Excited state dynamics of atmospheric species: Collisional quenching of OH A2Sigma+ and photodissociation of CH2I2 and CH2OO

This thesis focuses on the nonradiative disposal of energy following electronic excitation of atmospherically relevant species. Specifically, this work examines (1) removal of population from electronically excited OH A 2Σ+ through collisions with molecular partners via regions of conical intersection and (2) UV photodissociation of diiodomethane, CH2I2, and the simplest Criegee intermediate, CH2OO. By mapping out product translational and/or internal energy distributions from these events, insights are gained into the mechanism and/or dynamics by which such processes occur. Products from nonreactive and/or reactive quenching of OH/D A 2Σ+ by H2, O 2, CO, and Kr are studied using a laser pump-probe scheme with fluorescence detection, determining the OH/D X2Π product state distribution that results from nonreactive quenching and/or the product translational energy release to H, D, or O-atoms as a result of reactive quenching. By comparing with complementary theory, the outcomes of collisional quenching are shown to provide experimental observables that reflect on the nonadiabatic coupling as the system evolves to nonreactive or reactive products through regions of conical intersection (CI). Significant rotational excitation of the OH X 2Π nonreactive products is indicative of a strong torque placed on OH in the vicinity of the CI, while minimal vibration shows that the OH bondlength is essentially unchanged in the quenching process. The kinetic energy release to H, D, and O-atom reaction products and the corresponding internal energy of the correlated fragments provide additional insights on the forces or geometric configurations in the vicinity of the CIs. The photodissociation dynamics of CH2I2 and CH 2OO are also examined using a laser pump-probe scheme with ionization detection of the I- or O-atom fragments. The ionized products are detected with a newly built velocity map ion imaging apparatus giving mass and 2D spatial resolution. The product translational energy distributions derived from the image yield the nascent internal energy distributions of the molecular cofragments as well as the ground state binding energy for CH2OO. The angular distributions are indicative of prompt dissociation and the character of the electronic state(s) involved in dissociation process