Beyond the helium buffer: ¹²C⁻₂ rotational cooling in cold traps with H₂ as a partner gas: interaction forces and quantum dynamics

Abstract

The scattering cross-sections and corresponding rate coefficients for rotationally inelastic collisions of 12 C − 2 ( 2 + g ) with H 2 ( 1 + g ) are presented over a broad range of cold-trap temperatures. They have been calculated using quantum scattering theory that employs a new ab initio potential energy surface. The rate coefficients for the inelastic processes in the anionic partner are used to model the thermalisation dynamics of 12 C − 2 using H 2 as a buffer gas, a trap partner which is found here to be far more efficient than the typical buffer gas He and even more so than when using Ar as a partner gas. The microscopic physics underlying these findings is discussed in some detail. We additionally compute and discuss 12 C − 2 quadrupole transitions by spontaneous emission and use the newly computed rates to show that the anion's rotational levels should be in local thermal equilibrium at typical interstellar conditions.

Collisional excitation of C2H− by H2: New interaction potential and scattering calculations

“…24,25 Collisions with a He buffer gas in the trap play a major role for the preparation of the ions in specific vibrational and rotational states. However, in a recent study, Mant et al 26 consider possible cooling by H 2 instead of He as collisional cross sections by H 2 are much larger than cross sections with noble gas atoms, leading to faster thermalization times. 27 The C 2 H − -H 2 collisional data are then needed in cold physics studies and they can also 2 This is the author's peer reviewed, accepted manuscript.…”