Vibrational energy transfer between I2 and He at very low temperatures: impulsive vs complex formation mechanisms assisted by tunnelling through the centrifugal barrier

IVÁN CABANILLAS-VIDOSA; CARLOS A. RINALDI; GUSTAVO A. PINO; JUAN C. FERRERO

JOURNAL OF CHEMICAL PHYSICS

American Institute of Physics

Año: 2008 vol. 129 p. 1443031 - 1443039

0021-9606

The temperature dependence of the state-to-state (s-t-s) vibrational relaxation rate constant  (kv21-Dn)  for collisions between I2(B,n?=21) and He at very low kinetic energies was studied. The fluorescence from I2(B,n?=21) with Dn= (1-5) indicates that in the temperature range 0.6 - 8.2 K these states are populated by only one collision with He. The behavior of kv21-Dn with temperature can be divided into two groups. The group with quantum changes Dn= (1-3) shows scattering resonances in the low temperature region, with a general monotonical decrease of the rate constant with temperature, suggesting the importance of van der Waals interactions. This behavior is supported by the calculation of the probability of tunneling through the centrifugal barriers. For collisions in which 4 to 5 quanta are lost in a single event, there are no evidences of scattering resonances and the values of the relaxation rate constants could be determined only at the highest temperatures of this study. This suggests that relaxation occurs via impulsive collisions. The branching ratios for each channel are also temperature dependent and this behavior also suggests that the energy transfer mechanism changes with Dn`.