The Laboratory was set up in 1970, first as a sector that was later transformed into a laboratory. It first chief was Dr. R.V. Ambartzumian, and since 1984 it has been headed by Prof. E.A. Ryabov. At the present time (2003) its permanent staff includes ten researchers. In addition, there are usually a few students and post graduates working at the Laboratory. Since the very beginning, the main field of activity of the Laboratory has been the study of excited states in molecules. This has largely been due to the need to develop methods of selective laser action on molecules, specifically laser isotope separation techniques. The development of the molecular approach in laser isotope separation (MLIS) and the associated fundamental investigations on the spectroscopy of excited states in molecules have long been the main lines of inquiry at the Laboratory

These works have always been performed in close collaboration with the Laboratory of Laser Spectroscopy.

Investigations under the MLIS program have from the very beginning been oriented towards the use of vibrational transitions for selective excitation of molecules. This choice was conditioned by the essential individuality of the IR spectra of various molecules and the substantial isotope shift manifest in these spectra. The development of this approach led to the development of two most promising MLIS techniques. The first of them is based on the two-step IR-UV dissociation of molecules. It was for the first time implemented in the experiment on the laser separation of nitrogen isotopes (Fig. 1) upon dissociation of NH 3 (R.V. Ambartzumian, V.S. Letokhov, G.N. Makarov, A.A. Puretzky, 1972) [1].

Fig. 1.

Left: schematic diagram of isotope-selective two-step photodissociation of ammonia molecules. The molecules selectively excited by IR laser radiation are dissociated by UV radiation. Right: results of laser separation of nitrogen isotopes. Mass spectra of N 2 at: ( а ) nonselective photodissociation of the ¹⁴NH₃+¹⁵NH₃ mixture (1:1 ratio) and ( b) selective (as to ¹⁵NH₃) two-step photodissociation of the mixture. The shaded lines correspond to the mass spectra of the mixture prior to irradiation (background lines).

Practically simultaneously with the first method, work was started on developing the second approach based on the IR multiple-photon dissociation (IR MPD) of molecules. The effect of isotope selectivity of the IR MPD technique was for the first time demonstrated (Fig. 2) in the experiments with the ¹⁰BCl₃ and ¹¹BCl₃ molecules (R.V. Ambartzumian, V.S. Letokhov, E.A. Ryabov, N.V. Chekalin, 1974) [2]. Immediately thereafter experiments were conducted on the macroscopic enrichment of SF 6 gas in sulfur isotopes (Fig. 3) upon its IR MPD [3].

Fig. 2.

Observation of isotope selectivity of the IR MPD of molecules: the chemiluminescence of the BO * radical when irradiating with a СО₂ laser a mixture of ¹⁰BCl₃ and ¹¹BCl₃ in the presence of oxygen. Shown at the top are model spectra of the ¹⁰ВО and ¹¹ВО radicals taken separately.

Fig. 3.

Enrichment of SF₆ gas with ³⁴SF₆ and ³⁶SF₆ upon the IR MPD of ³²SF₆ molecules by СО₂ -laser radiation, observed from their IR absorption spectra: ( а ) IR spectrum prior to irradiation; ( b ) after irradiation.

Following the first successful demonstrations, a great cycle of investigations was conducted into the selective laser excitation of molecules. Special attention was paid to the study of the mechanism of the IR MP excitation (IR MPE) of molecules, as well as the reasons behind the isotope selectivity of this process. The results obtained were reflected in a number of monographs and reviews [4–8]. At the same time, work was being performed in collaboration with a number of organizations (TRINITI, Scientific Research Institute of Stable Isotopes, D.V. Efremov NIIEFA) on the scaling of the process based on IR MPD. All this made it possible to develop an industrial process for laser separation of carbon isotopes on the basis of the IR MPD of CF₂ HCl molecules by a pulsed СО₂ -laser radiation. This process was used by the “Gas-Oil” Company at their pilot plant in the city of Kaliningrad for the production of the ¹³С isotope [9]. The block diagram of one of the modules of this plant is presented in Fig. 4.

