Application of semiempirical electronic structure methods to studies of structures and dynamics of molecular clusters and condensed phase systems

A semi-empirical electronic structure method, diatomics-in-molecules (DIM), and a version specifically for dealing with ionic systems, diatomics-in-ionic-systems (DIIS), are applied to studies of highly excited I₂ doped argon and krypton matrices in an attempt to reproduce the ion-pair state emission bands and explore the influence of the solvation environment on the electronic states involved in the transitions giving rise to these bands. The calculated emission bands are compared with experimental results and the agreement and differences are discussed.; These methods are also applied to the $Xe+nI-$ excimer states of iodine doped Xe clusters to study the equilibrium geometries and excited state properties as a function of the cluster size. The properties calculated with this model are compared to experimental findings when they are available. The equilibrium geometries and properties are also studied in connection with the nature of the chemical bonding in these “charge transfer to solvent” (CTTS) clusters.; Building on previous theoretical studies of the charge transfer dynamics of excited $I-2$ in small CO₂ clusters, we use both DIM (for probed neutral states) and DIIS (for initial molecular ion states), together with non-adiabatic molecular dynamics trajectories, to compute the transient photo-electron detachment spectra of these ionic cluster systems. We compare our results directly with experimental femtosecond photoelectron spectra (PES) observed by Neumark and coworkers, and interpret the rich excited state dynamics revealed by the PES results in light of our MD simulation results.; Finally, the DIM model, together with a mixed quantum-classical surface hopping algorithm, is employed to study the non-adiabatic electronic relaxation of a neutral I₂ molecule initially photo-excited to its B(3Π_u) state in condensed phase rare gas solvent environments. We show that, when both the anisotropy of the non-adiabatic coupling and the quasi-degeneracy of electronic states in solids are correctly accounted for, these mixed quantum classical simulations reveal the source of the order of magnitude difference observed in electronic relaxation time-scales between solid and liquid environments. The pump-probe signals obtained from our MD simulations are compared with experimental spectra.