We report the observation of a $\pi$-type dipole-bound state ($\pi$-DBS) in cryogenically cooled deprotonated 9-anthrol molecular anions ($9{\mathrm{AT}}^{-}$) by resonant two-photon photoelectron imaging. A DBS is observed $191{\mathrm{cm}}^{-1}$ (0.0237 eV) below the detachment threshold, and the existence of the $\pi$-DBS is revealed by a distinct ($s+d$)-wave photoelectron angular distribution. The $\pi$-DBS is stabilized by the large anisotropic in-plane polarizability of 9AT. The population of the dipole-forbidden $\pi$-DBS is proposed to be via a nonadiabatic coupling with the dipole-allowed $\sigma$-type DBS mediated by molecular rotations.

• Received 9 April 2020
• Revised 7 June 2020
• Accepted 28 July 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.073003