DENSITY EVOLUTION OF THE ps TIME-DOMAIN CARS RESPONSES FROM THE CO2 GAS.

A. KOUZOV, V.MOROSOV*, S. MOCHALOV*, A. OLENIN*, V. TUNKIN*

* Department of Physics, Moscow State University, Moscow 119992, Russia

We report the results of a joint experimental and theoretical study of the vibrational CARS transients excited in the room-temperature CO2 gas under the binary-collision regime (r =0.0006-18 Amagat). Two overlapping pump pulses, each of the 30 ps duration, were produced by a dye laser and by the second harmonic of a Nd:YAG laser. To study the CARS coherence decay in a wide dynamic range (up to 8 decades), a delayed probe pulse (third harmonic of the Nd:YAG laser) of the same duration was scanned from zero time up to 12 ns. Isotropic CARS signals recorded from the Fermi-dyad components at 1285 and 1388 cm-1 were analyzed to deduce the relaxation parameters.

The fast-collision approximation [1] and the HITRAN CO2 data [2] were used in a FORTRAN code to simulate the responses. Besides, we extended the theory [1] on the case of excited bending states without an introduction of additional model parameters. The simulations undertaken account for the finite pump/probe pulse durations as well as for the collision-perturbed Doppler effect, the vibrational dephasing and the collisional mixing of rotational states. The velocity relaxation was approximated by an exponential decay with a known diffusion constant [3]. The relaxation parameters were deduced by nonlinear least-squares fitting. Some fits of the 1285 cm-1 signals are displayed below.
The Russian Foundation for Basic Research (projects 01-02-16044 and 01-03-32744) is thanked for the support of this study.

References
[1] A.P. Kouzov, Phys. Rev., A60, 2931 (1999).
[2] L.S. Rothman, Applied Optics, 25, 1795 (1986).
[3] D.S. Kuznetsov, V.B. Morozov, A.N. Olenin, and V.G. Tunkin, Chem. Phys., 257, 117 (2000).





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