Polar molecules with sufficiently large dipole moments can form highly diffuse dipole-bound anions, and possess noncovalent dipole-bound states (DBSs) just below the electron detachment threshold by the long-range electron-dipole interaction. DBSs have been proposed as the “doorway” to the formation of valence-bound anions and exploited as a means to develop high-resolution resonant photoelectron spectroscopy. We have developed an experimental technique coupling electrospray ionization (ESI), cryogenic ion cooling in a 3D Paul trap, and high-resolution photoelectron imaging. Using this third-generation ESI-PES apparatus, DBSs of a wide variety of cryogenically cooled anions have been investigated. The π-type DBS (π-DBS) with one additional angular node in the wave function was predicted to exist as an excited electronic state in addition to the σ-type ground state. However, π-type noncovalent states have never been observed in isolated molecular systems. Recently, we reported the observation of a π-DBS in cryogenically-cooled 9-anthrolate anions (9AT⁻, C₁₄H₉O⁻) by resonant two-photon photoelectron imaging. A DBS is observed 191 cm⁻¹ (0.0237 eV) below the detachment threshold, and the existence of the π-DBS is revealed by a distinct (s + d)-wave photoelectron angular distribution. The π-DBS is stabilized by the large anisotropic in-plane polarizability of 9AT. The population of the dipole-forbidden π-DBS is proposed to be via a nonadiabatic coupling with the dipole-allowed σ-type DBS mediated by molecular rotations.

Brenda Rubenstein

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