Probing the Quantum Nature of Intramolecular Vibrational Energy Redistribution in HCCH-H2CC Isomerization Using High-Resolution Photoelectron Spectroscopy.

High resolution photoelectron spectra of vinylidene anions (H2CC- and D2CC-) are reported using variations on slow electron velocity-map imaging of cryogenically cooled anions (cryo-SEVI). This work builds on previous studies aimed at exploring the coupling between neutral H2CC and HCCH isomers. One-photon photodetachment (PD) spectra with improved cooling and signal levels probe very weak transitions to vibrational levels of H2CC in which the ν6 rocking mode is excited; this is the vibrational mode most strongly coupled to isomerization. In addition, vibrationally resonant two-photon detachment (VR2PD) experiments are carried out that probe PD from vibrationally excited anion levels to the same neutral ν6 states and other low-lying vibrational levels of neutral vinylidene. The experimental spectra show a clear boundary in spectral complexity near the predicted barrier height for H2CC→HCCH isomerization, indicating that vinylidene and acetylene states above the barrier are strongly mixed whereas states below the barrier are largely localized in their respective wells. This mixing is, however, largely absent in the cryo-SEVI spectra of D2CC-. Comparison with quantum dynamics calculations based on an accurate full dimensional potential energy surface provides insight into the connection between the H2CC-HCCH isomerization reaction and the vibrational structure of vinylidene, revealing pathways of intramolecular vibrational energy flow in the quantum state space of this prototypical system.