Astro spectra in the laboratory

Whatever data astronomers and astrophysicists receive about celestial bodies, it is possible to decipher these data, as a rule, only relying on the regularities derived in terrestrial laboratories when studying terrestrial objects.

An ingenious method for modeling planetary atmospheres in an absorption tube and possible applications of this method is described in this article.

Spectra of planetary atmospheres

Spectral study of planetary atmospheres is one of the urgent problems of modern astrophysics. However, this complex, large task cannot be successfully solved only by astronomers, without the involvement of specialists in related sciences. For example, astronomers cannot do without the results of laboratory studies of spectroscopists-physicists to study molecular absorption spectra, without determining the physical constants of molecules and their structure. Only having at our disposal a sufficient number of molecular constants and spectral atlases of molecules, it is possible to identify the spectra of planetary atmospheres and other celestial bodies. This applies to any method of observation, be it ground-based astronomy (optical or radio astronomy methods) or the results obtained with rockets launched outside the Earth's atmosphere.

The spectra of planetary atmospheres consist mainly of molecular bands that belong to molecules of carbon dioxide (CO2), carbon monoxide (CO), methane (SND of ammonia (NH3), nitrogen (N2), oxygen (O2), i.e., mainly two -, three- and four-atomic molecules. At present, we can almost confidently speak about the qualitative chemical composition of the atmospheres of most planets. It was established after careful study of astronomical spectrograms obtained by optical methods and using radio astronomy observations. In addition, the results of the Soviet space station " Venus-4 "allowed not only to give information about a more accurate qualitative chemical composition of the atmosphere of Venus, but also to clarify its quantitative composition, temperature and pressure.

As for the quantitative chemical composition of the atmospheres of other planets, it still requires serious verification and clarification. Until now, astronomers encounter great difficulties in identifying and studying the stripe spectra of the atmospheres of planets. These difficulties, as a rule, are caused by the fact that our laboratory and theoretical knowledge of the structure and properties of even simple molecules is limited. Therefore, when studying the astronomical spectrum, we must first of all determine which of the molecules gave it, and then, according to laboratory studies, clarify the properties and structure of the bands of this molecule.

Polyatomic molecules, and in particular triatomic ones that are found in comets and planets, are even less studied.

It should be noted that it is not always possible to easily and simply obtain in laboratory conditions the same molecules that are found, say, in stellar atmospheres. Let's look at one interesting example.

In 1926, P. Merrill and R. Sanford observed very strong absorption bands in some carbon stars of the RV Dragon type, but they could not be confidently identified for decades. True, for theoretical reasons, it was assumed that these bands are caused by a complex molecule - the triatomic S1C2.

For the correct solution of the problem, laboratory experiments were set. In 1956 W. Clement tried to obtain these bands in the laboratory. When setting up the experiments, he proceeded from the following consideration: the spectra of the Cr molecule are observed in a number of stars and are well studied. The spectrum of the silicon molecule is well studied in the laboratory, but has not been noted among the astronomical spectra.Therefore, Clement suggested that in the presence of carbon and silicon, a unipolar SiC molecule is formed, which should be observed in astronomical spectra, as well as in the laboratory, although this was not possible until 1961. Then Clement reasoned as follows: if S1 is added to the King's high-temperature furnace, which is made of pure compressed coal, then at a certain furnace heating temperature (a temperature of 2500-3000 ° K can be obtained in the furnace), an absorption spectrum belonging to the SiC molecule should be observed. However, the spectrum obtained by Clement turned out to be more complex and unlike that expected for SiC. Then they compared the spectrum obtained in the laboratory with the unidentified spectrum of one of the cool stars of the RV Dragon type, and it turned out that the bands matched well. Only one thing became clear from the experiment, that Clement was able to reproduce the stellar spectrum in the laboratory. However, it was impossible to determine which molecule gave this spectrum.

The molecule remained unknown. Only there was more reason to believe that only carbon and silicon could provide such a spectrum.

In addition, vibrational analysis showed that the desired molecule contains one heavy atom, combined with two associated lighter ones. From this, a conclusion was made (requiring more confirmation): most likely, this complex spectrum is provided by the S1C2 molecule. In his research, Clement obtained spectrograms at a high temperature of the spectrum source, so the fine structure of the bands could not be determined in detail. Such an imperfection of the experiment carried out did not allow the definitive identification of the Merrill and Sanford bands.

