radiation in the case of using low-inertia receivers can be achieved by employing the method of selective modulation of the luminous flux^46. In this method the modulator is a rotating sector disk made of materials transparent

in the region up to 25–50 μ and absorbing the longer-wavelength region (for example, KBr or CsJ crystals). When such a disk, placed in the path of the light flux, is rotated, those regions of the spectrum that are not transmitted by the crystalline sectors of the modulator will be modulated with the greatest depth. The visible, near, and middle regions of the IR spectrum, which are not absorbed by the crystal, will not be modulated, and the constant signal of the receiver will not be passed by the AC amplifier. Thus, the crystalline modulator serves as a filter that blocks radiation of short wavelengths and transmits the long-wavelength region up to the wavelength at which the given crystal again becomes transparent.

Since the short-wavelength radiation transmitted by the crystal, on passing through a sector of the modulator, will be attenuated as a result of partial absorption and double reflection from the air–crystal–air interfaces, it will be partially modulated, and at the output of the amplifying device a signal will appear whose magnitude, because of the large energy in the short-wavelength region of the spectrum, may exceed the useful signal. To eliminate this harmful background, metal diaphragms are introduced into the open gaps of the modulator, compensating the energy losses upon reflection.

To obtain the purest long-wavelength spectra, free from superposition of short-wavelength radiation, modern spectrometers use combinations of the methods described above.

B. Radiation receivers

At present the spectral resolving power and operating accuracy of infrared spectrometers are limited by the sensitivity of the receiving-amplifying and recording devices and, first of all, by the sensitivity of the IR radiation receivers.

In long-wavelength IR spectrometers intended for recording extremely small energies of monochromatic radiation (of the order of \(10^{-8}\)—\(10^{-10}\) W), the threshold sensitivity of the receiver plays a very substantial, if not determining, role \(^{71}\).

Another important characteristic of a receiver is the time constant, which determines the possibility of its use with modulation of the light flux, as well as the specific sensitivity and the magnitude of the area of the receiving surface, which, because of the large apertures of long-wavelength spectrometers and the difficulties arising in concentrating the energy in a small volume, must not be too small.

The requirements of high specific \((S_0)\) and threshold \((Q_{\min})\) sensitivity, a small time constant \((\tau)\), allowing a modulation frequency of the order of 10 Hz, and a receiving surface \((F)\) that is not too small are satisfied by Golay pneumatic receivers \(^{72}\), having the characteristics: \(Q_{\min} = 7.6 \cdot 10^{-11}\) W, \(\tau = 0.02\) sec., \(F = 4—7\ \text{mm}^2\), as well as by Schwarz thermocouples \(^{73}\) \((Q_{\min} = 4.2 \cdot 10^{-11}\) W, \(\tau = 0.03\) sec., \(S_0 = 24\ \text{V/W}\), \(F = 0.4\ \text{mm}^2)\) and Berd metal bolometers \(^{74}\) \((Q_{\min} = 1 \cdot 10^{-10}\) W, \(\tau = 0.0041\) sec., \(F = 0.2\ \text{mm}^2)\), which, however, have small receiving surfaces. These receivers, together with bolometers and thermoelements of other firms, are currently used in foreign spectrometers intended for the mid-wave and long-wave regions of the IR spectrum.

In the USSR sensitive metal bolometers \(^{75}\) have also been developed with the parameters: \(Q_{\min} = 6 \cdot 10^{-11}\) W, \(\tau = 0.02\) sec., \(S_0 = 28\ \text{V/W}\),

\(F = 1.0\ \mathrm{mm}^2\)*), as well as highly sensitive thermoelements^77. Compared with bolometers, the latter have a somewhat lower threshold sensitivity (down to \(2 \cdot 10^{-10}\ \mathrm{W}\)), but have larger receiving surfaces (\(2.5\ \mathrm{mm}^2\) and more). The considerable time constants of these thermoelements (0.2–0.3 sec) do not permit their use in the intermittent-illumination method. Nevertheless, with suitable amplifiers they can be used in long-wave IR spectrometers^64.

