The electronic states of Si2 and Si−2 as revealed by photoelectron spectroscopy

We have measured the photoelectron spectrum of Si−2 and find that the molecular electron affinity is EA(Si2)=2.199±0.012 eV. This spectrum apparently involves multiple electronic states of the Si−2 ion as well as several electronic states of the final neutral, Si2. In order to unravel our experimental findings, we have carried out ab initio MCSCF+1+2 CI calculations on both species. These calculations suggest that there are two nearly degenerate states for both Si2 and Si−2. We can fit our experimental data by assuming detachment from two states of Si−2 :X (2Π) [re=2.187 Å] and A (2Σ+g) [T0=0.117±0.016 eV, re=2.127 Å]. We observe two final states of Si2: X (3Σ−g) [re=2.246 Å] and A (3Πu) [T0=0.053±0.015 eV, re=2.171 Å]. These assignments are confirmed by an experimental study of the angular distributions of the photodetached electrons.