High-Resolution Photoelectron Spectroscopy of Vibrationally Excited Vinoxide Anions

High-resolution photoelectron spectra of vibrationally pre-excited vinoxide anions (CH₂CHO^-) are reported using the recently developed IR-cryo-SEVI technique. This method is combined with a newly developed implementation of vibrational perturbation theory that can readily identify relevant anharmonic couplings among nearly degenerate vibrational states. IR-cryo-SEVI spectra are obtained by resonant infrared excitation of vinoxide anions via the fundamental C-O (ν₄, 1566 cm^-1) or isolated C-H (ν₃, 2540 cm^-1) stretching vibrations prior to photodetachment. Excitation of the ν₄ mode leads to a well-resolved photoelectron spectrum that is in excellent agreement with a harmonic Franck-Condon simulation. Excitation of the higher-energy ν₃ mode results in a more complicated spectrum that requires consideration of the calculated anharmonic resonances in both the anion and the neutral. From this analysis, information about the zeroth-order states that contribute to the nominal ν₃ wave function in the anion is obtained. In the neutral, we observe anharmonic splitting of the ν₃ fundamental into a polyad feature with peaks at 2737(22), 2 835(18), and 2910(12) cm^-1, for which only the center frequency has been previously reported. Overall, 9 of the 12 fundamental frequencies of the vinoxy radical are extracted from the IR-cryo-SEVI and ground-state cryo-SEVI spectra, most of which are consistent with previous measurements. However, we provide a new estimate of the ν₅ (CH₂ scissoring) fundamental frequency at 1395(11) cm^-1 and attribute the discrepancy with previously reported values to a Fermi resonance with the 2ν₁₁ overtone (CH₂ wagging).