Millimeter-wave time-resolved studies of HCO+-H2 inelastic collisions.

A novel ion cell has been constructed for the purpose of studying rotationally inelastic collisions involving truly thermal molecular ions at low temperatures. With this ion cell, time-resolved double resonance (pump-probe) spectroscopic experiments have been performed to determine the cross sections for relaxation of the J = 2 state of HCO+ in collisions with normal-H2 at temperatures around 40 and 77 K. The HCO+ is pumped through the J = 2<--1 transition and probed via the J = 3<--2 transition. The cross sections at the lower temperature are slightly below those predicted by the simple Langevin theory, while those at the higher temperature are in good agreement with this theory.