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

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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.