Dipole-bound excited states and resonant photoelectron imaging of phenoxide and thiophenoxide anions.

Abstract

We report photodetachment and resonant photoelectron-imaging studies of cryogenically cooled phenoxide (C6H5O-) and thiophenoxide (C6H5S-) anions. In a previous study [H. T. Liu et al. Angew. Chem., Int. Ed. 52, 8976 (2013)], a dipole-bound excited state was observed for C6H5O- at 97 cm-1 below the detachment threshold. Eight resonant photoelectron spectra were obtained via excitations to eight vibrational levels of the dipole-bound state (DBS) followed by autodetachment. Here we present a complete photodetachment spectrum of C6H5O- covering a spectral range 2600 cm-1 above the detachment threshold and revealing nine additional vibrational resonances of the DBS. We also report the first observation of a dipole-bound excited state for C6H5S-, 39 cm-1 below its detachment threshold of 18 982 cm-1. Photodetachment spectroscopy covering a spectral range 1500 cm-1 above the threshold reveals twelve vibrational resonances for the DBS of C6H5S-. By tuning the detachment laser to the vibrational resonances in the DBS of C6H5O- and C6H5S-, we obtain highly non-Franck-Condon resonant photoelectron spectra, as a result of mode-selectivity and the Δv = -1 propensity rule for vibrational autodetachment. Five new fundamental vibrational frequencies are obtained for the ground state of the C6H5O (X 2B1) radical. Intramolecular inelastic scattering is observed in some of the resonant photoelectron spectra, leading to the excitation of the Franck-Condon-inactive lowest-frequency bending mode (ν20) of C6H5O. The first excited state of C6H5O (A 2B2) is observed to be 0.953 eV above the ground state. Twelve resonant photoelectron spectra are obtained for C6H5S-, allowing the measurements of seven fundamental vibrational frequencies of the C6H5S radical, whereas the non-resonant photoelectron spectrum exhibits only a single Franck-Condon active mode. The current study again demonstrates that the combination of photodetachment spectroscopy and resonant photoelectron spectroscopy is a powerful technique to obtain vibrational information about polar radical species.