Рис . 4.

Block diagram of one of the modules of the plant for the laser separation of carbon isotopes. The module consists of a high pulse repetition rate СО₂ laser (average power up to 1.8 kW), a separation reactor; an enriched product extraction unit; and a unit converting the enriched product into the final product – ¹³СО₂.

The output of such a module is around a gram of 13 С per hour, with enrichment from the natural abundance of 1.108% to 40-50%. If necessary, further enrichment to 99% can be achieved both by the traditional methods and by using a second laser separation stage. At present work is being carried out within the framework of this program on the improvement of the existing separation process and the development of processes for separating isotopes of other elements (Dr. V.B. Laptev and others).

Along with the study of the isotope-selective IR MP excitation and dissociation of molecules, a research program was accomplished at the Laboratory on the spectroscopy of vibrational states in the vicinity of and above the dissociation limit. Also studied were inverse electronic relaxation processes and the unimolecular decay of molecules. Specifically the shape of IR absorption lines was for the first time measured at the Laboratory for molecules overexcited far (up to two times) above the dissociation limit [10]. Some results of these investigations were summarized in the collective monographs [11, 12].

Thereafter fundamental investigations at the Laboratory were conducted along the following main lines.

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1. Spectroscopy of Excited States of Molecules in Molecular Beams (Dr. G.N. Makarov, Prof. E.A. Ryabov, and others).

Using the double IR-IR resonance technique, a cycle of works was accomplished on the study of IR MPE in a discrete spectrum region below the quasicontinuum boundary, which allowed us to make substantial progress in the understanding of this process [13].

A number of new methods were suggested for controlling the parameters of intense pulsed molecular beams, such as the pulse duration and the kinetic energy of the molecules in the beams. An entirely new method was developed for producing accelerated neutral pulsed molecular beams with IR-laser-radiation-controllable kinetic energy. The effect is based on the IR MP excitation of molecules and subsequent transfer of the vibrational excitation energy to translational degrees of freedom. This method makes it possible to obtain molecular beams with a kinetic energy in the range from 0.1-0.2 eV to 2-3 eV [14, 15]. This effect also allows one to obtain beams of cold radicals.

Also studied were the processes of multiple-photon ionization (MPI) and fragmentation of molecules and radicals under the effect of intense UV radiation. Using the UV MPI effect, investigations were conducted into the UV absorption spectra of highly vibrationally excited (~15000 cm –1 ) SF₆ molecules and unimolecular decay kinetics of a number of molecules and the velocity distribution of the radicals formed in the process [16, 17].

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2. Problems Associated with the Formation of Selectivity in the IR MPE Process

The investigations carried out at the Laboratory substantially bettered the understanding of the mechanisms responsible for the formation of selectivity in the IR MPE process. In particular, this made it possible to attain record-high isotope selectivity values (~10 4 ) in the IR MPD process (Dr. A.A. Puretzky and others) [18]. Under study at the present time is the isotope selectivity of the IR MPD process under gas-dynamic cooling conditions in molecular beams [17], those interacting with a solid surface included [19]. In the latter case, the loss of the radicals formed as a result of IR MPD can be substantially reduced.

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3. Molecular Dynamics and Vibrational Energy Exchange (Prof. E.A. Ryabov and others).

Using time-resolved Raman spectroscopy techniques, we studied energy redistribution in highly excited vibrational states of polyatomic molecules. It was demonstrated that the intramolecular vibrational relaxation (IVR) resulted from stochastization of vibrational motion produces the equilibrium energy redistribution over all vibrational modes even when only one of them is subjected to a resonant excitation. Stochastization was found to have an energy boundary (Fig. 5), and the energy values corresponding to this boundary were measured for the first time for a number of molecules (Chap. 2 in [11, 12] and [20]).

It was also found [21] that intramolecular interactions can substantially accelerate (by a few orders of magnitude) the rate of collisional vibrational relaxation in polyatomic molecules. In a number of cases, vibrational relaxation takes up but a few collisions to end [22]. This effect was theoretically interpreted (Dr. A.A. Makarov) [22].

