Vibrational relaxation of small molecules in the liquid phase: Liquid nitrogen doped with H2 or D2

Abstract

Experimental results, using a pulsed laser technique to study the vibrational relaxation of nitrogen in solutions with H₂ or D₂ show that the system N₂–H₂ relaxes 100 times faster than N₂–D₂. The rate constants are k_N2–H2= (1.5±0.2) ×10⁵ and k_N2–D2 = (1.8±0.4) ×10³ s⁻¹ (mol fraction)⁻¹. From the experimental rate constants, transition probabilities 〈P〉_N2–H2=9.0×10⁻⁹ and 〈P〉_N2–D2=1.5×10⁻¹⁰ were calculated using a simple liquid model. This model treats the liquid as a dense gas and assumes that isolated binary collisions are causing the vibrational relaxation. Theoretical calculations of transition probabilities based on the first order distorted wave approximation were used to estimate the importance of V–T, V–T,R, or V–V channels to the relaxation of vibrationally excited nitrogen in the presence of H₂ or D₂. These calculations favor the V–T channel over the V–T,R channel for both N₂–H₂ and N₂–D₂ systems at 77 K. Relaxation of N₂ by H₂ is more effective than by D₂ because collisions of the lighter isotopes occur with higher velocities. Relaxation by the V–V channel is unimportant in these systems. Our conclusions are consistent with those of others who have shown that the V–T channel is primarily responsible for vibrational relaxation for similar systems, CO–H₂ and CO–H₂ in the gas phase at low temperatures.