On the Refractive Index of a Gas under High-Thermal-Nonequilibrium Conditions
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Journal of Engineering Physics and Thermophysics, Vol. 93, No. 4, July, 2020
KINETIC THEORY OF TRANSFER PROCESSES ON THE REFRACTIVE INDEX OF A GAS UNDER HIGH-THERMAL-NONEQUILIBRIUM CONDITIONS B. I. Lukhovitskii,a,b A. S. Sharipov,a,b I. V. Arsent′ev,a V. V. Kuzmitskii,c and O. G. Penyazkovc
UDC 535.327
An analysis has been made of the influence of the excitation of internal degrees of freedom of molecules on the refractive index (n – 1) under the conditions of high thermal nonequilibrium using the problem on relaxation of molecular nitrogen and oxygen behind the front of a strong shock wave as an example. It has been shown that the processes of vibrational relaxation and the processes of electronic translational relaxation and dissociation alike may exert a substantial influence on the refractive index of the gas. The change in the refractive index has been shown to occur primarily because of the change in the gas density in the course of these processes. Also, it has been found that a slight but detectable influence (about 1–2% for n – 1) on the refractive index is exerted by the change in the polarizability of the gas as a result of the change in vibrational temperatures of the mixture′s molecules and due to the excitation of electronic states and the dissociation of the starting components. Keywords: thermal nonequilibrium, molecular vibrations, electronically excited states, polarizability, refractive index, Gladstone–Dale constant, optical diagnostics of flows, oxygen, nitrogen. Introduction. It is common knowledge that the excitation of internal degrees of freedom of molecules (vibrational, rotational, and electronic) may substantially influence their electric and optical properties (polarizability, hyperpolarizability, dipole and quadrupole moments, and others) [1–4]. Such excitation is implemented both in thermally equilibrium heating of a gas to high temperatures and under substantially nonequilibrium conditions occurring behind strong shock waves (SWs), in divergent jets of high-temperature gas, in the upper and middle atmospheres of various planets, in the course of intense chemical reactions, in electric discharges of certain type, or in absorbing powerful laser radiation by a gas [4–12]. As far as the mechanism of influence of the excitation on electric properties is concerned, it is fundamentally different for vibrational (in addition to rotational) and electronic degrees of freedom. Thus, in exciting vibrational and rotational degrees of freedom, this influence is caused by the fact that the observed values of electric properties in an assigned vibrational (rotational) state are determined by the averaging of the electric characteristic over a relevant vibrational (rotational) wave function [2, 13]. Moreover, even zero vibrations may exert a pronounced influence on the dipole moment and the polarizability of molecules [14]. The influence of electronic excitation on electric properties is in turn manifested in populating more diffuse and higher lying molecular orbitals and hence in improved respons
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