Modeling of the dynamics of the change in the index of refraction during resonance interaction of radiation with vibrational-rotational transitions

Abstract

It is well known that resonance absorption of radiation by a gas can lead to excitation of molecular internal degrees of freedom, and this determines, to a significant degree, the nonlinear response of the medium. The dielectric permittivity of the medium has both imaginary and real parts. The imaginary part is associated with the absorption coefficient tq, and the real part is associated with the index of refraction n. A change of n in the beam channel causes the light rays to be deflected from the initial direction and determines, to a significant degree, the character of laser propagation in a nonlinear medium [1]. This is why there are so many works on the mechanisms responsible for the change in the index of refraction under the action of resonance radiation. Special attention is devoted to the analysis of the mechanism of the change in n during propagation of an IR pulse (radiation in this region of the spectrum is usually absorbed on vibrational-rotational transitions) [2][3][4][5][6][7]. This is because it is with IR lasers that the high intensities at which nonlinear effects become very significant are achieved.It has been shown that the main mechanisms responsible for the change in n are the change in molecular polarizability of the medium due to excitation of molecular vibrations and the change in the density of the medium due to hydrodynamic effects produced by the nonuniform heat transfer from vibrational into translational degrees of freedom. Hydrodynamic effects have been analyzed primarily within the model of a nonviscous thermally nonconducting gas, neglecting the influence of diffusion and heat conduction, which in a vibrationally nonequilibrium gas depends significantly on the degree of excitation and can significantly influence the behavior of the concentrations of the mixture components even over times shorter than the characteristic times of these processes [8]. It is thus of interest to make a comprehensive analysis of the mechanisms of the change in the index of refraction of a mixture of gases, taking into account all processes associated with the excitation of molecular vibrations by resonance radiation. This paper is devoted to such an analysis.The analysis is performed for a two-component mixture of gases, consisting of molecules of different types, for example A (1) and B (2). The molecules of type A have at least two different types of vibrations k and q with frequencies u k < Uq, while type-B molecules have vibrations of one type s with frequencies u s _< Uq or u s _> Uq. Let the vibrationaltranslational relaxational time for the mode q be much longer than the vibrational-vibrational exchange times Uq ---u k and Uq --, u s, and let the frequency u I of the acting radiation be in resonance with the frequency at the line center of the vibrationalrotational transition m --, n, whose upper n and lower m states are vibrations of the types k and q, respectively: ~, = (~v. -Ev, + ~. -~,)/hwhere E v. and E v, are the vibrational energies of the excited states n and m of a type-A molecule, E...