Dynamics of rovibrational energy transfer from excited molecular iodine: New crossed beam studies

Rovibrational inelastic scattering has been studied for the collisions between helium and excited molecular iodine (I{dollar}sb2{dollar}) in a crossed beam apparatus. I{dollar}sb2{dollar} was initially prepared in two vibrational states, {dollar}upsilonprime{dollar} = 15 and 35, in the B O{dollar}sb{lcub}rm u{rcub}sp +{dollar} 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 {dollar}vert Deltaupsilonvert{dollar} = 6 in {dollar}upsilonprime{dollar} = 35 and {dollar}vert Deltaupsilonvert{dollar} = 3 in {dollar}upsilonprime{dollar} = 15 were observed. Nearly 200 relative vibrational state-to-state inelastic scattering cross sections were measured.; At each Ecm, all the cross sections for both {dollar}upsilonprime{dollar} = 15 and 35 can be fitted by a single exponential function {dollar}sigma propto{dollar} exp(-{dollar}vert Deltarm Evert/beta{dollar}). At high Ecm, {dollar}betasb{lcub}rm Vto T{rcub}{dollar} is equal to {dollar}betasb{lcub}rm Tto V{rcub}{dollar}. At low ECM, {dollar}betasb{lcub}rm Vto T{rcub}{dollar} is larger than {dollar}betasb{lcub}rm Tto V{rcub}{dollar}. However, all {dollar}beta{dollar}'s are linear functions of Ecm. Also the cross sections for the {dollar}Deltaupsilon{dollar} = {dollar}pm{dollar}1 scattering are nearly independent of Ecm. Considering that the collisions are not adiabatic, these results are not consistent with the well-known Landau-Teller 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 {dollar}to{dollar} T but not for T {dollar}to{dollar} V transitions. The calculation also shows that the bulb energy transfer is mainly induced by collisions with velocities {dollar}sim{dollar}2 times larger than the most probable velocity.; From the cross sections, mean energy transfer per vibrationally inelastic collision, {dollar}langle Deltarm Erangle{dollar}, was also obtained. The results show that {dollar}langle Deltarm Erangle{dollar} increases linearly with Ecm and levels off to near-zero at high collision energy. At low Ecm, {dollar}langle Deltarm Erangle{dollar} in {dollar}upsilonprime{dollar} = 15 is larger than that in {dollar}upsilonprime{dollar} = 35.; The average rotational energy transfered increases almost linearly with Ecm but is small, only {dollar}sim{dollar}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 {dollar}vert Deltarm Evert{dollar} dependence of the cross sections is a result of 3-dimensional averaging.