DOI: 10.25549/usctheses-c40-359801
title: Photoionization/photodetachment spectroscopy and Dyson orbitals: theoretical tools to aid experimental studies

description: Photoionization and photodetachment spectroscopy are important experimental tools for studying electronic structure and dynamics. However, connecting the experimental observables with the underlying quantum mechanical picture and even interpretation of the experimental results are not straightforward without theoretical modeling. Despite great progress in computational methods, the most common analysis tools are still based on the Koopmans' theorem, employing canonical Hartree-Fock or Kohn-Sham orbitals. Although this approach often captures a qualitative picture, it has a limited predictive power, especially for systems beyond closed-shell species. ❧ In Chapter 2, we present a derivation of the generalized one-electron wave function that provides a quantitative yet simple and transparent description of the electronic structure part of the problem. This description is applicable to a wide variety of possible experimental setups. We employ a high-level robust electronic structure approach, equation-of-motion coupled-cluster family of methods, and describe a black-box implementation for computing Dyson orbitals, an overlap of many-electron wave functions that differ by one electron. Our choice of electronic structure method enables analyzing multiple transitions from one calculation and provides a direct connection to Koopmans' theorem predictions. ❧ In Chapter 3, we analyze limits of the simplest single-center expansion approach for modeling outgoing electron by comparing computed cross-sections with reliable experimental data. We show that the plane-wave description is valid for photodetachment from molecular anions, but molecular photoionization cross-section profiles often contain features observed both in plane-wave and in Coulomb-wave descriptions. We then proceed to utilize Coulomb waves with partial charge to show that a single fitted parameter makes it possible to quantitatively reproduce absolute cross-sections in a wide range of photon energies. Finally, we propose a simplified one-dimensional model for obtaining partial charge without fitting to experimental data. ❧ In Chapter 4, we apply the formalism developed in Chapter 2 to describe solvated electrons. Our approach enables implicit inclusion of the effects of the bath (solvent) within an orbital (i.e., one-electron) picture. Analysis of Dyson orbital shapes suggests that in small sodium-doped ammonia and dimethyl ether clusters, the electron is surface-bound. The analysis of the computed properties reveals the effect of hydrogen bonding in solvent on stabilization of the charge-separated system. We further explain the experimentally observed trend in anisotropy parameter dependence on cluster size, including the appearance of “magic numbers”. We also make connections to popular mean-field orbital descriptions and illustrate the consequences of self-interaction error of Kohn-Sham density functional theory in this type of systems. ❧ Finally, in Chapter 5 we consider photoionization of arginine in the gas phase and in water. This amino acid has long and complex side chain, which participates in multiple non-covalent inter- and intramolecular interactions. Our calculations suggest that while the photoionization in the gas phase results in fast fragmentation of the cations, the solvation stabilizes them, changing the photoionization picture qualitatively.