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 <i>et al.</i> Angew. Chem., Int. Ed. <b>52</b>, 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 (<i>X</i> 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 (<i>A</i> 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.