The vibrational deactivation of CO(v=1) by inelastic collisions with H-2 and D-2

Calculations of the relaxation rate constants, k(CO-H2), for the vibrational deactivation of CO(upsilon = 1) by pH(2) and oH(2) 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 H-2 is included using the coupled states (CS) approximation. A near-resonant energy transfer process, in which the H-2 molecule is rotationally excited from J = 2 to J = 6 on relaxation of CO(upsilon = 1), is found to account for the experimental observation that k(CO-pH2)/k(CO-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(upsilon = 1)-pH(2) system, thermal depopulation of the J = 2 rotational state in pH(2) reduces the importance of the near-resonant energy transfer process in the determination of k(CO-pH2). For T much less than 80 K the ratio k(CO-pH2)/k(CO-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-nH(2) system below 60 K. Rate constants for the deactivation of CO(upsilon = 1) by oD(2) and pD(2) have also been calculated and compared with experimental data. (C) 1997 American Institute of Physics.