At present, researchers have returned to this issue again. Canadian physicists are paying great attention to the search for a light source that gives a molecular spectrum similar to the striped spectra of carbon stars. Prof. G. Herzberg reports that he and his collaborator R. Verm in the laboratory were able to observe the bands of the SiC2 molecule at low temperatures - Herzberg expresses the hope that a thorough study of the new spectra at a higher resolution will make it possible to more confidently analyze the rotational structure and determine the moment of inertia of this mysterious molecule.

Many scientists await the results of this study with great interest and hope that the source of the molecular spectrum will finally be found, which will make it possible to finally identify the Merrill and Sanford bands. Molecule SiC2 will then be the first polyatomic molecule confidently found in the atmosphere of a star.

In the atmospheres of stars and comets, other molecules, such as CH +, C3, NH2, are currently identified, which can only be obtained with great difficulty and very rarely in laboratories under specially controlled conditions. In general, molecular spectra, due to their complex structure, have been studied much less well than atomic ones.

The spectra of atoms of various chemical elements have been studied almost well, although there are a number of questions that remain unresolved. Now we have the necessary amount of completely reliable information about the physical constants of the spectra of atoms. Perhaps due to this, atomic spectra will play a dominant role over molecular ones in various fields of science for a long time.

The laboratory study of the spectra of molecules of astrophysical interest has received particular attention since the forties of this century. However, there are still no good, complete reference books of the molecules under study until now.

Absorption pipes with a large absorption path

Molecular absorption spectra are more complex than atomic ones. They are made up of a number of bands, and each band is made up of a large number of individual spectral lines. In addition to translational motion, a molecule also has internal motions, consisting of the rotation of the molecule around its center of gravity, the vibrations of the nuclei of the atoms that make up the molecule relative to each other and the movement of the electrons that make up the electron shell of the molecule.

To resolve molecular absorption bands into individual spectral lines, it is necessary to use high-resolution spectral devices and transmit light through absorption (absorbing) tubes. Initially, the work was carried out with short pipes and at pressures of the studied gases or their mixtures of several tens of atmospheres.

It turned out that this technique does not help to reveal the structure of the spectrum of molecular bands, but, on the contrary, washes them out. Therefore, they immediately had to abandon it. After that, we took the path of creating absorption tubes with multiple passage of light through them. The optical scheme of such an absorption tube was first proposed by J. White in 1942. In tubes designed according to White's scheme, it is possible to obtain equivalent optical paths of absorbing layers from several meters to several hundred thousand meters. The pressure of the investigated pure gases or gas mixtures varies from hundredths to tens and hundreds of atmospheres. The use of such absorption tubes for studying molecular absorption spectra has proven to be very effective.

So, in order to resolve the spectra of molecular bands into separate spectral lines, it is necessary to have a special type of equipment, which consists of high-resolution spectral devices and absorption tubes with multiple passes of light through them. In order to identify the obtained spectra of the atmospheres of planets, it is necessary to compare them directly with laboratory ones and in this way find not only the wavelengths, but also confidently determine the chemical composition, and estimate the pressures in the atmospheres of the planets from the broadening of spectral lines. The measured absorption in absorption tubes can be compared in magnitude with the absorption in the atmosphere of a planet. Consequently, in absorption tubes with multiple passes of light, when the pressure of the studied pure gases or their mixtures changes, one can, as it were, simulate the atmospheres of the planets. It has become more realistic now that it is possible to change the temperature regime in the pipes within a few hundred degrees Kelvin.

Optical layout of the J. White absorption tube

The essence of J. White's invention boils down to the following: three spherical concave mirrors of strictly equal radii of curvature are taken. One of the mirrors (A) is installed at one end inside the pipe, and the other two (B, C), which are two equal parts of the cut mirror, are at the other end. The distance between the first mirror and the other two is equal to the radius of curvature of the mirrors. The pipe is hermetically sealed. The vacuum in the pipe is created to tenths or hundredths of a mm Hg. Art., and then the pipe is filled with the test gas to a certain (depending on the task, pressure. Mirrors in the pipe are installed in such a way that the light entering the pipe is reflected from the mirrors, passing a specified number of times in forward and backward directions.

At present, all absorption tubes are made according to J. White's scheme with a change in the design of the front mirror introduced by G. Herzberg and N. Bernstein in 1948. Herzberg used an optical scheme to obtain a long light absorption path in an absorption tube with a radius of curvature of mirrors of 22 m and pipe diameter 250 mm. The pipe is made of electrolytic iron. In one of Herzberg's works on the study of absorption spectra of carbon dioxide (CO 2), the absorbing path of light was 5,500 m, which corresponds to 250 passages between mirrors. Such a large absorbing path, i.e., a large optical depth, was obtained only thanks to the ingenious optical scheme proposed by White.

