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 states of CS₂. The two-dimensional photoelectron spectra for the CS₂⁺(X̃ ²Π_g) and CS₂⁺(B̃ ²Σ_u⁺) bands show remarkable formation of vibrational levels excited with one quantum of the antisymmetric vibrational mode at Ehv ∼ 14.88 eV. The vibrational excitation is attributable to autoionization from a dipole-forbidden superexcited state which is formed through vibronic interaction with the 5oσ_u Rydberg state converging to CS₂⁺(C̃ ²Σ_g⁺). The forbidden superexcited state is assigned as the υ = 1 vibrational state in the ν₃ mode of the 3dσ_g Rydberg member converging to CS₂⁺(C̃ ²Σ_g⁺). Preference in the autoionization of the forbidden superexcited state is discussed.