Vibrational relaxation of water molecules in H2O+Ar collisions between 200 and 1000 K: 001→020, 020→010, and 010→000 transitions

Abstract

The vibrational relaxation of the stretching levels of H2O in the H2O+Ar collision is studied using a model of vibration-to-rotation (VR) energy transfer in the temperature range of 200–1000 K. The dominant relaxation pathway from the rapidly equilibrating ν1 and ν3 levels is deexcitation to the 2ν2 level followed by the 020→010 and 010→000 relaxation processes. At 300 K, the probabilities of 001→020, 020→010, and 010→000 are 2.2×10−3, 2.6×10−3, and 1.4×10−3, respectively. In the model the energy release ΔE of each deexcitation process is preferentially removed by rotation. Transition probabilities calculated by a semiclassical procedure indicate that when ΔE is large, the VR mechanism is of major importance at lower temperatures. The VR model correctly predicts both the temperature dependence and magnitude of the 020→010 probability over the temperature range of 200–1000 K. A model which assumes the removal of the energy release by translation seriously underestimates the probability of this process, especially at lower temperatures, and predicts a temperature dependence which is too steep. The contribution of translational motion to the relaxation process becomes important at higher collision energies, especially for 001→020, where ΔE is small. Calculations are extended to the D2O+Ar collision in order to study isotope effects.