Laboratory of Excited Molecular States Spectroscopy

    The laboratory was set up in 1970 and its first head was Prof. R.V. Ambartzumyan. Since 1984 Prof. E.A. Ryabov is in charge of the laboratory which has 12 researchers with 3 groups in the laboratory (Prof. G.N. Makarov, Prof. A.A. Puretsky, Prof. E.A. Ryabov) which carry out experiments on several directions of excited molecular states spectroscopy (1993).
     From the very beginning the main field of research in the laboratory was he spectroscopy of excited vibrational and electronic states in polyatomic molecules. It was caused mainly by necessity of development of new methods of selective laser action on molecules, in particular, creation of laser separation methods. The development of Molecular approach to Laser Isotope Separation (MLIS) and relevant basic researchers on the spectroscopy of excited states of molecules were the main line of the work in the laboratory for a long time. All the time these research works were done together with the laser spectroscopy laboratory.
    The MLIS program was always orientated towards using vibrational transitions for selective excitation of molecules because of essential individuality of IR spectrum of different molecules and display of considerable isotope shift in it. The development of this approach led to the creation of two most promising MLIS methods. The first method is based on two-step IR-UV dissociation of molecules. For the first time it was demonstrated in experiments on laser separation on nitrogen isotopes (Fig. 19) at dissociation of NH3 (R.V. Ambartzumyan, V.S. Letokhov, G.N. Makarov, A.A. Puretsky, 1972) [1].

Fig.19.

    On the left: The scheme of isotope-selective two-step photodissociation of NH3 molecules. Molecules selectively excited by IR laser radiation are dissociated by UV radiation.
    On the right: The results of laser separation of nitrogen isotopes. Mass-spectra of N2 at:
a) non-selective photodissociation of the mixture 14NH3+15NH3 (in ratio 1:1) and
b) selective (in regard to 15NH3) two step photodissociation of the mixture. Shaded lines correspond to mass-spactra of the mixture before the irradiation (background lines).
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    Approximately at the same time the second method based on IR multiple photon dissociation (MPD) started to be developed. The first successful demonstration of isotopic selectivity of IR MPD was carried out (Fig. 20) in the experiment with 10BCl3 and 11BCl3 molecules (R.V. Ambartzumyan, V.S. Letokhov, E.A. Ryabov, N.V. Chekalin, 1974) [2]. Just after this experiment the macroscopic enrichment of sulfur isotopes (Fig. 21) was accomplished by IR MPD of SF6 [3].

Fig.20.

    The observation of isotopic selectivity of IR MP dissociation of molecules: chemiluminescence of radical BO* at the irradiation of the mixture 10BCl and 11BCl by CO2 laser at the presence of oxygen.
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Fig.21.

    The enrichment of the gas SF6 by 34SF6 and 36SF6 isotopes after IR MPD of 32SF6 molecules by the CO2 laser radiation observed in spectra of IR-absorption:
a) IR-spectrum before the irradiation;
b) IR-spectrum after the irradiation.
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    After first successful demonstrations a number of studies of selective laser excitation of molecules were done. Particular attention was given to the study of mechanism of IR multiphoton excitation (MPE) of molecules and to causes of isotope selectivity of that process. At the same time research works on large scale realization of the separation process on the basis of IR MPD were done together with other institutes. Finally the technology of laser separation of carbon isotopes was worked out and made ready for industrial realization. At present isotope separation of other elements is developed within the framework of this program.
    Besides the study of isotope selective IR MPE and MPD, the research program on the spectroscopy of vibrational states close to the dissociation limit and above it was carried out in the laboratory. In particular, processes of inverse electron relaxation and dynamics of unimolecular decay of molecules were studied. Together with Center of Technological Laser of Russian Academy of Sciences the method was developed, and the shape of IR absorption band (Fig. 22) for molecules of strongly overexcited (up to the double value) above the dissociation limit was measured [4].

Fig.22.

    The spectrum of linear IR absorption of (CF3)3Cl molecules with value of vibrational energy E=35000 cm-1. On the right: the spectrum of IR absorption of unexcited molecules at T=300 K is shown.
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    During of previous decade main directions of  basic researchers of the laboratory are as follows:
 1. Spectroscopy of excited vibrational states of molecules in the region of discrete spectrum below the onset of quasi-continuum (group of Prof. G.N. Makarov).
 The technique of double IR-IR resonance in a molecular beam is used. These investigations are connected with the problem of the excitation of molecules in the system of lower vibrational levels at their IR MPE, which is not solved yet. The group managed to advance in understanding of this process and clear up the effect of the width of laser pulse spectrum, of its shape and its detuning on the efficiency of IR MPE [5].
 2. Mechanisms of IR MPE selectivity (group of Prof. A.A. Puretsky).
 The researchers done made it more clear the mechanism of interaction of intensive IR laser radiation with polyatomic molecules. It allowed in its turn  to reach a record value of isotopic selectivity of IR MPD > 104 [6] at rather small values of isotope shift. At present the possibility of realization not only intermolecular (molecules of different sorts) but also intra-molecular selectivity is studied. Here we mean the possibility to achieve mode-selective or regio-selective excitation and dissociation of molecules.
 3. Intra-molecular dynamics and vibrational energy exchange (group of Prof. E.A. Ryabov).
    The redistribution of energy in highly excitated vibrational states of main electron term of polyatomic molecules is investigated by methods of time-resolved Raman spectroscopy. It was shown that the stochastization of vibrational motion resulted in statistical energy distribution over all the modes of the molecule even at resonance pumping of one of them. It was proved the existence of energy threshold of stochastization (Fig. 23) and its value was measured for a number of molecules [7]. At present the peculiarities of collisional V-V exchange for highly excited vibrational states are investigated, [9].
    At present, basic research is being conducted along two main avenues of inquiry. The first is the spectroscopy of the vibrational quasicontinuum. A theory was developed (Dr. A.A. Makarov and others) for calculating the spectra of IR transitions of polyatomic molecules in the quasicontinuum in the case where the statistically inhomogeneous broadening is predominant [10]. The measurements taken demonstrated its good agreement with experiment.
    Based on these results, the theory is being developed of the IR MP excitation of molecules. The second is the continuation of investigations into intermolecular dynamics and the solution of the problem of realization of bond-group selectivity (regio-selectivity) of IR excitation and dissociation of molecules. One of the lines of investigation here is the search for site-selectivity in extended chain-like molecules (A.V. Dem yanenko and others) [11].

