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

Abstract

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 CH2CN(-) 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(-).

Channel branching ratios in CH2CN− photodetachment: Rotational structure and vibrational energy redistribution in autodetachment

“…26 These are a triplet ( 3 A ) and a singlet ( 1 A ), which we calculate to be bound by 3 meV and 1 meV, respectively, in excellent agreement with the experimentally measured 5 meV. 8 At the optimized geometry of the neutral (C 2v ), the vertical electron affinity (VEA) is 1.506 eV (EOM-IP-CCSD/aug-cc-pVTZ). This structure has a (slightly) larger dipole moment of 3.666 D. EOM-EE-CCSD calculation using the aug-cc-pVTZ(+9s,9p,3d) basis set shows that the triplet ( 3 B 1 ) and singlet ( 1 B 1 ) dipole-bound states still lie 3 and 1 meV below the neutral state.…”

“…The spectra are plotted in the electron binding energy (eBE) domain, and the lowest energy feature, labeled A, corresponds to the vibronic origin band (0 0 0 direct detachment transition) with an eBE at the center of 12 468 cm 1 . 8 The eBE domain spectra of Fig. 3 are converted (unsmoothed) directly from velocity domain spectra extracted via the BASEX technique, 18 calibrated for electron kinetic energy (eKE = E hν -eBE) using the peak of feature A and scaled for intensity using the appropriate Jacobian transformation.…”

“…Assignments of these levels can be made according to low temperature SEVI measurements. 8 Band origins are summarized in Table I, along with a description of the most relevant modes. The greatest change in geometry from the non-planar anion to the planar neutral occurs along the ν 5 umbrella mode coordinate, 4,6,9,10 but 5 0 v…”

“…A series of Gaussian functions of the same width (approximating the instrument response and rotational envelope) are used to represent the individual vibrational transitions and summed to recreate the whole, velocity domain spectrum. Only transitions with eBE lower than the photon energy are considered in each case; the centers of these transitions are fixed (for a given photon energy) using conservation of energy and the SEVI reported vibrational band center (Table I), 8 and a single width is used for all transitions. This reduces the fitting parameters to an area for each Gaussian and a common width for all.…”

“…6,7 Additionally, several direct detachment transition vibrational origins have been measured with high accuracy in recent slow electron velocity map imaging (SEVI) experiments. 8 However, the primary reason for choosing this species is the documented existence of a dipole-bound state just below the neutral ground state. Figure 1 shows calculated energies of the anion and neutral ground states at their respective equilibrium geometries (black horizontal lines).…”