Reactions of Atoms and Radicals in Pulsed Molecular Beams

Abstract

The final report describes studies of unimolecular reactions of transient species and radicals relevant to combustion processes. Specifically, the dynamics of predissociation of free radicals for which multiple pathways, including molecular rearrangements, compete. These small, prototypical, systems are amenable to treatment by high level theory, and close collaboration with theory continues to be a cornerstone of the program. The chemistry of hydroxyalkyl radicals is important in atmospheric and combustion environments, because cleavage of the C-H and O-H bonds is implicated in the reactions of several atoms and radicals with alcohols and alkanes. In particular, the hydroxymethyl radical affords many opportunities to study both isomerization and dissociation on the ground and excited potential energy surfaces. During this funding period, high vibrational levels of the ground electronic state of the hydroxymethyl radicals were accessed in two ways: (1) by using internal conversion from the lowest excited electronic state, the 3s Rydberg state; and (2) via direct vibrational excitation of the hydroxymethyl radical accessed by pumping OH overtones as ''bright'' states. In the former method, levels that are {approx} 3 eV above the H + formaldehyde asymptote are reached. In the latter, the region of the dissociation barrier is gradually approached from below, while examining the role of energy flow from OH overtones to other vibrational levels. The first task was to characterize the electronic absorption of the radical, in order to develop diagnostics and reach the ground state via excitation of the lowest-lying state, the 3s Rydberg state. To this end, excitations to the three lowest 3s and 3p Rydberg states were studied and the dissociation channels identified. These were predominantly O-H and C-H fission, while isomerization to the methoxy radical was not observed. Collaborations with theory identified the molecular dynamics on the excited electronic potential energy surfaces and how the ground state is reached via nonadiabatic transitions. Establishing the relevant dissociation and isomerization pathways on the ground electronic state requires studying energy flow patterns with increasing internal energy. Two energy regimes corresponding to distinct unimolecular dynamics were reached: (1) low energy spectroscopic studies of IR transitions in the region of the fundamental and first overtone of the OH-stretch, where intramolecular dynamics is likely to be weak and first manifested; (2) Studies of the region near the dissociation barrier, i.e. the third OH-stretch overtone, where dissociation occurs mostly by tunneling. The experimental approach relies on pulsed molecular beams, state-selective reactant preparation and product detection, and good resolution in both frequency and velocity domains. Radicals are prepared in molecular beams by using photoinitiated bimolecular reactions in a quartz tube attachment to the pulsed valve. Excitation is achieved using either overtone pumping of OH stretch vibrations, or excitation to an electrically excited state followed by radiationless transitions. Detection relies on time-of-flight methods with core sampling, which provide kinetic energy and angular distribution.