Vibrational relaxation of water vapor

The method of laser-excited vibrational fluorescence is used to measure the vibrational relaxation rates of H2 18O in collisions with H2 18O, Ar, N2, and O2. Pumping of various vibration–rotation lines of the ν1 and ν3 stretching levels with a tunable infrared laser leads to the same measured rate constants. In pure H2 18O the stretching levels are found to relax together with a rate constant of 2.2×10−11 cm3 molecule−1 sec−1, the bend overtone relaxes with a rate constant of 9.1×10−11 cm3 molecule−1 sec−1, and the bend fundamental relaxes with a rate constant between 5×10−11 and 9×10−11 cm3 molecule−1 sec−1. Relative intensity measurements show that the dominant relaxation pathway is V→V transfer from the stretching vibrations to the v2=2 bending level followed by stepwise V→T, R relaxation to v2=1 and then v2=0.