Fig. 5.

Investigation into vibrational energy stochastization in polyatomic molecules. Top: time-resolved Raman scattering probing technique. Bottom: Average energy E of the molecules subject to stochastization as a function of pumping. The stationary E values reached at low pumping values correspond to the stochastization boundary.

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4. Spectroscopy of Vibrational Quasicontinuum (Prof. E.A. Ryabov, Dr. A.A. Makarov, and others).

A cycle of investigations was performed at the Laboratory with a view to studying the spectral properties (the shape, position, and intensity of IR and Raman bands) of the vibrational quasicontinuum of polyatomic molecules. Using XY 6 -type spherical tops as an example, it was demonstrated that the main part in the formation of the shape of IR transition and Raman bands is played by inhomogeneous statistical broadening and homogeneous broadening. The former leads to a Gaussian band shape and the latter, to a Lorentzian band shape. A theory was developed (Dr. A.A. Makarov and others) for calculating the spectra of vibrational transitions in polyatomic molecules when the dominant role is played by statistical inhomogeneous broadening [23]. The calculations performed demonstrated good agreement with experiment [24, 25]. A method was developed for determining the magnitude of homogeneous broadening. Using the SF₆ molecule as an example, it was measured within a wide range of energies [26]. Based on these results, a theoretical model was developed to quantitatively describe the IR MP excitation and dissociation processes [16].

Fundamental investigations are now being carried out at the Laboratory along the following main lines.

•  Research into intramolecular dynamics by picosecond laser spectroscopy techniques. We plan to change over to femtosecond-long laser pulses in the near future.

•  Spectroscopy of molecular clusters, including studies into the multiple-photon processes induced by UV and IR laser radiation.

•  Interaction between beams of vibrationally excited molecules and a cold surface.

Besides, in connection with the development of tunable sources of pico- and femtosecond-long radiation pulses in the middle IR region, investigations are being carried out into the optical and nonlinear properties of nonlinear crystals from the chalcogenide family.

Over the course of the past years, five doctoral and fifteen Ph.D. dissertations were written and defended at the Laboratory.

Five monographs were published on the basis of the investigations conducted at the Laboratory.

Monographs
Of the Laboratory of the Spectroscopy of Excited States in Molecules

1. V.N. Bagratashvili, V.S. Letokhov, A.A. Makarov, E.A. Ryabov. Multiple-Photon Processes in Molecules in an Infrared Laser Field (VINITI, Moscow, 1981).
2. E.P. Velikhov, V.Yu. Baranov, V.S. Letokhov, E.A. Ryabov, A.P. Starostin. Pulsed СО₂ Lasers and their Application in Isotope Separation (Nauka, Moscow, 1983).
3. V.N. Bagratashvili, V.S. Letokhov, A.A. Makarov, E.A. Ryabov. Multiple-Photon Infrared Laser Photophysics and Photochemistry (Harwood Acad. Publ., Chur, 1985).
4. Laser Spectroscopy of Highly Vibrationally Excited Molecules, ed. by V.S. Letokhov (Adam-Hilger, Bristol, 1989).
5. Laser Spectroscopy of Vibrationally Excited Molecules, ed. By V.S. Letokhov (Nauka, Moscow, 1990).

