ELECTRONIC STRUCTURE AND REACTION DYNAMICS OF MOLECULAR AND CLUSTER ANIONS VIA PHOTOELECTRON IMAGING

Abstract

The electronic structure and reaction dynamics of molecular and cluster anions in the gas phase has been investigated using negative ion velocity-map imaging photoelectron spectrometer. Photoelectron images provide important information about both energies and symmetries of the parent anion orbitals from which photoelectron originates. The symmetry and the ordering of several low-lying electronic states of neutral nitromethane (X¹A′, a³A″, b³A″, and A¹A″) are assigned based on a group theoretical analysis of the transitions angular distributions and the results of DFT calculations. The through-bond electronic coherence in meta- and para-dinitrobenzene anions is explored by recording a series of photoelectron images in 532-266 nm wavelength range. Photoelectron angular distributions for both isomers exhibit oscillatory behavior characteristic of the quantum interference effect, suggesting that dinitrobenzene anions retain their high symmetry electronic structures in the gas phase. Photoelectron imaging experiments on [O(N₂O)(n)]⁻, n =0–9 at 266 and 355 nm provide clear evidence of a switch from the cova)lent NNO₂⁻ cluster core to the atomic O⁻ core occurring between n = 3 and 4. The experimental results and theoretical modeling indicate that despite the greater stability of NNO₂⁻ relative to the O⁻ + N₂O⁻ dissociation limit, an O⁻ cluster core becomes energetically favored over NNO₂⁻ for n > 3, due to the more efficient solvation of the atomic anion. The photodissociation dynamics of I₂⁻ and IBr⁻ anions on the respective A' excited-state anion potentials is effectively unraveled in 780 nm pump - 390 nm probe time-resolve experiments. The time-dependent photoelectron spectra and classical trajectory calculations of the IBr⁻ dissociation provide the first rigorous dynamical test of the recently calculated A′ potential for this system. The photoelectron anisotropy cyclic variation observed in photodissociation of I₂⁻ is interpreted in the context of dual-center quantum interference model. The 390 nm pump – 390 nm probe experimental data reveal fast (≤100 fs) and delayed (~ 700 fs) appearance of the I⁻ channel in the photodissociation of I₂Cl⁻ and BrICl⁻ anions respectively. The difference in the reaction time-scales is attributed to the distinct dissociation pathways available for the anions to form I⁻ product.