Molecular anion formation in 9,10-anthraquinone: dependence of the electron detachment rate on temperature and incident electron energy.

Attachment of low-energy electrons to gas phase 9,10-anthraquinone (AQ) was observed with Electron Transmission Spectroscopy (ETS), and interpreted with the support of quantum chemical calculations. The ET spectrum displays three shape resonances at 0.45, 0.7 and 2.2 eV, associated with temporary electron capture into empty * molecular orbitals of AQ, the first two anion states being stable. According to TD-B3LYP calculations, the first -* core-excited resonance lies at about 1.8 eV, although no experimental evidence for this anion state was found. The long-lived parent molecular anion [AQ]– was observed by means of Electron Attachment Spectroscopy (EAS) using two different mass spectrometers and also by measuring the total anion current at the collision chamber walls. The molecular anion current shows maxima at zero energy, around 0.6 eV and at 1.8 eV. Association of these maxima with the corresponding resonant anion states is discussed. The experimentally measured electron detachment times from [AQ]– as a function of the incident electron energy and the temperature of the target molecule show a pronounced change of slope around 1.5 eV, regardless of the temperature. This unexpected behavior can be qualitatively reproduced within the framework of a multi-exponential approach which describes the electron detachment event in terms of a redistribution of the anion excess energy, regardless of the initial mechanism of temporary anion formation.