Probing the Critical Dipole Moment To Support Excited Dipole-Bound States in Valence-Bound Anions.

Abstract

We report photodetachment spectroscopy and high-resolution photoelectron imaging of para-halogen substituted phenoxide anions, p-XC6H4O- (X = F, Cl, Br, I). The dipole moments of the p-XC6H4O neutral radicals increase from 2.56 to 3.19 D for X = F to I, providing a series of similar molecules to allow the examination of charge-dipole interactions by minimizing molecule-dependent effects. Excited DBSs ([XC6H4O]*-) are observed for the four anions with binding energies of 8, 11, 24, and 53 cm-1, respectively, for X = F to I, below their respective detachment thresholds. The binding energies exhibit a linear correlation with the dipole moments of the neutral radicals, extrapolating to a critical dipole moment of 2.5 D for zero binding energy. Because of the small binding energy of the excited DBS of [FC6H4O]*-, rotational autodetachment is observed to compete with vibrational autodetachment in the resonant photoelectron spectra, resulting in electrons with near zero kinetic energies.