**DENSITY EVOLUTION OF THE ps TIME-DOMAIN CARS RESPONSES FROM THE
CO_{2} 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 *CO _{2}* 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

The fast-collision approximation [1] and the HITRAN *CO _{2}* 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

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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