Cryogenic Photodetachment Spectroscopy and High-Resolution Resonant Photoelectron Imaging of Cold para-Ethylphenolate Anions

Abstract

Valence-bound molecular anions with polar neutral cores (μ > 2.5 D) can support highly diffuse dipole-bound states (DBSs) as electronically excited states just below the detachment threshold. Such weakly bound nonvalence excited states have little influence on the structure of the neutral core, and they usually have the same vibrational frequencies. DBSs can be systematically searched using photodetachment spectroscopy (PDS), which can yield the binding energies of the DBSs, the electron detachment threshold of the anion, and above-threshold vibrational levels of the DBSs (Feshbach resonances). We have shown that the combination of PDS and resonant photoelectron spectroscopy (rPES) at the Feshbach resonances is a powerful approach to obtain rich vibrational information for complex molecular radicals. A prerequisite for this technique is to produce vibrationally cold anions, made possible by a cryogenically controlled Paul trap. In this article, we report a PDS and rPES study of cold para-ethylphenolate anions (p-EP–). The electron affinity of the p-EP radical is measured to be 17425 ± 3 cm–1 (2.1604 ± 0.0004 eV), and a DBS is found at 145 cm–1 below the detachment threshold of p-EP–. Thirty-four vibrational levels are observed for the DBS, including two bound levels and 32 Feshbach resonances. Frequencies for 17 vibrational modes of the p-EP radical are measured from the combination of PDS and rPES, including six symmetry-forbidden modes with A″ symmetry. The current study confirms again the power of combining cryogenic ion cooling with PDS and high-resolution rPES to obtain spectroscopic information on complex molecular radicals.