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Structure and Dynamics of Small Molecules and Anions: Negative-ion photoelectron spectroscopy

TitleStructure and Dynamics of Small Molecules and Anions: Negative-ion photoelectron spectroscopy
Publication TypeThesis
Year of Publication2012
AuthorsMiller, EM

Photoelectron spectroscopy of gas-phase anions is utilized to study the spectroscopic and dynamic properties of anions and anionic clusters. The photoelectron spectrum of IBr⁻(X̃2+) leads to more accurate measurements of a number of molecular properties, such as the electron affinity of IBr (EA(IBr) = 2.512±0.003 eV), equilibrium geometry of the anion ground state (Re(I–Br⁻) = 3.01±0.01 Å), and dissociation energy of the anion ground state (D0(I–Br⁻) = 0.966±0.003 eV). The photoelectron spectrum of IBr⁻(CO2)n, n = 1 – 3, demonstrates minimal perturbation to the IBr⁻ electronic structure, and the EAs for solvated IBr(CO2)n, n = 1 – 3, are determined. The presence of the CO2 influences the dissociation dynamics along two excited states of IBr⁻ by enabling nonadiabatic transitions. Time-resolved photoelectron spectra of IBr⁻ and IBr⁻(CO2) are taken to measure these dissociation dynamics, which when combined with theory leads to a mechanism. The solvation energy of CO2 and the ability of CO2 to temporarily acquire partial charge as it bends facilitate the charge transfer in the two photoexcitation/photodissociation channels studied.

Photoelectron spectroscopy of ICN⁻ (X̃2+) probes transitions to the ground state and first five excited states of neutral ICN. The first three excited states and a conical intersection region between the 30+ and 11 states are spectroscopically resolved. Through thermochemical cycles involving narrow transitions to excited states, the EA(ICN) is found to be 1.34(+.04/-.02) eV and the D0(ICN⁻) equals 0.83(+.04/-.02) eV. In addition, four spectral peaks are observed with photoelectron kinetic energies of ~0, ~45, ~70, and ~150 – 200 meV, and the kinetic energy is unchanged as the photodetachment photon energy is varied from 2.5 to 4.2 eV. These autodetachment features are a result of photoexcited ICN⁻ converting internal energy into CN rotation followed by a quasi-thermionic emission of electrons to produce neutral ICN/INC in its ground electronic state. The autodetachment features persist when ICN⁻ is solvated by Ar or CO2, indicating that solvation does not modify the autodetachment mechanism. In addition to these investigations, current projects are in progress, such as HO3⁻, that require abundant Ar cooling and an entrainment block as part of the anion source.