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-H-2 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. (C) 2001 Elsevier Science B.V. All rights reserved.