Millimeter-wave time-resolved studies of HCO+-H-2 inelastic collisions

A novel ion cell has been constructed for the purpose of studying rotationa lly inelastic collisions involving truly thermal molecular ions at low temp eratures. With this ion cell, time-resolved double resonance (pump-probe) s pectroscopic experiments have been performed to determine the cross section s for relaxation of the J=2 state of HCO+ in collisions with normal-H, at t emperatures around 40 and 77 K. The HCO+ is pumped through the J=2 <--1 tra nsition and probed via the J=3 <--2 transition. The cross sections at the l ower temperature are slightly below those predicted by the simple Langevin theory, while those at the higher temperature are in good agreement with th is theory. (C) 2001 Elsevier Science B.V. All rights reserved.