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

We have observed a vibrational progression of C2H22+ with double charge tra nsfer (DCT) spectroscopy. From an exploratory calculation, the progression is assigned to the symmetric C-C stretching mode of the lowest singlet (1)D elta (g) state with linear geometry. The observed structure is well reprodu ced with the theoretical Franck-Condon (FC) factors obtained from the poten tial curves of the (1)A(g) and first-excited (1)Sigma (+)(g) states along t he normal C-C stretching coordinate. Multi-reference CI calculations reveal a predominant contribution of the (1)Delta (g) state. These results sugges t that the DCT process can probe dicationic states of polyatomic molecules like acetylene with resort to the FC principle. (C) 2001 Elsevier Science B .V. All rights reserved.