Rovibronic structure in slow photoelectron velocity-map imaging spectroscopy of CH₂CN^- and CD₂CN^-

We report high-resolution anion photoelectron spectra of the cryogenically cooled cyanomethide anion, CH₂CN^-, and its isotopologue, CD₂CN^-, using slow photoelectron velocity-map imaging (SEVI) spectroscopy. Electron affinities of 12 468(2) cm^-1 for CH₂CN and 12 402(2) cm^-1 for CD₂CN are obtained, demonstrating greater precision than previous experiments. New vibrational structure is resolved for both neutral species, especially activity of the ν₅ hydrogen umbrella modes. The ν₆ out-of-plane bending mode fundamental frequency is measured for the first time in both systems and found to be 420(10) cm^-1 for CH₂CN and 389(8) cm^-1 for CD₂CN. Some rotational structure is resolved, allowing for accurate extraction of vibrational frequencies. Temperature-dependent SEVI spectra show marked effects ascribed to controlled population of low-lying anion vibrational levels. We directly measure the inversion splitting between the first two vibrational levels of the anion ν₅ umbrella mode in both species, finding a splitting of 130(20) cm^-1 for CH₂CN^- and 81(20) cm^-1 for CD₂CN^-. Franck-Condon forbidden activity is observed and attributed to mode-specific vibrational autodetachment from the CH ₂CN^- and CD₂CN^- dipole bound excited states. We also refine the binding energy of the anion dipole bound states to 39 and 42 cm^-1, respectively, for CH₂CN^- and CD₂CN^-.