An essential matter in developing new effective receivers intended exclusively for detecting long-wave IR radiation is the choice of absorbing coatings for their receiving elements, since the coatings used in ordinary receivers (soot, metallic black, etc.) become increasingly “transparent” as the wavelength increases. In this direction, additional special investigations are required.

3. Laboratory-type spectrometers

As noted above, from 1938 to the present several laboratory spectral installations have been built for the investigation of long-wave IR spectra with high resolution. However, an industrial type of such instruments has not yet been developed. This is explained by the fact that the long-wave IR region has not yet found such broad application as the medium-wave and short-wave regions of the IR spectrum possess.

Let us briefly consider the schemes of several laboratory installations built in recent years.

1. Randall spectrometer (1932–1938). The optical scheme of the monochromator (Fig. 13) is very simple and at the same time free from aberrations, which are eliminated by the use of an off-axis parabolic mirror \(M_1\) with a relative aperture of \(1:1.5\) and an axial elliptical mirror \(M_3\), having focal distances of 89 and 445 mm (reduction \(1:5\)). The entrance and exit slits \(S_1\) and \(S_2\), 5 cm high, are located above the grating \(G\), which has dimensions of the ruled surface \(250 \times 550\ \mathrm{mm}\). Two gratings were used, 14 and 5 lines per 1 mm, covering the spectral region from 18 to 200 μ and operating in the first order within an angle of \(6^\circ\). The radiation source was a platinum strip coated with thorium oxide and heated by current. Short-wave radiation was eliminated with the aid of “residual reflectors” made of KBr and KJ and filters made of paraffin and crystalline quartz. As the receiver \(T\), vacuum thermoelements with windows of potassium bromide or paraffin were used, continuously pumped out with the use of liquid air. The absorbing coatings of the receiving surfaces of the thermoelements—

Fig. 13. Scheme of Randall’s spectrometer^35.

* The parameters of domestic and foreign receivers are taken from the article by N. M. Markov^76, devoted to a comparison of low-inertia receivers of IR radiation; the values of the threshold sensitivity \(Q_{\min}\) are determined at a time constant of 1 sec.

tions were selected depending on the wavelength of the spectral region under investigation.

Amplification of the thermocurrents was carried out by means of a Firestone resonance amplifier with an amplification factor of 100 and higher. The spectra were recorded by photographic registration of galvanometer oscillations and took from 3 to 8 hours, depending on the required resolution. The entire optical system of the instrument was enclosed in vacuum.

1. The Mac-Rabbin and Sinton spectrometer (1952) was the instrument with the greatest light-gathering power. Its relative aperture was \(1:1\), with a parabolic collimating mirror diameter of \(440\ \text{mm}\) and echelle dimensions of \(300 \times 300\ \text{mm}\).

The spectrometer was built according to Pfund’s autocollimation scheme (Fig. 14), without an additional plane mirror; the entrance and exit beams passed through an aperture in the center of the echelle. The exit slit was circular, with a diameter of \(3\ \text{mm}\); the receiving area was that of a Golay pneumatic detector.

Two echelles were used, with a “blaze” angle of \(10^\circ\) in first order: one with 2.6 lines per \(1\ \text{mm}\) for the region \(100\text{–}300\ \mu\), and the other with 1 line per \(1\ \text{mm}\) for the interval \(300\text{–}700\ \mu\).

As sources, high-pressure quartz mercury lamps were used: an H-3 lamp (for the region \(100\text{–}300\ \mu\)) and a powerful liquid-cathode lamp of the “Hanovia” type (\(300\text{–}700\ \mu\)).

Elimination of short-wavelength radiation was achieved by focal isolation with the aid of a quartz lens, selective reflection from a KRS-5 crystal, and also by smoking with turpentine soot the surfaces of the quartz lens, the echelle, and the quartz plate placed in front of the receiver.

Fig. 14. Diagram of the Mac-Rabbin and Sinton spectrometer\(^{44}\).

Fig. 15. Diagram of Bohn’s spectrometer\(^{54}\).