Fig.23.

    The investigation of the stochastization of vibrational energy in polyatomic molecules. Top: the method of time resolved Raman probing. Bottom: the dependence of average energy E of  stochastized  molecules on the CO pump. The stationary value of E at low values of pumping corresponds to the threshold of the stochastization.
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    Four thesis for a Doctor of Science degree and 12 thesis for a Ph.D. degree were prepared in the laboratory. Four monographs are published.

    Monographs of Laboratory of Excited Molecular States Spectroscopy

1. V.N. Bagratashvili, V.S. Letokhov, A.A. Makarov,  E.A. Ryabov.  Multiphoton processes in molecules in an IR laser field . (VINITI, Moscow, 1981) (in Russian).
2. E.P. Velikhov, V.Yu. Baranov, V.S. Letokhov. E.A. Ryabov, A.P. Starostin.  Pulsed CO2-lasers and their application for isotope separation . (Nauka Publ., Moscow, 1993) (in Russian).
3. V.N. Bagratashvili, V.S. Letokhov, A.A. Makarov, E.A. Ryabov.  Multiple photon infrared laser photophysics and photochemistry . (Harwood Academic Publ., Chur, 1985).
4. a)  Laser spectroscopy of highly vibrationally excited molecules . Ed. by V.S. Letokhov (Adam Hilger, Bristol, 1989).
b) ibid. (Nauka Publ., Moscow, 1990) (in Russian).

References

1. R.V. Ambartzumyan, V.S. Letokhov, G.N. Makarov, A.A. Puretzkii. Laser Separation of Nitrogen Isotopes. Pis ma v ZhETF. 17, 91 (1973); [JETP Lett. 17, 63 (1973)].
2. R.V. Ambartzumyan, V.S. Letokhov, E.A. Ryabov, N.V. Chekalin. Isotope-selective Chemical Reaction of BCl3 in a Strong IR Laser Field. Pis ma v ZhETF 20, 597 (1974); [JETP Lett. 20, 273 (1974)].
3. R.V. Ambartzumyan, Yu.A. Gorokhov, V.S. Letokhov, G.N. Makarov. Separation of sulfur isotopes with an enrichment ratio higher than 103 by acting on the SF6 molecule with CO2-laser radiation. Pis ma v ZhETF 21, 375 (1975); [JETP Lett. 21, 171 (1975)].
4. V.N. Bagratashvili, S.I. Ionov, V.S. Letokhov, V.N. Lokhman, A.A. Makarov, A.A. Stuchebrukhov. Homogeneous vibrational transition spectrum and intramolecular relaxation time of high excited polyatomic molecule. Pis ma v ZhETF 44, 450 (1986).
5. V.N. Lokhman, G.N. Makarov. Pis ma v ZhETF 66, 172 (1995).
6. A.V. Evseev, V.S. Letokhov, A.A. Puretzky. Highly selective and efficient multiphoton dissociation of polyatomic molecules. Appl. Phys. 36B, No. 2, 93 (1985).
7. A.L. Malinovsky, V.S. Letokhov, E.A. Ryabov. Laser time-resolved Raman spectroscopy of mode selectivity and vibrational energy distribution for IR MP excited polyatimic molecules. Chem. Phys. 139, No. 1, 229 (1989).
8. A.A. Kosterev, A.L. Malinovsky, E.A. Ryabov. Pis ma v ZhETF. 54, 16 (1991).
9. A.A. Kosterev, A.A. Makarov, A.L. Malinovsky, E.A. Ryabov. Chem. Phys. 219, 305 (1997).
10. A.A. Makarov, I.Yu. Petrova, E.A. Ryabov, V.S. Letokhov. Statitstical Inhomogeneous broadening of infrared and Raman transitions in highly vibrationally excited SF6 molecule. J. Phys. Chem. 102, 1438 (1998).
11. A. Demianenko, V. Letokhov, A. Makarov, E. Ryabov. Multiphoton IR Laser-Induced Unimolecular Decay of (CF3)3CBr Molecule. Faraday Discuss. 102, 301 (1995).