Vibrational relaxation of CO by collisions with 4He at ultracold temperatures

Quantum mechanical coupled channel scattering calculations are performed for the ro-vibrational relaxation of CO in collisions with ultracold He atoms. The van der Waals well in the interaction potential supports a number of shape resonances which significantly influence the relaxation cross sections at energies less than the well depth. Feshbach resonances are also found to occur near channel thresholds corresponding to the j = 1 rotational level in the v = 0 and v = 1 vibrational levels. Their existence influences dramatically the limiting values of the elastic scattering cross sections and the rotational quenching rate coefficients from the j = 1 level. We present complex scattering lengths for several low lying rotational levels of CO which characterize both elastic and inelastic collisions in the limit of zero temperature. Our results for the vibrational relaxation of CO (v = 1) are in good agreement with available experimental and theoretical results. (C) 2000 American Institute of Physics. [S0021-9606(00)01526-9].