Observation of Mode‐Specific Vibrational Autodetachment from Dipole‐Bound States of Cold Anions

Abstract

Molecules with sufficiently large dipole moments were predicted to form weakly bound negative ions in the dipolar field [1][2][3][4] and such dipole-bound anions have been observed and characterized experimentally. [5][6][7][8][9][10] Negative ions with dipolar molecular cores can have excited dipole-bound states (DBSs) near the detachment threshold, analogous to Rydberg states in neutral molecules. DBSs in excited anions were first observed as resonances in photodetachment crosssections. [11][12][13] Ultrahigh resolution spectroscopy has been reported on excited DBSs of a number of anions near their detachment threshold by autodetachment. [14][15][16] The theory for autoionization from Rydberg states was first developed by Berry, [17] who predicted a Dv = À1 vibrational propensity rule, that is, the Rydberg state undergoes a vibrational relaxation by one vibrational quantum in the molecular core during autoionization, in which the vibrational energy is transferred to the Rydberg electron. The Dv = À1 propensity rule has been observed in autoionization of numerous molecules [18][19][20] and mode-specific autoionization has been observed by photoelectron spectroscopy, [21] in which the kinetic energies of the outgoing electrons are measured. The same Dv = À1 propensity rule should apply in autodetachment from DBSs of excited anions [22] and has been inferred in previous studies.[ 13,16] However, the electron kinetic energies of the outgoing autodetached electrons from the excited DBSs have not been measured by electron spectroscopy.All prior studies of DBSs in excited anions are dominated by rotational effects [23,24] and pure vibrational autodetachment from DBSs has not been reported, even though vibrational autodetachment has been observed in weakly bound anions and has been used effectively as a spectroscopic tool for anions. [25][26][27] Here we report the direct observation of pure vibrational autodetachment from DBSs of cryogenically cooled phenoxide anions using high-resolution photoelectron imaging. Autodetachment from eight vibrational levels of the DBS in optically exited phenoxide anions are observed and the Dv = À1 propensity rule is found to be strictly obeyed.

Communication: Observation of dipole-bound state and high-resolution photoelectron imaging of cold acetate anions

“…This is the main character of DBSs of anions observed previously. 36,39 Hence, features 1 and 2 are due to the 14 ′1 0 and 8 ′1 0 vibrational levels, respectively, of the excited DBS of CH 3 COO − , as we expected.…”

“…[33][34][35] However, the studies of autodetachment dynamics of DBSs were hindered by congested spectra at room temperature. Recently, we directly observed mode-specific vibrational autodetachment from DBSs of cold phenoxide anions, 36 which were cooled in a temperature-controlled ion trap to eliminate vibrational hot bands and minimize rotational broadening. The ∆v = −1 vibrational propensity rule, which was initially developed for autoionzation from Rydberg states 37 and extended to autodetachment, 38 was directly observed.…”

“…We expected that transitions to the 14 ′ 0 1 and 8 ′ 0 1 vibrational levels of the DBS should occur near the 14 1 0 and 8 1 0 levels of CH 3 COO•, because the binding energy of the DBS was expected to be small. 36,39 Since the photodetachment spectrum was taken over ∼300-530 cm −1 above the detachment threshold, the overall baseline represents the direct photodetachment from the electronic ground state of CH 3 COO − to that of CH 3 COO•. In addition to the weak direct detachment signals, two peaks were observed and labeled as 1 and 2, which must be due to autodetachment by resonant excitation to an electronic excited state of CH 3 COO − .…”

Anion photoelectron spectroscopy of deprotonated ortho-, meta-, and para-methylphenol

“…Gunion et al 39 and more recently Liu et al 62 found autodetachment in the photoelectron spectroscopy of phenoxide.…”

Perspective: Electrospray photoelectron spectroscopy: From multiply-charged anions to ultracold anions

“…The double arrows below the images indicate the laser polarization. 150 the DBS ground state by resonant two-photon detachment. The binding energy of the DBS was measured to be 97 cm −1 relative to the detachment threshold at 18 173 cm −1 , i.e., the EA of phenoxy.…”