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 mon ochromatized synchrotron radiation source. The principal focus is on the me chanisms 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 autoionizi ng transitions of excited atomic sulfur, S*, into the ground S+(S-4 degrees ) state. These S* atoms are in the singlet Rydberg states converging to S*( D-2 degrees). The precursor molecular states, SO2*, are considered to be mu ltiple-electron excited Rydberg states lying at the photon energy above sim ilar to 22 eV. The onset of the photoelectron yield due to the atomic autoi onization accords with that expected from the thermochemical threshold for the formation of S* through three-body dissociation SO2*-->S*+O+O. The two- dimensional photoelectron spectrum of CS, provides tangible evidence for th e formation of a dipole-forbidden Rydberg state (6 sigma (g))(-1)(3d sigma (g))(11)Sigma (+)(g) at the photon energy of 14.88 eV which autoionizes int o the v(3) = 1 vibrational state of the antisymmetric stretch v(3) mode of CS2+ (X(2)Pi (g,Ohm), Ohm = 1/2 and 3/2). This Rydberg state is expected to borrow substantial oscillator strength from the (6 sigma (g))(-1)(5p sigma (u))(11)Sigma (+)(u) state through vibronic coupling involving the v(3) vi bration. (C) 2000 Elsevier Science B.V. All rights reserved.