Collisional quenching of electronically excited hydroxyl A2sigma+ radicals by molecular partners: Product branching and quantum state distributions

Hydroxyl (OH) radicals are important in atmospheric and combustion environments where they are often detected by laser-induced fluorescence (LIF) on the A2Σ+ - X2Π band system. Previous kinetic studies have shown that collisional quenching of electronically excited OH A2Σ+ radicals impacts on LIF measurements of ground state populations in these environments. This thesis examines the dynamical outcomes of collisional quenching of OH A2Σ+ radicals by several molecular collision partners, namely H2, D2, N2, O2, and CO2. The experiments utilize a pump-probe scheme to determine the OH X2Π product state distribution (PSD) following non-reactive quenching. A pump laser prepares OH A2Σ+ (ν\u27=0, N\u27=0) in the collisional region of the pulsed supersonic region, which results in a significantly reduced fluorescence quantum yield due to quenching by collisions with the molecular partner, as evidenced by the shortened fluorescence lifetime. After a short time delay, a probe laser monitors the collision-induced population in OH X2Π (ν , N ) levels under single collision conditions via LIF. The population in specific OH X2Π (ν , N ) product states is also compared to that in the initially prepared OH A2Σ + (ν\u27=0, N\u27=0) state to determine the branching fraction for OH X2Π. For these quenchers, the OH X2Π products are released with little vibrational excitation, predominantly in ν =0, with monotonically decreasing population in higher vibrational levels. The OH X2Π products from quenching with H2, D2, N2 and O 2 are found to be highly rotationally excited. In contrast, the OH X 2Π products from quenching with CO2 show a modest degree of rotational excitation indicative of a long-lived complex. Branching fraction measurements reveal that the branching to OH X2Π products varies significantly with collision partner, accounting for 12(5)% of the total quenching with H2, 15(8)% with D2, \u3e88(3)% with N2, 40(1)% with O2, and 64(5)% with CO2. These observations provide new insight on the fundamental mechanism (or mechanisms) by which quenching occurs for OH A2Σ+ with molecular partners. For H2, D2 and N2, collisional quenching occurs through conical intersections (CI) that couple the excited- and ground-state potential energy surfaces (PES). The vibrational, rotational, and fine structure distributions of the OH X2Π products as well as the branching fractions are interpreted as dynamical signatures of nonadiabatic passage through the CI region. For these systems, the experimental measurements are supplemented by theoretical calculations examining the topography of the PESs in the vicinity of the CI. As a result, they are emerging as benchmark systems for understanding the nonadiabatic dynamics of collisional quenching