Theory of vibrational relaxation in the inverting molecules, NH3 and ND3

Abstract

The exceptionally rapid vibrational energy relaxation in ammonia has led to the speculation that the inversion motion is responsible for the higher rate of collisional energy exchange than in related molecules. Exact numerical solution of the scattering equations has been carried out utilizing an atom–atom exponential repulsive potential for one dimensional collisions, with emphasis upon the comparison of models whose internal energy is described by (a) an ordinary harmonic oscillator (noninverting model) and (b) the states that accurately describe the vibrational motion for double minimum potential (inverting model). Although the transition probabilities for the 0+, 0− and 1+, 1− states are high at thermal energies, such transitions do not make a large contribution to the relaxation time found from acoustic experiments. The speed of the observed processes is attributed principally to (a) the ’’acceleration’’ effect which is a consequence of the intermolecular potential of polar molecules and (b) the high impact velocities of rotating molecules with low moments of inertia.