Partial elimination of absorption by atmospheric water vapor was carried out by filling the monochromator and illuminator with dry nitrogen.

1. Bohn spectrometer (1953) is also built according to an autocollimation scheme, consisting of a plane mirror \(M_4\) with an aperture at the center, a parabolic mirror \(M_3\) of diameter 250 mm, and an echelette \(G\), of size \(185 \times 200\) mm (Fig. 15).

Radiation from the source \(O\) (a Silit rod) is focused by the cylindrical “residual reflector” \(M_1\) into the plane of the intermediate image, in which a sector disk of the crystalline (selective) modulator \(Ch\) is placed; it then enters the gas cuvette \(C\) with a polyethylene window, and is focused by the spherical mirror \(M_2\) onto the entrance slit of the monochromator \(S_1\). The radiation leaving the exit slit, by means of the plane mirror \(M_5\) and the elliptical mirror \(M_6\), is concentrated on the receiver—a thermoelement (bismuth–antimony, resistance 20 ohms), connected to the input transformer of an electronic a.c. amplifier (amplification factor \(10^7\), passband width from 2 to 20 cps). The amplified signal is mechanically rectified and enters an \(RC\) filter, whose maximum time constant was 60 sec with a frequency passband width of 0.003 cps. The spectra were recorded on a Brown recorder having a sensitivity of 10 mV full-scale.

For the spectral region from 22 to 77 \(\mu\), two echelettes were used, having 15 and 24 lines per 1 mm. The crystalline modulator was made of sodium chloride (for the region 22–40 \(\mu\)) and potassium bromide (40–77 \(\mu\)), while the cylindrical reflector was made of fluorite, as well as of sodium chloride and sodium bromide.

1. Meyer spectrometer (1953). The basis of this instrument, intended for obtaining spectra in the region from 150 to 600 \(\mu\), is a double Pfund scheme, including two spherical mirrors \(H_2, H_3\) (diameter 250 mm, focal length 600 mm), a plane mirror \(R_2\), and an echelette \(G\) with apertures (Fig. 16). The source \(Q\) is a high-pressure mercury

Fig. 16. Diagram of the Meyer spectrometer \(^{42}\).

lamp (PRK-2 or PRK-4), the receiver \(R\) is a radiometer with a quartz window 2 mm thick and a conical light guide \(T\); \(F_1, F_2\), and \(F_3\) are filters of black paper 0.13 mm thick; \(KL\) is a rock-salt plate. Spectral measurements were made point by point.

1. Tenzel and Eckhardt spectrometer (1955–1956) is intended for recording IR spectra in the longest-wavelength region—from 100 to 1600 \(\mu\).

The monochromator of the instrument, built according to the Ebert–Fastie scheme (Fig. 17), consisted of two spherical mirrors of diameter 200 mm, with a relative aperture of \(1:3\), an echelette, and two slits, 60 mm high and allowing an opening of up to 30 mm.

For operation in the spectral regions 100–350 \(\mu\), 250–700 \(\mu\), 350–950 \(\mu\), and 500–1600 \(\mu\), four echelettes were used in first order, with a blaze angle of \(20^\circ\) and grating constants: 0.315, 0.625, 0.833, and 1.250 mm.

A quartz mercury lamp of type PRK-2, with the aid of a parabolic mirror (diameter 200 mm, relative aperture \(1:0.6\)), was focused on the entrance slit with 4-fold magnification.

Modulated at a frequency of 12.5 Hz, the radiation, after passing through the exit slit, was reflected from an echelette filter and a plane mirror and was focused by a concave mirror (1 : 0.5) on the receiving surface of a bismuth bolometer. To match the dimensions of the slits, which have different widths depending on the isolated wavelength, to the dimensions of the receiving surface, four bolometers were used, having different receiving areas: \(0.6 \times 6\ \mathrm{mm}^2\), \(1.5 \times 6\ \mathrm{mm}^2\), \(3 \times 10\ \mathrm{mm}^2\), and \(6 \times 12\ \mathrm{mm}^2\), and, respectively, different resistances: 2; 0.9; 0.8; 0.5 kΩ.

