Spectroscopic detection and decay dynamics of the hydroxyl radical and its reactant complexes with carbon monoxide and acetylene

Excited electronic states of the hydroxyl radical are investigated using two types of measurements. First, laser-induced excitation originating from high vibrational levels (primarily v = 3, 4, and 5) of the X 2Π ground state to the repulsive 1 2Σ − potential results in D(2S) and O(3P 2) photodissociation products. The angle-speed distribution of the products, and thus their total kinetic energy release, are ascertained using velocity map imaging. Experimental results and complementary theory provide a consistent description of the photodissociation dynamics. The second experiment is focused on the spectroscopic characterization and dynamics of the D 2Σ− and 3 2Σ − Rydberg states of OH through resonance enhanced multiphoton ionization (REMPI). A jet-cooled and hexapole state selected beam of OH radicals yields simplified two-photon spectra of the D 2Σ − (v′ = 0–3) and 3 2Σ− (v′ = 0,1) vibronic levels, extending previous studies. First-principle theoretical studies inspired by this experiment are used to interpret its findings and to examine the utility of this REMPI scheme for sensitive detection of OH radicals. The structure, stability and/or dynamics of the OH-acetylene and OH-CO reactant complexes are studied in their ground and excited electronic states. Laser-induced fluorescence and fluorescence depletion methods provide a broad and relatively unstructured electronic excitation spectrum for the OH-CO reactant complex in the OH A-X (1,0) region. The electronic spectrum is consistent with a Franck-Condon mapping of the ground state radial distribution on a strongly attractive excited state potential that undergoes rapid decay dynamics. OH-acetylene is characterized by infrared action spectroscopy utilizing both the OH overtone and asymmetric CH stretching excitations of the complex. Analysis of the rotational band structure (2νOH) reveals a T-shaped π-hydrogen bonded structure. Investigation of the predissociation dynamics upon 2ν OH excitation establishes an upper limit of 956 cm−1 for the complex stability. The spectrum observed upon excitation of the asymmetric CH stretching mode displays unexpected rotational band structure. The unusual spectrum arises from a partial quenching of the OH orbital angular momentum upon complex formation. The partial quenching of the electronic orbital angular momentum of OH indicates that the electronic changes accompanying the evolution of reactants into products have begun to occur in the OH-acetylene complex