References

1. R.V. Ambartzumian, V.S. Letokhov, G.N. Makarov, A.A. Puretzky. Sov. Phys. JETP Lett. 17 , 63, (1973).
2. R.V. Ambartzumian, V.S. Letokhov, E.A. Ryabov, N.V. Chekalin. Sov. Phys. JETP Lett. 20 , 273, (1974).
3. R.V. Ambartzumian, Yu.A. Gorokhov, V.S. Letokhov, G.N. Makarov. Sov. Phys. JETP Lett. 21 , 171, (1975).
4. V.N. Bagratashvili, V.S. Letokhov, A.A. Makarov, E.A. Ryabov. Multiple-Photon Processes in Molecules in an Infrared Laser Field (VINITI, Moscow, 1981).
5. E.P. Velikhov, V.Yu. Baranov, V.S. Letokhov, E.A. Ryabov, A.P. Starostin. Pulsed СО 2 Lasers and their Application in Isotope Separation (Nauka, Moscow, Москва , 1983).
6. V.N. Bagratashvili, V.S. Letokhov, A.A. Makarov, E.A. Ryabov. Multiple-Photon Infrared Laser Photophysics and Photochemistry (Harwood Acad. Publ., Chur, 1985).
7. V.S. Letokhov, E.A. Ryabov, in: V.Yu. Baranov, ed. Isotopes (IzdAT, Moscow , 2000), Sect. 7.3.
8. V.S. Letokhov, E.A. Ryabov. Laser Isotope Separation, in: Optics Encyclopedia ( Wiley-VCH , Berlin , 2003).
9. V.Yu. Baranov, A.P. Dyad'kin, D.D. Malyuta, V.A. Kuzmenko, S.V. Pigulski, V.S. Mezhevov, V.S. Letokhov, V.B. Laptev, E.A. Ryabov, I.Yu. Yarovoi, V.B. Zarin, A.S. Podoryashy. Proceedings of SPIE 4165 , 314–323 (2000).
10. V.N. Bagratashvili, S.I. Ionov, V.S. Letokhov, V.N. Lokhman, G.N. Makarov, A.A. Stuchebryukhov. Pis'ma v ZhETF 44 , 450 (1986).
11. Laser Spectroscopy of Highly Vibrationally Excited Molecules , ed. By V.S. Letokhov (Adam-Hilger, Bristol, 1989).
12. Laser Spectroscopy of Highly Vibrationally Excited Molecules , ed. By V.S. Letokhov (Nauka, Moscow, 1990).
13. Yu.A. Kuritsyn, G.N. Makarov, I. Pak, M.V. Sotnikov. ZhETF 94 , 65 (1988).
14. V.N. Apatin, G.N. Makarov, V.V. Nesterov. Pis'ma v ZhETF 73 , 735 (2001).
15. G.N. Makarov. UFN 173, No. 9 (2003).
16. V.N. Lokhman, A.A. Makarov, D.D. Ogurok, E.A. Ryabov, V.S. Letokhov. Chem. Phys. 286 , 385 (2003).
17. V.N. Lokhman, D.D. Ogurok, E.A. Ryabov. Chem. Phys. 271 , 357 (2001).
18. A.V. Evseev, V.S. Letokhov, A.A. Puretzky. Appl. Phys. 36B , №2, 93 (1985).
19. G.N. Makarov, A.N. Petin. ZhETF 119 , 5 (2000).
20. A.L. Malinovsky, V.S. Letokhov, E.A. Ryabov. Chem. Phys. 139 , No. 1, 229 (1989).
21. А . А . Kosterev, A.L. Malinovsky, E.A. Ryabov. Pis'ma v ZhETF 54 , 16 (1991).
22. A.A. Kosterev, A.A. Makarov, A.L. Malinovsky, E.A. Ryabov. Chem. Phys. 219 , 305 (1997).
23. A.A. Makarov, I.Yu. Petrova, E.A. Ryabov, V.S. Letokhov. J. Phys. Chem. 102 , 1438 (1998).
24. A.L. Malinovsky, I.Yu. Petrova, A.A. Makarov, E.A. Ryabov, V.S. Letokhov. J. Phys. Chem. 102A , 9353 (1998).
25. A.A. Kosterev, A.A. Makarov, A.L. Malinovsky, I.Yu. Petrova, E.A. Ryabov, V.S. Letokhov. J. Phys. Chem. 104A , 10259 (2000).
26. V.N. Lokhman, A.A. Makarov, I.Yu. Petrova, E.A. Ryabov, V.S. Letokhov. J. Phys. Chem. 103A , 11299 (1999).