Low Energy Collision Induced Vibrational Relaxation in B 3Pi0u+ Iodine

Andrew Boyd Rock B.Sc. (Hons)

Faculty of Science and Technology, Griffith University

February 1996

Understanding energy transfer processes is an essential prerequisite for the deep understanding of all chemical processes. This thesis investigates the process of vibrational relaxation (or deexcitation) of highly vibrationally and electronically excited molecular iodine (I2) induced by very low energy collisions in a supersonic free jet with six foreign gases. In an investigation of the state-to-field relaxation of I2 (B 3Pi0u+, u = 16) induced by collisions with He at temperatures of 2 to 12 K we find that the absolute relaxation rates are an order of magnitude smaller than those at 300 K and that the explanation of the magnitudes of these rates does not require enhancement due to low energy orbiting resonances. We find that the rates scale well with estimated collision encounter rates that account for the attractive part of the intermolecular potential. A second investigation with a much wider scope explores vibrational relaxation from u = 21 to 24 with six foreign gases: He, Ne, Ar, H2, D2 and N2. For this investigation a new type of experimental procedure has been designed and implemented that records a detailed and complete map of the fluorescence from B 3Pi0u+ I2 that is resolved with respect to both fluorescence frequency and time. These not only yield state-to-field rates, but coupled with a novel deconvolution method for growth curve fitting, yield absolute state-to-state rates for vibrational relaxation processes with delta u = -1, -2, -3 and -4. The dependence of the relaxation rates on the collision partner, temperature and delta u are discussed. An exponential dependence on the vibrational energy gap may be adequate to characterise the delta u dependence of vibrational relaxation. The frequency resolution of the experimental data also reveals that some of the energy released by vibrational de-excitation is transferred to the rotation of the I2 molecule. We find this process is best characterised by an exponential dependence on the change of I2 angular momentum and that its extent scales with the mass of the collision partner. Measurements of the low-energy collision-induced quenching of B 3Pi0u+ I2 are also reported for all six foreign gases. The possibility arises from the rates that the mechanism for quenching by H2 and D2 at low temperatures is different to that of the other gases and to that for H2 and D2 at high temperatures.