The Electronic Spectrum of Si 2 .

The optical spectrum of Si 2 + is presented. The two electronic band systems observed near 430 and 270 nm correspond to the two lowest optically allowed transitions of Si 2 + assigned to 4 Σ u - (I) ← X 4 Σ g - and 4 Σ u - (II) ← X 4 Σ g - . The spectra were measured via photodissociation spectroscopy of mass-selected ions at the level of vibrational resolution, and the determined spectroscopic constants provide detailed information about the geometric and electronic structure, establishing molecular constants of this fundamental diatomic cation that enable astrophysical detection on, for example, hot rocky super-Earth-like exoplanets.