UC Berkeley

Abstract

Investigations of Two- and Three-Body Molecular Photodissociation Dynamics by Fast Beam Photofragment Translational Spectroscopy

by

Aaron Woods Harrison

Doctor of Philosophy in Chemistry

University of California, Berkeley

Professor Daniel M. Neumark, Chair

Fast beam photofragment translational spectroscopy is used to study the two- and three-body photodissociation dynamics of neutral molecules and molecular anions. With this method, a time- and position-sensitive coincident measurement of the dissociation products is made. This allows an accurate determination of the masses and populations of the available product channels as well as the corresponding translational energy release and photofragment angular distribution. This information provides insight into the branching ratios between the product channels and the potential energy surfaces involved in the dissociation mechanism. In addition, it is possible to elucidate three-body dissociation mechanisms (concerted vs. sequential) through the use of Dalitz plot analysis.

The details of the experimental apparatus are outlined in Chapter 2. Most importantly, the time- and position-sensitive detector utilized on the instrument has been recently upgraded to a delay-line detector from the previous CCD/PMT coincidence imaging detector. The principles of operation and the advantages of using this detector are discussed as well as the spatial and the translational energy resolution of the new detection system.

The photodissociation of the thiophenoxy radical (Chapter 3) was studied by preparing a fast beam of the thiophenoxide anion followed by threshold photodetachment to form the neutral radical. The fragmentation of thiophenoxy was investigated at 248, 193, and 157 nm. At each wavelength, this study revealed that following absorption to an excited state, there is internal conversion to the ground state potential energy surface followed by statistical dissociation. CS loss and SH loss are found to be the major dissociation pathways with a minor contribution from S loss. Electronic strucuture and RRKM calculations are employed to support the experimental data.

The three-body dissociation of ozone at 193 and 157 nm was investigated (Chapter 4). At 193 nm, this dissociation pathway is just barely accessible at this photon energy and is a relatively minor pathway (5.2(6)%). However at 157 nm, it is found that three-body dissociation constitutes 26(4)% of the total fragmentation. Dalitz plot analysis showed the three-body decay to occur through a concerted mechanism.

The three-body dissociation of the ion-molecule complex I2¯(CO2) was also examined (Chapter 5). Photodissociation of this complex is initiated by excitation of the I2¯ chromophore to the repulsive A-state at 720 nm and B-state at 386 nm. The translational energy distributions showed excitation in the bending mode of CO2 photofragment and provided an accurate measurement of the CO2 binding energy (218 ± 10 meV). An asynchronous-concerted three-body dissociation mechanism was evidenced through Dalitz plot analysis.