Vibrational relaxation of the CH(D)(X2Π, v=1, 2) radical by helium and argon: evidence for a curve crossing mechanism

Vibrational relaxation of the CH and CD (v=2, 1) by the noble gases helium and argon has been monitored by laser-induced fluorescence following generation by pulsed laser photolysis of bromoform. Vibrational to translational relaxation is comparatively fast (k=0.5–6.8×10−14 cm3 molecule−1 s−1). Neither the isotopic dependence (kCH>kCD), or the variation with bath gas (kAr>kHe), is compatible with the impulsive mechanism for V–T energy transfer developed by Schwartz, Slawsky and Herzfeld (SSH). A semi-quantitative explanation of the data has been developed based on the ab initio CH–Ar potential energy surfaces calculated by Alexander et al. [J. Chem. Phys. 101 (1994) 4547].