Autoionization in photoelectron emission by solutions: Anthracene anion radicals

Abstract

Photoelectron emission (PEE) spectra (up to 5 eV) of anthracene anion radicals A− and A2− in tetrahydrofuran (THF) and hexamethyl phosphoric triamide (HMPA) exhibit three bands with maxima at photon energies (≈ 3.35, 3.8, and 4.8 eV) nearly coinciding with the energies at the maxima of absorption bands. PEE is interpreted as arising primarily from autoionization of excited states of the 3.35, 3.8, and 4.8 eV bands into the continuum located above the 1.7 eV absorption band. Direct transition to the continuum appears minor except possibly at the foot of the first PEE band. Quantum yields for A− in THF are lower than in HMPA because stronger ion pairing in THF favors geminate recombination of parents with emitted quasifree electrons. The single PEE band of A2− in THF at ≈ 3.7 eV, which nearly coincides with an absorption band, is also explained by autoionization into the continuum above the 2.1 eV absorption band. Quantum yields for A2− in THF are higher than for A− because of lower probability of geminate recombination and/or higher autoionization efficiency. Results for A2− in HMPA were not conclusive, probably because of the presence of some A−. Anthracene in THF or HMPA and the cation A+ in dichloromethane and fluorosulfonic acid do not exhibit any PEE (≤ 5 eV) because the continuum is too high to allow autoionization or direct bound-continuum transitions. These are the first reported examples, to our knowledge, of autoionization in solutions.