PUB - Publikationen an der Universität Bielefeld

The title system is thoroughly investigated by high-level electronic structure techniques and nuclear quantum dynamics calculations. Equilibrium geometries and harmonic frequencies are determined by coupled-cluster singles doubles [CCSD(T)] calculations with large AO basis sets. A C-4 nu distorted geometry is found for the anion in contrast to previous assumptions. This is explained by the bonding situation in the electronic ground state and possible vibronic interactions with higher electronic states. The computed adiabatic electron affinity of 0.73 eV is considerably lower than the currently recommended value. Analysis of the electronic states of the anion shows that the sigma* ground state at equilibrium position corresponds to a highly excited state at the neutral's geometry where the ground state is either a very weakly bound or scattering state. If the electron is captured by this latter state, a nonadiabatic transition to the sigma* state followed by internal vibrational redistribution could explain the formation of a stable anion. The C-4 nu distortion of the equilibrium geometry is essential for the explanation of recently measured photodetachment spectra. Since the distortion leads to six equivalent minima with very low barriers, an anharmonic potential energy surface (PES) of the four relevant vibrational modes is constructed and fitted to CCSD(T) computed energies. The remaining 11 modes are treated as harmonic oscillators. The vibrational dynamics of the anion is studied by diagonalization of the Hamiltonian in the basis of the neutral's eigenstates. The computed photoelectron spectra are in good agreement with recent experiments and demonstrate the quality of the PES and that C-4 nu distortion is responsible for the observed irregularities. However, thermal effects play a significant role for the shape of the spectra because many low-lying initial states are populated. (C) 2011 American Institute of Physics. [doi:10.1063/1.3544213]