Vibrational relaxation of small molecules in the liquid phase: Liquid nitrogen doped with CO2, CD4, or N2O

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Abstract
The vibrational relaxation times of liquid nitrogen doped with CO2, CD4, or N2O at 77 K were measured. Stimulated Raman scattering was used to excite N2 into the ν=1 level and its concentration as a function of time was monitored by spontaneous anti‐Stokes Raman scattering. For these solutions rapid V–V energy transfer occurs between N2 (ν=1) and the near resonant vibrational levels of the solute molecules. On a longer time scale is the relaxation of the vibrationally excited solute molecule by V–T, R energy transfer. Comparison of the nitrogen solution data for V–T,R relaxation of CO2 by N2 and N2O self‐relaxation with gas phase results is consistent with the suggestion that isolated binary collisions are responsible for vibrational relaxation in the liquid state. Therefore, for these systems as far as vibrational relaxation processes are concerned the liquid may be treated simply as a high density gas. No direct comparisons between liquid state and gas phase measurements were possible for V–T,R relaxation of vibrationally excited N2O or CD4 by N2. However, extrapolations from available data suggest that there may be a change in the vibrational relaxation mechanisms for these and other systems when the phase is changed from gas to liquid.