Ro-vibrational quenching calculations of C2- in collision with H2
Abstract
The molecular anion C2- has been of interest in the last few years as a candidate for laser cooling due to its electronic structure and favourable branching ratios to the ground electronic and vibrational state. Molecular hydrogen has been used by the Wester group in Innsbruck as a buffer gas to cool the molecule's internal ro-vibrational motion. In the present work, we generate a new, five dimensional (5D) interaction potential for the system by considering the H2 as a rigid rotor and the C2- as a rotating-vibrating diatomic molecule. We thereafter calculate the cross sections and rate coefficients for ro-vibrational inelastic collisions of C2- with both para- and ortho-H2 on this new 5D ab initio potential energy surface using quantum scattering theory for the dynamics. The rates for vibrational quenching are obtained over the range of temperatures which covers the single value measured by the experiments. A comparison is also made with the earlier results using a simpler 3D interaction potential. Furthermore, para-H2 is found to be more efficient than ortho-H2 (with or without undergoing rotational excitation) in cooling C2-. The rate coefficients for cooling the anions has been computed by appropriately weighting the ortho- and para-H2 and compared with the available experimental result at 20 K. When the vibrational de-excitation rate coefficients are taken to be the ones not causing any concurrent rotational excitations in the final C2- anions, the properly averaged results are found to get smaller and to become very close to the experimental measurements. The implications of these new results for laser cooling of C2- are analyzed and discussed.
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