Multiphoton Ionization/Dissociation of Molecular Sulfur S₂ in the UV Region

The multiphoton ionization/dissociation dynamics of molecular sulfur (S₂) in the ultraviolet range of 205-300 nm is studied using velocity map ion imaging (VMI). In this one-color experiment, molecular sulfur (S₂) is generated in a pulsed discharge and then photodissociated by UV radiation. At the three-photon level, superexcited states are accessed via two different resonant states: the B³Σ_u^- (v' = 8-11) valence states at the one-photon level and a Rydberg state at the two-photon level. Among the decay processes of these superexcited states, dissociation to electronically excited S atoms is dominant as compared to autoionization to ionic states S₂⁺ (X²Π_g) at wavelengths λ < 288 nm. The anisotropy parameter extracted from these images reflects the parallel character of these electronic transitions. In contrast, autoionization is found to be particularly efficient at S(¹D) and S(¹S) detection wavelengths around 288 nm. Information obtained from the kinetic energy distributions of S atoms has revealed the existence of vibrationally excited S₂⁺ (X²Π_g (v⁺ > 11)) that dissociates to ionic products following one-photon absorption. This work also reveals many interesting features of S₂ photodynamics compared to those of electronically analogous O₂.