Resonant states at threshold observed in electron photodetachment cross sections of polyatomic negative ions

Abstract

We have measured high resolution electron photodetachment cross sections for substituted acetophenone enolate anions using an ion cyclotron resonance spectrometer as an ion detector and a cw dye laser as a light source. In each case we find a series of narrow resonances (half-width∼1 nm) beginning at threshold which do not correlate with states appearing in limited basis set MO calculations. These resonances are attributed to the vibrational levels of a Rydberg-like electronic state supported by the dipole of the product neutral. We also report the observation of higher energy resonances in the low resolution cross sections. These resonances do correlate with states appearing in limited basis set MO calculations and we assign them as π–π* states of the anion. From the high resolution data, we report electron affinities for the following substituted acetophenone enolate radicals: m-fluoro: 2.218+0.005−0.010eV, p-fluoro: 2.176+0.005−0.010eV, p-H: 2.057+0.005−0.010eV, p-C(CH3)3: 2.032+0.005−0.010eV, m-methyl: 2.030+0.005−0.010eV.