Observation of a Symmetry-Forbidden Excited Quadrupole-Bound State

Abstract

We report the observation of a symmetry-forbidden excited quadrupole-bound state (QBS) in the tetracyanobenzene anion (TCNB − ) using both photoelectron and photodetachment spectroscopies of cryogenically-cooled anions. The electron affinity of TCNB is accurately measured as 2.4695 eV. Photodetachment spectroscopy of TCNB − reveals selected symmetry-allowed vibronic transitions to the QBS, but the ground vibrational state was not observed because the transition from the ground state of TCNB − (A u symmetry) to the QBS (A g symmetry) is triply forbidden by the electric and magnetic dipoles and the electric quadrupole. The binding energy of the QBS is found to be 0.2206 eV, which is unusually large due to strong correlation and polarization effects. A centrifugal barrier is observed for nearthreshold autodetachment, as well as relaxations from the QBS vibronic levels to the ground and a valence excited state of TCNB − . The current study shows a rare example where symmetry selection rules, rather than the Franck−Condon principle, govern vibronic transitions to a nonvalence state in an anion.

Probing the Dipole-Bound State in the 9-Phenanthrolate Anion by Photodetachment Spectroscopy, Resonant Two-Photon Photoelectron Imaging, and Resonant Photoelectron Spectroscopy

“…Since there are no low-lying excited electronic states in 9PT – according to our theoretical calculation, the low kinetic energy electrons must come from detachment from vibrationally excited levels of the ground electronic state of 9PT – , via internal conversion from the DBS followed by intramolecular vibrational energy redistribution (IVR) . Similar internal conversion processes from nonvalence excited states to the ground electronic states of anions seem to be quite common and have been observed before. ,,, Recently, the autodetachment dynamics from vibrational Feshbach resonances of the DBS in the phenoxide anion has been studied using a pump–probe experiment . It is also interesting to directly probe the dynamics of the intersystem crossing and internal conversion from the DBS to valence excited states in anions.…”

“…74 Similar internal conversion processes from nonvalence excited states to the ground electronic states of anions seem to be quite common and have been observed before. 38,53,75,76 Recently, the autodetachment dynamics from vibrational Feshbach resonances of the DBS in the phenoxide anion has been studied using a pump−probe experiment. 77 It is also interesting to directly probe the dynamics of the intersystem crossing and internal conversion from the DBS to valence excited states in anions.…”

Velocity Map Imaging Spectroscopy of the Dipole-Bound State of CH2CN–: Implications for the Diffuse Interstellar Bands

“…While the existence of DBSs has been long known, the second step of this problemautodetachment from DBS to neutralhas only recently received attention. Numerous new studies have highlighted how velocity-map imaging can be a powerful technique for directly probing the autodetachment pathways and dynamics. − ,− More exotic bound state systems have also been reported, including quadrupole-bound − and temporary-bound − states. These studies provide an insight into the rich chemistry of weakly bound systems that is still yet to be uncovered.…”

Mode-Specific Autodetachment Dynamics of an Excited Non-valence Quadrupole-Bound State

“…Compared to the DBS, other non-valence anionic states such as the correlation-bound state (CBS) or quadrupole-bound state (QBS) have been relatively less studied to date. Although distinctly classifying the prevalence of any particular long-range force acting in the specific non-valence bound anions seems to be a nontrivial task, ,, there have been quite a few successful experimental and theoretical studies regarding the CBS − and QBS. ,− The most definitive experimental example of the QBS, for instance, seems to be the case of the trans -succinonitrile anion. , Using high-resolution photoelectron spectroscopy and theoretical calculations, the origin of the small binding energy of ∼20 meV of the trans -succinonitrile anion has been ascribed to the electron-quadrupolar interaction. A recent study of the cryogenically cooled 4-cyanophenoxide (4-CP) anion, on the contrary, provides the first example of the excited QBS anion.…”