Laser double-resonance mesurements of rotational relaxation rates of HF( J=13) with rare gases, H2, and D2

Rotational relaxation rates for HF(v=0, J=13) colliding with rare gases (He, Ne, Ar, Kr, Xe), H2, and D2 are measured using a transient-absorption double-resonance technique. The relaxation rate constants with rare gases (rotation-to-translation, R–T) decrease dramatically through the series He–Ne–Ar, then increase substantially through the series Ar–Kr–Xe, revealing the increased effectiveness of either highly impulsive or highly attractive collisions. Rate constants at 298 K are: (He), 1.3±0.2×10−11; (Ne), 2.2±0.4×10−12; (Ar), 9.3±0.9×10−13; (Kr), 4.4±0.5×10−12; (Xe), 6.2±0.6×10−12 cm3 molecule−1 s−1. The fact that the rate constants with hydrogen (1.1±0.2×10−10 cm3 molecule−1 s−1) and deuterium (1.2±0.2×10−10 cm3/molecule−1 s−1) are ten times faster than with helium underscores the importance of rotation-to-rotation (R–R) transfer in the rotational relaxation process. The decrease in rates from He to Ar can be understood by an impulsive-type model, analogous to the Schwartz, Slawsky, and Herzfeld (SSH) treatment of vibration-to-translation transfer. However, the increasing trend from Ar to Xe, while clearly due to the increasing magnitude of the attractive interaction, is not easily modeled.