Dynamics of Single and Multiphoton Ionization Processes in Molecules

Citation

Lynch, Diane Lynn (1987) Dynamics of Single and Multiphoton Ionization Processes in Molecules. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/7grh-zk17. https://resolver.caltech.edu/CaltechTHESIS:08012025-212202129

Abstract

Single-photon and resonant multiphoton ionization studies, which can now be carried out using synchrotron radiation and lasers, respectively, are providing important dynamical information on molecular photoionization. We have studied the underlying dynamical features of these ionization processes using Hartree-Fock continuum orbitals generated using the Iterative Schwinger Variational method for solving the photoelectron collisional equations. Our single-photon studies examine the important role that shape and autoionizing resonances play in molecular photoionization, while the multiphoton studies investigate the ionization dynamics of excited electronic states.

We have demonstrated the subtle nature of shape resonances in polyatomic systems such as C₂H₂ and C₂N₂, where the possibility of multiple resonances in a single channel is observed. In addition, these resonant states induce the breakdown of the Franck-Condon approximation as well as the frozen core approximation in K-shell ionization, as illustrated in our vibrationally resolved studies of CO and the K-Shell studies of N₂ and CO₂, respectively. Critical comparisons with other theoretical results and available experimental data have been made for these systems.

Molecular autoionizing resonances are known to dominate regions of the photoionization spectra. We have adapted and applied a generalization of the Fane treatment for autoionization to molecular systems. Results for H₂ and C₂H₂ autoionizing resonances are presented and discussed.

Resonant enhanced multiphoton ionization (REMPI) is currently being used to study the dynamics of state-selected photoionization processes by laser excitation to specific vibrational and rotational levels. As first applications of a quantitative analysis for molecular REMPI we have studied several ionization schemes in H₂, such as (3+1) REMPI via the C¹∏_u state, (2+1) REMPI via the B¹Σ_u⁺ state, and (2+1) REMPI via the double-well E,F¹Σ_g⁺ state. These ab initio studies are very useful in g understanding the underlying dynamics of molecular REMPI, e.g., the observed non-Franck-Condon behavior of the vibrational branching ratios in the C¹∏_u and E,F¹Σ_g⁺ states can be primarily attributed to the internuclear distance dependence of the electronic transition moments. We compare and discuss our theoretical results with the available experimental data.

Thesis Files