Resonant two-photon photoelectron imaging and adiabatic detachment processes from bound vibrational levels of dipole-bound states

Abstract

Anions cannot have Rydberg states, but anions with polar neutral cores can support highly diffuse dipole-bound states (DBSs) as a class of interesting electronically excited states below the electron detachment threshold. The binding energies of DBSs are extremely small, ranging from a few to few hundred wavenumbers and generally cannot support bound vibrational levels below the detachment threshold. Thus, vibrational excitations in the DBS are usually above the electron detachment threshold and they have been used to conduct resonant photoelectron spectroscopy, which is dominated by state-specific autodetachment. Here we report an investigation of a cryogenically-cooled complex anion, the enantiopure (R)-(−)-1-(9-anthryl)-2,2,2-trifluoroethanolate (R-TFAE⁻). The neutral R-TFAE radical is relatively complex and highly polar with a non-planar structure (C₁ symmetry). Photodetachment spectroscopy reveals a DBS 209 cm⁻¹ below the detachment threshold of R-TFAE⁻ and seven bound and eight above-threshold vibrational levels of the DBS. Resonant two-photon detachment (R2PD) via the bound vibrational levels of the DBS exhibits strictly adiabatic photodetachment behaviors by the second photon, in which the vibrational energies in the DBS are carried to the neutral final states, because of the parallel potential energy surfaces of the DBS and the corresponding neutral ground electronic state. Relaxation processes from the bound DBS levels to the ground and low-lying electronically excited states of R-TFAE⁻ are also observed in the R2PD photoelectron spectra. The combination of photodetachment and resonant photoelectron spectroscopy yields frequencies for eight vibrational modes of the R-TFAE radical.