The vibrational deactivation of CO(v=1) by inelastic collisions with H2 and D2

Abstract

Calculations of the relaxation rate constants, kCO–H2, for the vibrational deactivation of CO(v=1) by pH2 and oH2 are reported in the temperature range 30 K<T<300 K. The CO rotation is treated using the infinite-order sudden (IOS) approximation, while the rotation of H2 is included using the coupled states (CS) approximation. A near-resonant energy transfer process, in which the H2 molecule is rotationally excited from J=2 to J=6 on relaxation of CO(v=1), is found to account for the experimental observation that kCO–pH2/kCO–oH2>1 for this system at temperatures above 80 K. Evidence is presented to suggest that below this temperature, which represents the current lower limit of existing experimental data for the CO(v=1)-pH2 system, thermal depopulation of the J=2 rotational state in pH2 reduces the importance of the near-resonant energy transfer process in the determination of kCO–pH2. For T≪80 K the ratio kCO–pH2/kCO–oH2<1 is predicted on the basis of these calculations. At impact energies less than 60 cm−1, the relaxation cross sections increase at a rate which is insufficient to account for the observed upturn in the experimentally determined deactivation rate constants for the CO–nH2 system below 60 K. Rate constants for the deactivation of CO(v=1) by oD2 and pD2 have also been calculated and compared with experimental data.