Methylation effects in state-resolved quenching of highly vibrationally excited azabenzenes $(Evib∼38 500 cm−1).$ II. Collisions with carbon dioxide

To explore the role of molecular structure in collisions that relax highly excited polyatomic molecules, we have studied collisional deactivation of a series of highly vibrationally excited methylated pyridines $(Evib∼38 500 cm−1)$ in a $CO2$ bath. Complementary studies that investigated quenching by $H2O$ have been presented in Part I of this series [M. S. Elioff, M. Fang, and A. S. Mullin, J. Chem. Phys. 115, 6990 (2001)]. We have used high-resolution transient infrared absorption probing to measure rotational and translational energy gain in individual quantum levels of $CO2 (0000)$ that are populated via collisions with vibrationally excited picoline (2-methylpyridine) and lutidine (2,6-dimethylpyridine). Vibrationally excited picoline and lutidine were prepared by absorption of pulsed $λ=266 nm$ light and fast internal decay to the ground electronic state. The nascent distribution of $CO2 (0000)$ rotational states was measured for $J=60–78.$ Translational energy gain distributions were determined for the $J=60–80$ states of $CO2 (0000)$ using Doppler-broadened linewidth measurements. Energy transfer probabilities were determined by measuring absolute energy transfer rate constants for energy gain into specific $CO2$ quantum states. These results are compared to previous single-collision energy transfer studies on hot pyridine [M. C. Wall, B. Stewart, and A. S. Mullin, J. Chem. Phys. 108, 9658 (1998)] and hot pyrazine [M. C. Wall and A. S. Mullin, J. Chem. Phys. 108, 9658 (1998)] initially excited with 266 nm light and quenched via collisions with $CO2.$ We find that donor methylation reduces the amount of translational and rotational energy imparted to the $CO2 (0000)$ high-J states, but that the cross section for exciting the high-J states of $CO2 (0000)$ increases upon donor methylation. Fermi’s golden rule is used to describe the relaxation process, and the energy transfer distribution functions for $ΔE>4000 cm−1$ are found to correlate remarkably well to the energy dependence of the density of states of the hot donor molecule. This analysis is also successfully applied to earlier quenching data for vibrationally excited $C6F6$ [C. A. Michaels et al., J. Chem. Phys. 106, 7055 (1997)], suggesting that this may be a general approach for describing relaxation of highly excited molecules.