On the exponential energy gap law in He–I2 vibrational relaxation

Abstract

A comparison between coupled states, infinite order sudden, and classical path calculations is used to elucidate the origin of an exponential energy gap law recently observed for vibrational relaxation from highly excited states in the B 0+u state of I2 due to collisions with He. All three methods provide relaxation cross sections in good agreement with experiment. Anharmonic effects play an important role, with accurate results obtained with a Morse, but not harmonic, oscillator description of the I*2 molecule. The nearly exact agreement between rotationally summed coupled states cross sections and the IOSA is consistent with the view that the I*2 molecule does not rotate significantly during a collision. A closed form solution of the forced harmonic oscillator, valid for highly excited states, predicts a J2‖Δv‖ distribution of vibrationally relaxed states at a given collision angle and impact parameter. The vibrationally close coupled-infinite order sudden (VCC-IOSA) results bear this out and show that the observed exponential scaling law arises from a superposition of such distributions over θ and b.