Beyond rare gases: Cooling of trapped BN− ions by H2 molecules

Quantum dynamics of collisional cooling of the BN− anion in a cold trap is carried out by using the ortho- and para-H2 molecules as a buffer gas. The calculations are performed using the full four-dimensional interaction potential energy surface in which both BN− and H2 are treated as vibrationless rigid rotors, computed from ab initio methods and further accurately fitted using an artificial neural network approach. Relevant state-to-state rotationally inelastic cross sections are computed from quantum methods, and the ensuing inelastic rate coefficients for the temperatures of interest are obtained. The latter are employed in modeling the cooling process at a variety of trap conditions. The results are discussed in the context of earlier data obtained using the rare gases He and Ar as buffer gases in the same trap. The effects of having either para- or ortho-H2 as a collision partner are analyzed.