The 3Σu− ← X3Σg− electronic absorption spectrum of linear C4 in a neon matrix

The electronic absorption spectrum of linear C4 in a neon matrix has been detected. This was achieved by codepositing mass selected C4− ions with an excess of neon and UV irradiation of the matrix during or after deposition. Mass selection, experimental and spectroscopic evidence, available ab initio calculations and comparison with the spectra of C2n (n=3–7) chains lead to the conclusion that the observed band system with origin at ≈ 380 nm is due to the 3Σu−←X3Σg− transition of linear C4. Vibrational frequencies of C4 in the excited electronic state have been inferred.