Fig. 17. Diagram of the Genzel and Eckhardt spectrometer\(^ {60}\).

Short-wavelength radiation was eliminated with the aid of filters made of quartz, paraffin, soot, polyurethane, and Vinidur, and also by means of echelettes, operated in zero order, with constants of 0.211 and 0.625 mm.

The entire optical system of the spectrometer, placed in a steel tube, allowed evacuation to pressures of the order of 0.1 mm Hg.

6. DICS-1 spectrometer (1956). The spectrometer, intended for automatic recording of IR spectra in the region from 20 to several hundred microns, is based on the optical scheme (Fig. 18),

Fig. 18. Optical scheme of the DICS-1 spectrometer\(^ {64}\): 1, 2, 5, 9 — spherical mirrors; 6, 7, 8 — plane mirrors; 3 — gas cuvette; 4 — echelette; 10 — radiation receiver; 11 — device for obtaining the bias voltage of the electronic detector; 12 — radiation source; 13 — crystal modulator; 14 — reflecting filters.

which in the monochromator part is similar to the scheme of Bohn’s spectrometer\(^ {54}\), and in the illuminator part to the scheme of the instrument of Oetjen and co-workers\(^ {46}\). The radiation flux from source 12 (a globar rod or a quartz mercury lamp PRK-4), after being preliminarily reflected from one of four cry-

steel plates mounted on a rotating table, is focused in the plane of the intermediate image, where the crystalline sector disk of the selective modulator 13 is placed. After then being reflected from the matte mirror and the filter–echelette, the beam is focused by mirror 2 onto the entrance slit of the monochromator, consisting of collimating mirror 5 (diameter 350 mm, relative aperture 1 : 2.2), a plane mirror with aperture 4, and echelette 6. Beyond the exit slit are arranged plane mirrors 7 and 8 and spherical mirror 9, which concentrates the monochromatic beam through the aperture of mirror 8 onto receiver 10. Auxiliary device 11, consisting of an additional

Fig. 19. General view of the installation with the DIKS-1 spectrometer:
1—illuminator, 2—monochromator, 3—amplifier, 4—recorder, 5—power-supply unit with vacuum installation.

light source, lens, photocell, and metallic modulating disk, serves to generate the reference voltage required for operation of the synchronous rectifier.

In obtaining spectra in the region from 20 to 500 μ, three echelettes of size

Fig. 20. Absorption of water vapor in the region 300–500 μ.

250 × 250 mm (blaze angle 12°, number of grooves per 1 mm—12.6 and 2) [78], fabricated in the laboratory of F. M. Gerasimov, were used, as well as various reflecting and transmitting filters. The illuminator and monochromator are placed in chambers connected with each other by a sylphon-

Table III

Laboratory spectrometers for the long-wavelength infrared region

with a new tube and permitting evacuation to a pressure of 0.1–0.2 mm Hg. The general appearance of the entire setup is shown in Fig. 19. Figure 20 presents one of the first examples of spectrograms of atmospheric water vapor recorded by the instrument in the region from 300 to 500 μ. In recording the spectrum, the source was a PRK-4 mercury lamp, and the receiver was a six-junction compensated thermoelement made by Prof. B. P. Kozyrev, connected to two photoelectric amplifiers of the FEOU-15 and FEOU-17 type arranged in series,^77 having an EPSh-09 electronic potentiometer at the output. When such a receiving-recording system was used, device 11 with the modulator was disconnected. The slit used was 2 divisions per 1 mm; the transmitting filters were: a polyethylene plate coated with soot, 1 mm thick, and crystalline quartz.

In conclusion, Table III gives the main characteristics of laboratory-type spectrometers built recently and intended for obtaining and recording long-wavelength infrared spectra.

The estimate of the spectral resolution attained with these instruments, indicated in the last column of the table, was made for the long-wavelength and short-wavelength limits of the spectrum from the spectral curves presented by the authors.

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