Dynamics of Single and Multiphoton Ionization Processes in Molecules

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 B1Σu+ state, and (2+1) REMPI via the double-well E,F1Σg+ 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,F1Σg+ 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.