Double charge transfer spectroscopy of acetylene dication C2H22+ at vibrational resolution

We have observed a vibrational progression of C2H22+ with double charge transfer (DCT) spectroscopy. From an exploratory calculation, the progression is assigned to the symmetric C–C stretching mode of the lowest singlet 1Δg state with linear geometry. The observed structure is well reproduced with the theoretical Franck–Condon (FC) factors obtained from the potential curves of the 1Δg and first-excited 1Σg+ states along the normal C–C stretching coordinate. Multi-reference CI calculations reveal a predominant contribution of the 1Δg state. These results suggest that the DCT process can probe dicationic states of polyatomic molecules like acetylene with resort to the FC principle.