Resonant photodetachment via shape and Feshbach resonances: p-benzoquinone anions as a model system

Abstract

For p-benzoquinone anions, the photodetachment spectrum at 0.15–0.65 eV above detachment threshold shows sharp and broad resonances, which we assign to enhanced photodetachment via resonantly excited anion states. The experiment is performed at cold and mass-selected anions to exclude contributions of fragment anions and internally excited molecules. The most prominent, intense and broad spectral feature at 20 200 cm−1 is assigned to an allowed transition from the 2 B 2g anion ground state to the 2 A u shape resonance, which corresponds to a π LUMO * →π * electron promotion. By linewidth we determine an ultrashort lifetime of 25 fs in qualitative agreement with a one-electron autodetachment process. In contrast to this, for the narrow resonances lifetimes between 0.2 and 1.2 ps are determined, in agreement with a slower autodetachment by a two-electron process from Feshbach states. Because of their low photoexcitation cross section they are assigned to dipole and symmetry forbidden n→π LUMO * transitions which can both be only optically active in some vibrations by Herzberg–Teller coupling to the nearby 2 A u anion state. The photodetachmentphotoelectron spectra recorded with wavelengths resonant to some of these excited anion states show that the intensities of the neutral ground state vibrations are mostly determined by the autodetachment process via the excited anion resonance. This shows that the resonantphotodetachment is by far the predominant process and not excitation into continuum. The vibrational origin of the anion to neutral transition is situated at 1.860 eV±5 meV and gives directly the electron affinity of p-benzoquinone. The Feshbach and shape states form a dense electronic state ladder, which can enhance fast radiationless relaxation processes, making p-benzoquinone a very efficient electron acceptor in gas phase and in solution.