Formation and autoionization of a dipole-forbidden superexcited state of CS2

Two-dimensional photoelectron spectroscopy has been performed in the photon energy region of 14.60-15.35 eV, to investigate forbidden superexcited sta tes of CS2. The two-dimensional photoelectron spectra for the CS2+((X) over tilde (2)Pi (g)) and CS2+((B) over tilde (2)Sigma (+)(u)) bands show remar kable formation of vibrational levels excited with one quantum of the antis ymmetric vibrational mode at E-hv similar to 14.88 eV. The vibrational exci tation is attributable to autoionization from a dipole-forbidden superexcit ed state which is formed through vibronic interaction with the 5p sigma (u) Rydberg state converging to CS2+((C) over tilde (2)Sigma (+)(g)). The forb idden superexcited state is assigned as the upsilon = I vibrational state i n the upsilon (3) mode of the 3d sigma (g) Rydberg member converging to CS2 +((C) over tilde (2)Sigma (+)(g)), Preference in the autoionization of the forbidden superexcited state is discussed.