Dynamics of Rovibrational Energy Transfer from Excited Molecular  Crossed Beam Studies
Abstract
Rovibrational inelastic scattering has been studied for the collisions between helium and excited molecular iodine (I_2) in a crossed beam apparatus. I _2 was initially prepared in two vibrational states, upsilon' = 15 and 35, in the B O_{rm u }^ + electronic state. Dispersed single vibrational level fluorescence spectra revealed the vibrational inelastic scattering. The collision energy (Ecm) was varied from 35meV to 190meV. Vibrational state changes up to  Deltaupsilon = 6 in upsilon' = 35 and  Deltaupsilon = 3 in upsilon' = 15 were observed. Nearly 200 relative vibrational statetostate inelastic scattering cross sections were measured. At each Ecm, all the cross sections for both upsilon' = 15 and 35 can be fitted by a single exponential function sigma ~ exp( Delta rm E/beta). At high Ecm, beta_{rm Vto T} is equal to beta_{rm Tto V}. At low ECM, beta _{rm Vto T} is larger than beta_{rm Tto V}. However, all beta's are linear functions of Ecm. Also the cross sections for the Deltaupsilon = +/1 scattering are nearly independent of Ecm. Considering that the collisions are not adiabatic, these results are not consistent with the wellknown LandauTeller theory. Using the empirical dependence of the cross sections on Ecm, we calculated the thermal rate constants. The calculation at 300K agrees with the bulb experiment for V to T but not for T to V transitions. The calculation also shows that the bulb energy transfer is mainly induced by collisions with velocities ~2 times larger than the most probable velocity. From the cross sections, mean energy transfer per vibrationally inelastic collision, < Deltarm E>, was also obtained. The results show that < Deltarm E> increases linearly with Ecm and levels off to nearzero at high collision energy. At low Ecm, < Deltarm E> in upsilon' = 15 is larger than that in upsilon' = 35. The average rotational energy transfered increases almost linearly with Ecm but is small, only ~ 2% of the Ecm. This is a direct result of angular momentum conservation. Classical trajectory calculations were carried out with an available program. Using an exponential interaction potential, the calculated and experimental cross sections agree fairly well. The calculations reveal that the exponential  Deltarm E dependence of the cross sections is a result of 3dimensional averaging.
 Publication:

Ph.D. Thesis
 Pub Date:
 January 1990
 Bibcode:
 1990PhDT.......137D
 Keywords:

 ENERGY TRANSFER;
 Chemistry: Physical; Physics: Molecular