The limit to the number of light passages is set by the reflection loss and the number of images that can be obtained on mirror C. In the creation of absorption pipes, designers encounter great mechanical difficulties. First of all, this is the development of the frame of mirrors and their fastening, adjustment and focusing mechanisms, outputs of the control mechanisms to the outside.If the pipe is relatively short, the mirrors are located on a common plateau, which, after installing the mirrors on it, is pushed into the pipe; if the pipe is long, the installation of the mirrors becomes much more difficult.

It is very important what material the pipes are made of. Electrolytically pure iron, stainless steel and invar are used. The inside of the steel pipe is coated with electrolytically pure iron. As far as we know, the walls inside the pipes are not covered with any vacuum varnishes, especially recently. The choice of material for covering the surface of mirrors depends on the spectral region in which the work will be carried out. Accordingly, gold, silver or aluminum are used. Dielectric coatings are also used.

Absorption pipe of the Pulkovo Observatory

Our absorption pipe is steel, one-piece drawn, welded from separate lengths. 8-10 m. Its total length is 96.7 m, inner diameter 400 mm, wall thickness 10 mm. Temporarily, two aluminum-coated mirrors with a diameter of only 100 mm and a radius of curvature of 96 m are installed in the tube. The tube also contains objectives. With the help of two mirrors, we get a trip three times. If we take two more mirrors and place them appropriately in the tube, the light is transmitted five times, which we have done recently.

So, in our works, we have the following absorbing paths: 100 m, 300 m, 500 m.This is when taking into account the distances from the light source to the entrance window of the tube and the distance that the light beam travels from the exit window to the spectrograph slit.

In the future, the mirrors are supposed to be replaced by large ones - 380 mm in diameter and 100 m radius of curvature. The corresponding optical scheme will be replaced by the classical White scheme with a change introduced by Herzberg and Bernstein. All optical calculations must be carried out so that the effective length of the absorbing path becomes 5000–6000 m for 50–60 passages.

Our absorption pipe is one of the longest, so new solutions had to be found when designing some of its components. For example, should the mirrors be mounted on a base connected to the pipe body, or installed on separate foundations independent of the pipe? This is one of the very difficult questions (we will not give others), and the reliability and accuracy of alignment and orientation of the mirrors will depend on its correct solution. Since the mirrors are located inside the pipe, then, naturally, when pumping out or when creating pressure in the pipe, due to deformations of the mounting of the mirrors (even if they are minimal, a change in the direction of the light beam. This issue also requires a special solution, like determining the number of light passes through the pipe We will carry out the alignment and focusing of the mirrors using a laser.

A vacuum diffraction spectrograph is placed next to the absorption tube. It is assembled according to an autocollimation scheme. A flat diffraction grating with 600 lines per millimeter gives a linear dispersion in the second order of 1.7 A / mm. We used a 24 V, 100 W incandescent lamp as the continuous spectrum source.

In addition to the installation and investigation of the pipe, the study of the A band of the molecular absorption spectrum of oxygen (O2) has now been completed. The work was aimed at revealing changes in equivalent absorption line widths depending on pressure. The equivalent widths are calculated for all wavelengths from 7598 to 7682 A. Spectrograms 1 and 2 show the absorption spectra of band A. Work is also underway to reveal the effect of increasing the equivalent widths depending on the presence of an extraneous gas. For example, carbon dioxide (CO2) is taken and some nitrogen (N2) is added to it.

In our laboratory, work on the study of molecular absorption spectra is being carried out by L.N. Zhukova, V.D.Galkin and the author of this article.We try to direct our investigations so that their results would contribute to the solution of astrophysical problems, mainly in planetary astronomy.

The processing of both laboratory and astronomical molecular absorption spectra obtained by photographic or photoelectric recording methods is very laborious and time-consuming. To accelerate this work at the University of California, J. Phillips, back in 1957, began processing molecular absorption spectra using an IBM-701 computer. At first, the program was compiled for the spectra of C2 and NO. At the same time, tables for CN were prepared. Phillips believes that, first of all, the machine needs to process the spectra of molecules of astorophysical interest: C2, CN, NH, BH, MgH, AIH, SIF, BO, ZrO.

The advantages of computer technology are obvious, and it should be widely used for processing experimental results.

