VIBRATIONAL AND ROTATIONAL RELAXATION OF IODINE IN SEEDED SUPERSONIC BEAMS

Laser-induced fluorescence has been used to measure the vibration-rotation state distributions of molecular iodine in seeded beams expanded from a supersonic nozzle. Four monatomic, four diatomic, and eight polyatomic gases were used as diluents. For an 0.017-cm nozzle operated at 300 K and total pressures from 30 to 1000 torr, the observed vibration-rotation distributions (for v ″ = 0-3 and J″ ≃ 0-50) were always found to have a Boltzmann form, characterized by temperatures of Tvib ≃ 300-45 K and Trot ≃ 150-3 K. The functional dependence of Trot on the total source pressure is governed by the specific heat ratio for the diluent gas and nearly coincides with a simple model for the terminal translational temperature of the expansion. The observed Tvib data are shown to require cross sections for vibrational relaxation which remain constant or increase with decreasing temperature. This cannot be reconciled with the usual impulsive Landau-Teller model; at low temperatures the discrepancy is several orders of magnitude. Relative values for the vibrational relaxation cross sections for different diluent gases were determined by using a series of binary gas mixtures composed of approximately 700 torr of helium and 0-50 torr of another diluent gas. The results show the relaxation cross sections vary by more than a factor of 300 and increase markedly in the order atoms < diatomic molecules < polyatomic molecules. The very efficient vibrational relaxation may be a consequence of the low relative translational energy (typically, Ttrans = 30 K) achieved in the expansion. This fosters formation of metastable or van der Waals complexes, allowing multiple I2⋯M collisions which can strongly enhance the vibrational relaxation. © 1979 American Chemical Society.