Exploring Vinylidene/Acetylene Isomerization by Photoelectron Spectroscopy of Vibrationally Excited Vinylidene Anions

In order to explore the vibrational levels of vinylidene (H₂CC) and their possible couplings with acetylene (HCCH), we investigate the effect of infrared (IR) vibrational pre-excitation on the high-resolution photoelectron spectra of the vinylidene anion (H₂CC^-) and its deuterated isotopologue (D₂CC^-). The photoelectron spectra are obtained using slow electron velocity-map imaging of cryogenically cooled anions (cryo-SEVI); here, cold anions are vibrationally excited by an IR laser pulse prior to photodetachment (IR cryo-SEVI). Infrared action spectra of the anion CH₂ stretching fundamentals are measured by monitoring growth and depletion of features in photoelectron spectra as the IR laser is tuned, yielding excitation frequencies of the symmetric (ν₁) and antisymmetric (ν₅) CH₂ stretching modes of 2590 ± 2 and 2658 ± 2 cm^-1, respectively. We then use IR cryo-SEVI to explore the effect of vibrational excitation of the two modes on the anion photoelectron spectrum. Interpretation of these spectra is facilitated by quantum calculations performed for each isotopologue on accurate six-dimensional potential energy surfaces of both neutral and anionic vinylidene. IR cryo-SEVI spectra resulting from excitation of these two close-lying anion vibrations are noticeably different. Excitation of the ν₁ mode leads to several new features that appear in the photoelectron spectra which closely match the Franck-Condon allowed transitions predicted by theory. Excitation of the ν₅ mode in H₂CC^- reveals complicated spectral features in the vicinity of the 5₁¹ sequence band that are not seen for D₂CC^-. These are explained by a combination of anharmonic coupling between ν₅ and ν₆ (CH₂ rock) states in neutral H₂CC and possible coupling to the HCCH isomer.