Laboratory research and astronomical spectra

A large group of physicists are studying the molecular absorption spectra obtained in absorption tubes of multiple light transmission. First of all, I would like to note the great role and merit of prof. G. Herzberg (Ottawa, Canada). His experimental and theoretical works, like his monographs,
lie at the foundation of this area of ​​science. One of the leading places in research, and especially in the study of the spectra of quadrupole molecules, is occupied by the work of prof. D. Rank (Pennsylvania, USA). Among the younger researchers, one cannot fail to note the work of T. Owen (Arizona, USA) who very successfully combines his laboratory experiments with astrophysical observations.

We have already given one example of a fruitful combination of laboratory and astrophysical methods in the first part of this article. It concerns the identification of molecular bands in the spectrum of an RV Draco star. As a second example, consider the joint work of G. Herzberg and D. Kuiper on the study of planetary spectra based on direct comparison with laboratory ones.

Kuiper at the McDonald Observatory obtained the spectra of Venus and Mars with a high resolution in the wavelength interval 14-2.5 microns. A total of 15 bands were noted, identified with the molecular bands of carbon dioxide (CO2). One band around X = 2.16 microns was questionable. Herzberg and Kuiper conducted additional laboratory studies of CO2, which confidently showed that the absorption at X = 2.16 μ in the spectrum of Venus is due to the CO2 molecule. For laboratory studies of the absorption spectra of CO2 by Herzberg and Kuiper, a multi-pass absorption tube of the Ierki Observatory was used with a mirror curvature radius of 22 m, also 22 m long and 250 mm in diameter. The pipe is made of electrolytic iron. Before filling the tube with the test gas, it was pumped out to several mm Hg. Art. (later they began to get a vacuum up to tenths of a mm Hg. Art.). In their first work, Herzberg and Kuiper varied the CO2 pressure in the pipe in the range from 0.12 to 2 atm. The length of the absorbing layer was 88 m and 1400 m, i.e., in the first case, the light passed through the tube 4 times, and in the second - 64 times. From the tube, light was directed to the spectrometer. In this work, we used the same spectrometer with which the spectra of Venus and Mars were obtained. The wavelengths of the CO2 absorption bands were determined in laboratory spectra. By comparing the spectrograms, the unknown absorption bands in the spectra of Venus were easily identified. Later, bands in the spectra of Mars and the Moon were identified in a similar way. Measurements of the self-broadening of spectral lines, caused only by a change in gas pressure or due to the addition of another gas, will make it possible to estimate the pressure in the atmospheres of planets. It should be noted that there are pressure and temperature gradients in the atmospheres of the planets; this makes it difficult to model them in the laboratory. Third example. We pointed out the importance of the work headed by prof. D. Rank.Many of them are devoted to the study of the spectra of quadrupole molecules: nitrogen (N2), hydrogen (H2) and other molecules. In addition, Rank and his collaborators are engaged in the highly topical issues of determining the rotational and vibrational constants for various molecules, which are so necessary for physicists and astrophysicists.

In the study of molecular absorption spectra in the Ranque laboratory, a large absorption tube 44 m long and 90 cm in diameter with multiple light transmission is used. Made of stainless steel pipe. The pressure of the studied gases in it can be obtained up to 6.4 kg / cm2, and the length of the light path - up to 5,000 m. With this tube, Rank performed new laboratory measurements of the CO2 and H2O lines, which made it possible to determine the amount of precipitated water (H2O) and CO2 in atmosphere of Mars. The measurements were carried out at the request of the American astrophysicists L. Kaplan, D. Munch and K. Spinrad and had to confirm the correctness of their identification of the rotation bands of the H2O lines around X = 8300 A and CO2 about X = 8700 A.

Laboratory studies of molecular absorption spectra in the lunar and planetary laboratories of the University of Arizona are being carried out with great success. T. Owen takes an active part in these works. An absorption tube 22 m long and 250 mm in diameter with multiple light transmission is installed in the laboratory. ' Steel pipe, lined with electrolytic iron inside. Laboratory spectra are obtained on a diffraction spectrograph with a linear dispersion of 2.5 A / mm. The main investigations are methane (CH4) and ammonia (NHa). The study is carried out in a wide range of pressures and at a large absorbing length. The light source is either the sun or an incandescent tungsten lamp. So, for example, for the work "Determination of the composition of the atmosphere and pressure on the surface of Mars", which was carried out by Owen and Kuiper (1954), it was required in the laboratory to investigate the X = 1.6 μ band in pure carbon dioxide (CO2) under the following conditions:

Owen and Kuiper also conducted a study on the addition of foreign gas. The authors note that if the total CO2 content is determined from weak bands, one can empirically find atmospheric pressure, in particular on Mars, from measurements of the X = 1.6 μ band, and detect the presence of any other component. But an empirical determination of the effects of pressure in gas mixtures at this facility is impossible, because it is necessary to have a beam path length equal to two heights of the homogeneous atmosphere of Mars, i.e., approximately 40 km. In the experiments of Kuiper and Owen, the absorbing path was only 4 km, that is, 10 times less.

