Influence of vibrational non-equilibrium on the polarizability and refraction index in air: computational study

Abstract

Knowledge of optical effects accompanying hypersonic flight conditions is of critical importance to ensure reliable operation of on-board optical instrumentation. This paper presents results of calculations of scalar polarizability dependence on vibrational nonequilibrium in molecular nitrogen and oxygen. The non-equilibrium effects are evaluated on the basis of a semi-classical model for the molecular polarizability. It is shown that, depending on the vibrational and translational populations, changes in the scalar polarizability in excess of normal density and temperature scaling can be around 1%-5% at moderate gas temperatures and around 10% at high temperatures. The model was applied to predict the polarizability for non-equilibrium states in a nitrogen plasma formed by a single nanosecond pulse with different rise times.

A simple semiempirical model for the static polarizability of electronically excited atoms and molecules

“…Electric properties of atoms, molecules, and clusters, which have been widely studied over the years, [1][2][3][4] continue to attract the attention of numerous research groups involved in different areas of atomic, molecular, optics, and chemical physics as well as in relevant existing and emerging technological applications. [21,[27][28][29][30][31] This is because the electric properties (primarily, dipole polarizability and, if present, permanent dipole moment) specify the variety of optical [1,32,33] and electrical [9,[34][35][36] phenomena in molecular gases, underlie the theory of infrared and Raman vibrational spectra, [37] play a paramount role in intermolecular [11,19,23,[38][39][40][41] and electron-molecular [42][43][44] interactions, and serve as markers of thermodynamic and kinetic stability of atomic/molecular systems. [24,25,40,[45][46][47][48][49][50][51][52][53] In this respect, it is not surprising that as a result of numerous experimental and theoretical efforts, a vast body of knowledge on the electric properties of atoms, molecules, atomic and molecular clusters has been gathered to date, [6]…”

“…Indeed, electric properties of atoms and molecules in excited electronic states are much less known and less studied (both experimentally and theoretically) than their groundstate counterparts [14,24,[85][86][87][88] albeit these characteristics are of increasing interest for several distinctive issues, e.g., in the design of substances and (meta) materials with extreme nonlinear optical properties, [89][90][91] in understanding the solvatochromic shifts in electronic spectra, [86,[91][92][93] in the interpretation of resonance energy transfer processes and molecular exciton interactions, [14] in optical (mainly, laser) [16,30,94,95] and microwave [96,97] diagnostics of nonequilibrium gas.…”

“…[33,70,110] Similarly, the appreciable abundance of electronically excited species significantly affects the propagation of radio waves both in nonthermal (low-temperature) and thermal plasmas. [111,112] Specifically, the knowledge of polarizability and dipole moment variation of species in excited states in comparison with the ground states is essential for analyzing electromagnetic wave propagation in low-temperature plasma in relation to the problems of the reliable operation of onboard instrumentation during spacecraft launching and reentry, [30,112] remote sensing of atmosphere, [113,114] and robust satellite-based location tracking. [113,115] In addition, enhanced polarizability of electronically excited states governs to some extent the reaction kinetics and molecular transport in thermal and nonthermal reacting gas flows.…”

“…[21,22,88,122] Unfortunately, this promising methodology is expected to work well only for states of certain types of excitation, [22,122,129,130] with the success of TD-DFT in the reliable estimation of the molecular electric properties depending heavily on the choice of the DFT functional. [21,22,87] In light of the above, for diverse applications, especially, when constructing the complex nonequilibrium kinetic models with a large set of species, intended to precisely describe the observable optical refractivity (dielectric permittivity) [30,64,112,131] and transport properties [116][117][118][119] of nonthermal reacting gas flows, it would be highly convenient to have the simple theoretical tools that adequately predict the effect of electronic excitation on the polarizability and dipole moment (if present) of molecules and atoms. Such tools or analytical models would also be useful for critical evaluation of known and sometimes contradictory experimental and theoretical data on the electric properties of excited species.…”

A simple semiempirical model for the static polarizability of ions

“…[21,25,28,29] The accurate data on the polarizability of charged species are primarily of fundamental relevance to the interpretation of the respective spectra [18,29,[53][54][55][56] and for the calculation of ionmolecular potentials. [57][58][59] At the same time, these data are also essential for several existing and emerging applications directly based on the intrinsic properties of ions themselves (quantum computing, [18,60,61] quantum metrology, [18,60,62] and development of optical frequency standards [18,21,23,53,60,63] ) as well for a number of issues where an appreciable ionization of the gaseous medium, due to the specific physical properties of charged particles as compared with their neutral counterparts, critically affects the macroscopic characteristics (optical refractivity, [31,[64][65][66] dielectric permittivity, [69][70][71][72] electrical conductivity, [73] molecular transport properties [44,45,74,75] ) of ionized reacting gas or plasma.…”