Theoretical calculations for line-broadening and pressure-shifting in the ν 1 + ν 2 + ν 4 + ν 5 band of acetylene over a range of temperatures

Abstract

Semiclassical calculations of self-broadening and self-induced pressure shift coefficients in the ν 1 + ν 2 + ν 4 + ν 5 band of C2H2 have been performed by considering, in addition to the main electrostatic quadrupole–quadrupole interaction, a simple anisotropic dispersion contribution, leading to results in overall satisfactory agreement with recent measurements [C. Povey, A. Predoi-Cross and D.R. Hurtmans, J. Mol. Spectrosc., 268, 177 (2011)]. In these calculations we have used the mean relative velocity and also considered the Maxwell–Boltzmann distribution of relative velocities. From the theoretical results obtained at different temperatures ranging from 200 to 350 K, we have determined temperature exponents of the broadenings using a simple power law, as well as coefficients of empirical linear and quadratic temperature dependences for the line shifts. These theoretical exponents and linear coefficients, derived from averaging over the distribution of velocities and from the mean thermal velocity, are significantly different and they are compared with those obtained from measurements of broadening coefficients and line shifts performed in a comparable temperature range [C. Povey, A. Predoi-Cross and D.R. Hurtmans, J. Mol. Spectrosc., 268, 177 (2011)]. The theoretical variation of the self-shifts with temperature is not linear and can be well fitted by a quadratic polynomial.