Very low temperature (0.6 ? 6.0) K vibrational relaxation of with He: evidence for orbiting resonance enhancement

I. CABANILLAS-VIDOSA; G. A. PINO; C. A. RINALDI; J. C. FERRERO

Orleans - Francia

Congreso; 19th International Symposium on Gas Kinetics; 2006

The question of low temperature vibrational relaxation enhancement has been a topic of considerable interest over the last decades.[1] As early as 1979 McClelland et al.[2] inferred that efficient vibrational relaxation of I2 (X1S+g), due to collisions with He in a supersonic expansion, persisted down to very low collision energies, in apparent conflict with the predictions of  repulsive models of vibrational relaxations. Rice et al. corroborated this observation for I2 (B3P+0u) and developed a theory that ascribed the mechanism of vibrational relaxation at low translational temperatures to non-classical features of the scattering process.[3] In this work will show results on the state-to-state (STS) and state-to-field (STF) vibrational relaxation of I2(B, v?=21) with He at very low temperature (0.6 -6.0)K, studied in a jet expansion by means of  disperse and time resolved LIF spectroscopy. All the results that will be presented have been determined under single collision conditions.   From the experimental results several characteristics are pointed out:   1-   At the lowest collisional temperatures, only transitions with Dn = -1,-2 are observed as previously determined from dissociation of I2-He van der Waals complexes,[4] evidencing the influence of the short range attractive forces.   2-   At higher collisional temperatures, transitions from states generated by Dn up to -4 are detected under single collision conditions, showing the importance of the repulsive wall at higher collisional energies.   3-   At each temperature, the relaxation probability vs Dn can be satisfactory explained by the SSH-T theory.[5]   4-   A maximum of the STF rate constant is observed at around 1.2 K. This effect was assumed to be due to orbiting resonances at very low collision energies with orbital angular momentum l = 4 or 5.3     References [1] A. Boyd Rock ? PhD. Thesis ? Griffith University (1996) and references there in. [2] G. M. McClelland, K. L. Saenger, J. J. Valenini and D. R. Herschbach. J. Phys. Chem. 83, 947 (1979) [3] C. Cerjan and S. A. Rice, J. Chem. Phys.  78, 4952 (1983) [4] D. Levy, Adv. Chem Phys. XLVII, 323 (Part 1) (1981) [5] R. R. Waclawik and W. D. Lawrance, J.Chem.Phys.102, 2780 (1994)