When in 1966 J. Kuiper, R. Vilod and T. Owen obtained the spectra of Uranus and Neptune, it turned out that they contain a number of unidentified absorption bands. Since it is most likely that the atmospheres of these planets are composed of methane (CH4), laboratory studies were carried out with it. Laboratory spectra were obtained at very large optical paths and moderate rarefaction. For example, part of the spectra of CH4 in the wavelength range of 7671 and 7430 A were obtained at an effective absorbing length of 1 940 m atm, and a part of the spectra in the range of 7587, 7470 A and shorter - at a length of 2 860 m atm.

Only a comparison of the spectra of Uranus and Neptune with laboratory ones made it possible to confidently identify the unknown bands and prove that the absorption in the atmospheres of these planets is mainly caused by methane. With the Illinois Research Institute of Technology (ILI 12.5 m long, 125 mm diameter; made of stainless steel) multiple pass absorption tube, Owen did research on methane, water vapor, ammonia. The light path length was 1000 m, i.e. the forward and backward directions in the tube passed 80 times. The spectra of gases obtained in the laboratory were compared with the spectra of Jupiter, Venus and the Moon. In this way Owen carried out the identification of unknown bands in the spectra of these planets.The spectra of these planets were obtained at the McDonald Observatory with an 82 "reflector, an 84" reflector and a 60 "solar telescope at Kitt Peak National Observatory. A detailed study of the spectrograms allows us to conclude that absorption bands caused by methane, ammonia and hydrogen are confidently identified in the atmosphere of Jupiter. For other gases, a number of laboratory tests are required.

At the international symposium in Kiev (1968) Owen reported the results of spectroscopic determination of gases contained in the atmospheres of Jupiter, Saturn and Uranus.

We noted that it is not always possible to analyze and identify the obtained spectrograms of celestial bodies by direct comparison with laboratory spectra. This can be explained by the fact that the excitation and glow of gaseous media on celestial bodies often occur in very complex physicochemical conditions that cannot be accurately reproduced in ground-based laboratories. Therefore, when compared with laboratory spectra, the structure of molecular bands and their intensities remain ambiguous. Then you have to resort to indirect identification methods. Let us give, for example, the case with the spectrogram of the central peak of the lunar crater Alphonse, which was obtained by N.A.Kozyrev on November 3, 1958 and processed by him in the same year. The spectrogram was identified by the coincidence of a number of known C2 bands. However, the maximum brightness of the band at A = 4740 A required a special explanation, since it was not possible to obtain a similar spectrum in the laboratory. Kozyrev explains this shift by the fact that a complex molecule is ionized under the action of hard radiation from the Sun, and as a result, the C2 radical is formed, to which the displaced band belongs, which does not coincide with the bands known in this region. Since Kozyrev made a very bold conclusion on the basis of these results about the internal energy of the lunar interior and about the volcanic emission of gases, it was decided to re-process this unique spectrogram. This processing was carried out by A.A. Kalinyak, using the method of microphotometry. Kozyrev's conclusion was confirmed.

In connection with the development of rocket technology and the launch of rockets outside the Earth's atmosphere, it became possible to obtain fundamentally new physical parameters of planetary atmospheres and to study the properties of celestial bodies that were previously unobservable. But in the processing and analysis of observations obtained both with the help of rockets and ground means, great difficulties are encountered, which are due to the lack of laboratory research. These difficulties can be eliminated by the experimental work of spectroscopists-physicists and astrophysicists, whose interests not only coincide, but also overlap in the field of studying atomic and molecular absorption and radiation spectra. Consequently, the tasks facing them can be successfully solved only by joint work in ground-based laboratories. Therefore, despite the tremendous advances in the study of planetary atmospheres using rocket technology, ground-based laboratories should play an important role and in no way lose their importance for astrophysics.

L.A. Mitrofanova

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