Rotational relaxation time in free jets of He + N2 mixtures

Non-equilibrium rotational energy distributions of N2 in free jets of He + N2 (<5%) were measured by the electron beam technique. The rotational temperature was derived from the spectrum of the first negative system of N2+ induced by high energy (E ∼ 10 keV) electrons. The experimental data were compared with kinetic calculations of rotational energy in free jets based on the linear relaxation equation: dERdt = −(ER−Et)τ. The rotational relaxation times τ were derived from available transport collision integrals, evaluated with three different potential energy surfaces: HFD1, BTT and M3SV. The HFD1 surface leads to an overestimated rate of rotational relaxation. Two other surfaces agree satisfactorily with experiment, similarly as observed by Dickinson et al. for transport coefficients. To decide between BTT and M3SV, more precise measurements should be carried out over a wide temperature range.