Observation of a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>π</mml:mi></mml:math>-Type Dipole-Bound State in Molecular Anions
Dao-Fu Yuan, Yuan Liu, Chen‐Hui Qian, Yue-Rou Zhang, Brenda M. Rubenstein, Lai‐Sheng Wang
Abstract
We report the observation of a $\ensuremath{\pi}$-type dipole-bound state ($\ensuremath{\pi}$-DBS) in cryogenically cooled deprotonated 9-anthrol molecular anions ($9{\mathrm{AT}}^{\ensuremath{-}}$) by resonant two-photon photoelectron imaging. A DBS is observed $191\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ (0.0237 eV) below the detachment threshold, and the existence of the $\ensuremath{\pi}$-DBS is revealed by a distinct ($s+d$)-wave photoelectron angular distribution. The $\ensuremath{\pi}$-DBS is stabilized by the large anisotropic in-plane polarizability of 9AT. The population of the dipole-forbidden $\ensuremath{\pi}$-DBS is proposed to be via a nonadiabatic coupling with the dipole-allowed $\ensuremath{\sigma}$-type DBS mediated by molecular rotations.