Superexcitation and subsequent decay of triatomic molecules studied by two-dimensional photoelectron spectroscopy

Photoionization and photodissociation processes of SO₂ and CS₂ in vacuum UV are studied by using two-dimensional photoelectron spectroscopy with a monochromatized synchrotron radiation source. The principal focus is on the mechanisms of autoionization and neutral dissociation of superexcited states. Photoelectron spectra of SO₂ exhibit characteristic peaks at the electron kinetic energy below 1.8 eV which are assigned as resulting from autoionizing transitions of excited atomic sulfur, S*, into the ground S⁺(⁴S°) state. These S* atoms are in the singlet Rydberg states converging to S⁺(²D°). The precursor molecular states, SO*₂, are considered to be multiple-electron excited Rydberg states lying at the photon energy above approximately 22 eV. The onset of the photoelectron yield due to the atomic autoionization accords with that expected from the thermochemical threshold for the formation of S* through three-body dissociation SO*₂→S*+O+O. The two-dimensional photoelectron spectrum of CS₂ provides tangible evidence for the formation of a dipole-forbidden Rydberg state (6σ_g)^-1(3dσ_g)¹¹Σ_g⁺ at the photon energy of 14.88 eV which autoionizes into the v₃ = 1 vibrational state of the antisymmetric stretch v₃ mode of CS₂⁺ (X²Π_g,Ω, Ω = 1/2 and 3/2). This Rydberg state is expected to borrow substantial oscillator strength from the (6σ_g)^-1(5pσ_u)¹¹Σ_u⁺ state through vibronic coupling involving the v₃ vibration.