| Rotational energy transfer is an important process in a variety of astrophysical environments including the interstellar medium, photo-dissociation regions, and cool stellar atmospheres. The knowledge of the rates of rotational energy transfer in collisions at low temperatures is required to understand the relative intensities of rotational transitions observed in emission from dense interstellar clouds. Specifically, it is necessary to know the rate coefficients for energy transfer between different rotational levels of CO in collisions with the major interstellar species such as H2, H, and He in order to estimate the radiative cooling rate as applied to the cooling mechanism in the interstellar medium.; Accurate quantum mechanical calculations were carried out to investigate the excitation and de-excitations of rotational levels in a hetero-nuclear molecule at low temperatures by taking the He + CO system as an illustrative example. Collisions between CO molecule in its electronic ground state and He atom provide an ideal test case for comparisons between experiment and theory, because the potential energy surface that governs the collision dynamics has been calculated accurately. The collision dynamics calculations were performed by solving the time-independent Schrodinger equation employing the close coupling method. The rate constants for rotational energy transfer were obtained by Boltzmann averaging the corresponding cross sections. The calculations were performed to investigate the collision energy dependence of total removal of cross sections for rotational levels of ji = 0.....12 in the ground vibrational state (nu = 0) of CO and ji = 0, 1, 4, and 6 in the excited nu = 2 vibrational level of CO. It is found that 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. The computed results are compared with available experimental and theoretical data. |
