Rotational relaxation in diatomic gas at high temperature observed with Brillouin scattering spectroscopy

Abstract

It has recently become possible to observe Brillouin scattering spectra of gases at high temperatures as well as their rotational–translational (R–T) relaxation at room temperatures with optical beating Brillouin scattering spectroscopy. In this study, the sound velocity in nitrogen gas was measured at high temperature and a R–T relaxation in the frequency region of 100 MHz was observed. The obtained relaxation frequency value was found to be experimentally reasonable when compared to theoretical values. To our knowledge, optical beating Brillouin spectroscopy is the only method used to observe the total change in sound velocity in R–T relaxation at high temperature. Employing the spectroscopy allowed for a discussion of the temperature and frequency dependence on mechanical properties such as the specific ratio; that is, the internal energy degrees of freedom of the molecules.

A simple measurement method of molecular relaxation in a gas by reconstructing acoustic velocity dispersion

“…For single relaxation process, when ωτ = 1, namely f e = ω e /2π (ω e = 1/τ ) is inversely proportional to the relaxation time, the effect of molecular relaxation on acoustic propagation is the strongest. In equation (11), setting ωτ = 1, we can get…”

“…Thus, f e is the frequency corresponding to the mean value of dispersion and the frequency corresponding to inflection point of an S-shaped curve, which is defined as the relaxation frequency. (11). Thus, the relaxation strength is the normalized ratio of dispersion.…”

“…Nevertheless, many multi-relaxation processes in gases have one primary (single) relaxation process. In equation (11), the dispersion of a single relaxation process is represented by three parameters V 2 s (∞), ε and τ . Accordingly, the three parameters can be obtained using three-frequency measurements of acoustic velocity in practice.…”

“…Simple and real-time measurements of molecular relaxation in gases are important in many applications, such as determining gas compositions [1][2][3][4][5], investigating the dissociation mechanism in plasma discharges [6,7] and analyzing the rate of molecular energy exchange in physical chemistry [8][9][10], etc. Compared with optical technologies [11,12], a gas-coupled or airborne ultrasonic transducer [13,14] is simple and cost-effective for measuring the molecular relaxation properties of gases. In traditional acoustic measurement methods for molecular relaxation, the constant frequency-pressure variation technique [15][16][17][18] is used due to the fact that commercial transducers have fixed resonance frequencies.…”

An analytical model for Rayleigh–Brillouin scattering spectra in gases

“…Rayleigh-Brillouin scattering (RBS) has been widely used in aerospace and atmospheric remote sensing applications, i.e. the measurement of air temperature profiles and wind measurements [1][2][3][4][5][6]. In these applications, parameters such as gas temperature are obtained by comparing experimental spectrum data with RBS line shape models.…”