Inelastic collisions in molecular nitrogen at low temperature (2<T<50 K)

Abstract

Theory and experiment are combined in a novel approach aimed at establishing a set of two-body state-to-state rates for elementary processes ij->lm in low temperature N2:N2 collisions involving the rotational states i, j, l, m. First, a set of 148 collision cross sections is calculated as a function of the collision energy at the converged close-coupled level via the MOLSCAT code, using a recent potential energy surface for N2–N2. Then, the corresponding rates for the range of 2<T<50 K are derived from the cross sections. The link between theory and experiment, aimed at assessing the calculated rates, is a master equation which accounts for the time evolution of rotational populations in a reference volume of gas in terms of the collision rates. In the experiment, the evolution of rotational populations is measured by Raman spectroscopy in a tiny reference volume 2E-3 mm3 of N2 traveling along the axis of a supersonic jet. The calculated collisional rates are assessed experimentally in the range of 4<T<35 K by means of the master equation, and then are scaled by averaging over a large set of experimental data. The scaled rates account accurately for the evolution of the rotational populations measured in a wide range of conditions. Accuracy of 10% is estimated for the main scaled rates.