FROM THE HISTORY OF PHYSICS

P. N. LEBEDEV’S WORKS ON INFRARED SPECTROSCOPY

B. A. Kiselev

At the present time, instruments of infrared spectral technology have acquired a sufficiently perfected form and make it possible to solve a number of practical problems—in particular, to study gases, liquids, and solids rapidly. As is known, instruments of this kind consist chiefly of a source of infrared radiation, a monochromator, a receiver of radiant energy, and automation elements that make it possible to record absorption spectra over the entire spectral range under investigation.

It seems of interest to illuminate certain aspects of the history of the development of infrared-ray technology and, in particular, to recall the works of Russian scientists in this field.

The pioneers of infrared spectroscopy in Russia were Petr Nikolaevich Lebedev and his pupils.

Universally recognized for his work in the study of electromagnetic waves and for his brilliant proof of the existence of light pressure, P. N. Lebedev also worked with the infrared interval of electromagnetic waves; to him we owe the development of the design of the double monochromator and of a method for the automatic recording of spectra, and he also greatly increased the sensitivity of radiation receivers.

In order to characterize the scientific activity of Petr Nikolaevich Lebedev (1866–1912), it is necessary to recall the state of science in the 1890s of the last century.* At that time there was as yet no electron theory, no theory of relativity; quantum theory did not exist.

* For a number of facts from the biography of P. N. Lebedev I express my deep gratitude to Corresponding Member of the Academy of Sciences of the USSR T. P. Kravets.

Maxwell’s theory was known, but there was no clear physical understanding of it; this theory seemed a complex piling-up of mathematical derivations. Only after Hertz’s experiments did the electromagnetic field and electromagnetic waves become the focus of physicists’ attention.

It is quite natural that Pyotr Nikolaevich’s attention was attracted by electromagnetic waves, and in 1895 he produced his celebrated work on the double refraction of electromagnetic waves[^1].

He discovered waves with a length of 6 mm and even 4 mm, i.e., those waves which present-day radiolocation has mastered only with the greatest difficulty. This work brought P. N. Lebedev the fame of a brilliant experimenter—“the world champion of experiment” (P. P. Lazarev). It is characteristic that during Pyotr Nikolaevich’s lifetime no one was able to advance further into the infrared region from the side of long electromagnetic waves, and only in the twenties did the Russian scientists A. A. Arkad’eva-Glagoleva, as well as M. A. Levitskaya, advance further. Pyotr Nikolaevich assumed that the method he had developed would be taken up and used for investigations of a purely optical character in a new spectral range. But with the state of laboratory technique at that time, such tasks were within the power of him alone. Only now has the spectroscopic study of substances with the aid of microwaves undergone vigorous development in all countries.

Pyotr Nikolaevich deeply understood the common nature of electromagnetic waves and their continuity in the transition from radio waves to infrared waves. The popular work he wrote in 1901, “The Scale of Electromagnetic Waves in the Ether”[^2], sums up the electromagnetic understanding of radiation phenomena. This work became a classic and to this day remains an indispensable element of teaching in higher education.

Pyotr Nikolaevich began to study infrared spectra in the laboratory of the Physics Institute of Moscow University. He understood the advantages of automatic recording of spectra, but the Langley recording method known at that time did not satisfy him. This method had two major shortcomings. First, the galvanometer used, despite magnetic shielding, gave too unstable a zero position; second, in the investigation of long infrared rays (up to 15 microns) large errors could arise because of the superposition of short-wave scattered light (Langley used an ordinary monochromator). The apparatus created by Pyotr Nikolaevich, fundamentally different from the others then known, contained such elements of novelty which, in a somewhat improved form, are indispensably preserved in modern devices.

The main feature that distinguished this spectrophotometer, called by Petr Nikolaevich the “Spectrograph for Ultraviolet Rays”^3, was the use of a double monochromator, a vacuum microradiometer, and an original system for automatic recording of spectra.

The layout of P. N. Lebedev’s double monochromator is shown in Fig. 1. Radiation from the Nernst filament \(N\) falls on the mirror \(S\) \((f = 25\ \text{cm})\), passes through the narrow slit \(K\), and is directed by the mirror \(S_1\) \((f = 50\ \text{cm})\) onto the first rock-salt prism \(P_1\) \((\varphi = 30^\circ)\), to the larger cathetus of which a silvered glass plate is attached by optical contact. The parallel beam entering the prism proceeds at an angle close to the angle of least deviation; this improves the quality of the spectrum. The rays then go to the mirror \(S_2\), and from it to the slit \(M\), which cuts out the narrow beam of them that falls on the mirror \(S\). After passing once more through the prism, now \(P_2\), and being reflected from the mirror \(S_4\), the rays fall on the junction of the vacuum microradiometer \(R\), placed behind the narrow slit. For adjusting the instrument by the yellow sodium line there is a sighting tube \(F\), by means of which the position of the microradiometer and of the spectrum can be observed. In order to direct the required portions of the spectrum onto the microradiometer junction, both prisms \(P_1\) and \(P_2\) are turned through the necessary angle by means of the hinged parallelogram \(P_1B_1B_2P_2\), rigidly connected on one side with the lever \(H\).

