Vibrational energy transfer between I2(BΠu+3,ν′=21) and He at very low temperatures: Impulsive versus complex formation mechanisms assisted by tunneling through the centrifugal barrier

Abstract

The temperature dependence of the state-to-state vibrational relaxation rate constant (k(nu)(21-Delta nu)) for collisions between I(2)(B,nu(')=21) and He at very low kinetic energies was studied. The fluorescence from I(2)(B,nu(')=21-Delta nu(')) with Delta nu(')=1-5 indicates that in the temperature range of 0.6-8.2 K these states are populated by only one collision with He. The behavior of k(nu)(21-Delta nu) with temperature can be divided into two groups. The group with quantum changes Delta nu(')=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 4-5 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 Delta nu(').

The role of orbiting resonances in the vibrational relaxation of I2(B,v′ = 21) by collisions with He at very low energies: a theoretical and experimental study

“…2 (with an increasing energy shifting) the averaged peak positions remain similar seems to be a strong indication that including the contributions of low J values still will keep a structure of peaks with similar positions to those found for J = 0. In this work we show that only j 0 o 8 are populated in the experiment, and in addition, in previous papers 30,31 it was reported that the enhancement of the collisional cross-section at the low temperatures involved in the experiment is due mainly to collisions with orbital momenta with l = 2-5. Therefore, relatively low J values are expected to contribute in the experiment.…”

“…Theoretical total cross sections and partial cross sections for the Dv 0 = 0 inelastic channel Before presenting and discussing the results a comment is due. In the experimental cross sections for the I 2 (B,v 0 = 21,j 0 ) + He collisions 30,31 important contributions of total angular momenta J 4 0 are expected. Thus, the cross sections obtained from the present simulations, which are restricted to J = 0, cannot be directly compared to the experimental cross sections, since the intensities of the calculated cross sections are not converged due to the lack of J 4 0 contributions.…”

“…Details of the experimental setup have been described in previous papers. 30,31 Briefly, a gaseous mixture He : I 2 (0.999 : 0.001) was expanded through a 500 micrometre diameter (d) nozzle of a pulsed valve at a stagnation pressure of 760 Torr, into a vacuum chamber. The jet was excited by a dye laser tuned to the (B;v 0 = 21 ' X;v = 0) transition of I 2 at different distances from the nozzle (x = 20, 18, 16, 14, and from 11 to 3 mm, in 1 mm steps) to explore different collision energies regions.…”

“…It is more recently that measured cross sections for I 2 (B,v 0 = 21) vibrational relaxation upon low temperature collisions with He that exhibit clear peaks at given collision energies have been reported for the first time. 30,31 The peaks of the cross section were attributed to orbiting resonances of the vdW I 2 (B,v 0 = 21) À He complex formed upon the low energy collisions. It is noted that (direct or indirect) evidence of the presence of orbiting resonances in vdW complexes has been found in spectroscopic experiments for the Br 2 -Ne, 32 ICl-He, 33 Br 2 -He, 34 and I 2 -He 35 complexes, and in time resolved experiments for the Br 2 -Ne complex.…”

“…30 Although the spectrum of vdW resonances in a triatomic complex typically changes slowly with increasing vibrational state v, the states v = 59 and 60 are rather far from the v = 21 state of I 2 (B)ÀHe probed experimentally. 30,31 It would be desirable to get a closer comparison between the theoretical and the experimental results, and this is the aim of the present work. To that purpose, the I 2 (B,v 0 = 21) + He low energy collisions have been simulated by means of a wave packet method, and the cross sections corresponding to the collisional channels of interest have been calculated.…”