Vibrational relaxation of CO(n=1) in collisions with He

We present a full ab initio study of vibrational relaxation of CO(n=1) in collisions with 3He and 4He. The vibrational coordinate dependence of the interaction potential is calculated directly. Dynamical calculations are performed within the IOS and the CS approximation. The IOS cross sections agree very well with the corresponding CS results for both He isotopes, however, the ratio IOS/CS shows a different energy behavior for 3He and 4He, respectively. The CS rates for 3He agree extremely well with the experimental relaxation rates. Although only the reduced mass is changed for 4He the agreement with experiment is less favorable in this case. It is explicitly demonstrated that the often used procedure to extract the vibrational coordinate dependence of the interaction potential from a dumbbell fit of the rigid-rotor surface is wrong for He–CO. For 3He the corresponding cross sections are 3–10 times smaller than those obtained form the ab initio potential although the vibrationally elastic parts of the interaction potential are identical.