Temperature dependence of vibrational relaxation in the very-low-energy collision regime: The ground electronic state of p-difluorobenzene prepared by stimulated emission pumping

Abstract

Stimulated emission pumping is used to selectively populate the υ″3=1 vibrational level (εvib=1258 cm−1) in the S0 state of p-difluorobenzene cooled in a supersonic free-jet expansion of argon. The time-dependent population of the υ″3=1 level, as it is depleted by collisions with the argon carrier gas, is probed using single vibronic level fluorescence. By varying the point along the expansion axis (X/D) at which state preparation and population probing are carried out, the rate coefficient for vibrational relaxation of the 31 state is measured as a function of temperature in the range 1–12 K. The temperature dependence of the rate coefficient is compared with the temperature dependence of hard-sphere (HS) and Lennard-Jones (LJ) elastic encounter rates. Comparison with data for the same collision system (p-difluorobenzene–Ar) at room temperature in both the S1 and S0 electronic states suggests that the temperature dependence of the Lennard-Jones elastic rate provides a useful means of scaling the temperature dependence of polyatomic vibrational relaxation over a wide range of temperatures, i.e., from 300 K down to near absolute zero.