Fig. 1.

The mirror of the microradiometer is illuminated by the small lamp \(L\). Rays from the mirror, through a narrow long slit, fall on the photographic plate \(A\). The joint motion of the light spot from the mirror and of the photographic plate gives, as a result, a curve that characterizes the transmission of the object.

The drive, uniformly moving the photographic plate and rotating the prisms through a definite angle, is very interestingly designed. For the automatic execution of these motions Petr Nikolaevich developed a special oil pump, which he called the “Clepsydra.” Its arrangement is shown in Fig. 2. Before work begins the weight \(G\) is raised to its highest position, the steel ribbon \(T\) is wound onto the wheel \(K\), and the latter is secured. The cassette with the plate \(A\) is set in its place and fixed with the screw \(V\).

If the wheel \(K\) is released, the ribbon \(T\) unwinds and the load \(G\) begins to descend uniformly. At the same time the piston presses on the liquid, which is forced through an opening in the replaceable sleeve \(J\) into the upper cavity of the cylinder. The photographic plate descends; the edge of the wedge \(E\) presses on the end of the lever \(H\), raised by the spring \(D\), and the prisms rotate. By replacing the sleeve with another one having a different opening, the recording speed can be varied.

Fig. 2.

The most favorable speed makes it possible, on a plate 18 cm long, to record the spectrum in the interval \(0.6\div 15\) microns over the course of 25 minutes.

The drawing does not show one further device, consisting of a disk with sectors, which, being connected with the piston, descends and thereby regulates the intensity of the radiation of the Nernst pin. This was done in order to weaken the excessively strong short-wave radiation.

Placing in the path of the rays, for example, a cuvette with the liquid under investigation, one records the first curve. Repeating this process, but now without the cuvette, one obtains another curve. The ratio of the ordinates of the curves gives the transmission for each given wavelength.

A very essential part of the instrument is the vacuum microradiometer\(^4\), which differs substantially from Boys’s instrument\(^5\), first, by the use of so-called diamagnetic astasia and, second, for the first time in history, by the use of a vacuum to increase sensitivity\(^6\). The latter circumstance increased the sensitivity so much that special damping of the instrument became necessary, carried out after the death of P. N. Lebedev by K. P. Yakovlev, who used the entire setup and described it in his master’s dissertation\(^7\). The vacuum microradiometer built by P. N. Lebedev far anticipated similar proposals by foreign scientists, who managed, strange as it may seem, even to patent them.

In the totality of the described elements of the “ultrared spectrograph” we see the prototype of modern recording infrared spectrophotometers.

Thus, in this matter as well, the priority of Russian science and the merits of the Russian scientist, one of the greatest physicists in the world—P. N. Lebedev—are evident.

Unfortunately, Pyotr Nikolaevich was unable to complete all the work he had planned in infrared spectroscopy. An untimely death cut short his life in the very flowering of his talent.

In conclusion it is fitting to quote here the words of Pyotr Nikolaevich’s contemporary, Professor I. I. Borgman: “All the investigations that attracted attention and then brought Lebedev fame were carried out by Pyotr Nikolaevich in Russia, in the laboratory of Moscow University. Pyotr Nikolaevich was, in the full sense, a Russian scientist”^8.

CITED LITERATURE

1. P. N. Lebedev, Collected Works, pp. 37–53, 1913, Moscow. Previously published in Wied. Ann. 56, 1–17 (1895); ZhRFKhO (phys. part) 27 (1), 213–220 (1895).

2. P. N. Lebedev, Collected Works, pp. 303–326, 1913, Moscow. Previously published in Fiz. ob. 2, 49–60, 217–230 (1901).

3. P. N. Lebedev, Collected Works, pp. 202–206, 1913, Moscow. Previously published in ZhRFKhO (phys. part) 43 (1), 125–130 (1911) and in Phys. Zeits. 13, 465–468 (1912).

4. P. N. Lebedev, Collected Works, pp. 148–152, 1913, Moscow. Previously published in Ann. d. Phys. 9, 209–213 (1902).

5. C. V. Boys. Phil. Trans., 180, A, 149 (1889).

6. This idea was first expressed in the work “On the Double Refraction of Electric Force Lines,” Collected Works, p. 49 (see 1).

7. K. P. Yakovlev, Infrared Absorption Spectra of Certain Organic Compounds. An Experimental Study, Moscow, 1913.

8. I. I. Borgman, “Pyotr Nikolaevich Lebedev,” in New Ideas in Physics, Collection 4. Obrazovanie Publishing House, St. Petersburg, 1912.