Spectroscopic identification of the low-lying electronic states of S2 molecule

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 B3Σu- (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+ (X2Πg) 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+ (X2Πg (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.

Theoretical study on predissociation of B3Σu- of sulfur dimer

A high level ab initio study on electronic states of sulfur dimer S₂ is carried out by utilizing multi-reference configuration interaction plus Davidson correction (MRCI+Q) method. The modification to the electronic structures caused by spin-orbit coupling (SOC) effect is taken into consideration by the state-interacting method with the full Breit-Pauli Hamiltonian. The potential energy curves (PECs) of 19 Λ-S states and 52 Ω states generated from the Λ-S states are calculated. With the aid of calculated SOC matrix elements and the PECs of the Ω states, we discuss the predissociaiton of the B³Σ_u^- state. The variation of line width of B³Σ_u^-- X³Σ_g^- transitions with vibrational quantum number v' of the B³Σ_u^- are determined from Fermi golden rule calculations. Our study indicate that the predissociation of the B³Σ_u^- state is induced by the strong spin-orbit coupling with different electronic excited states, resulting in the abnormal dependence of the dissociation rate on vibrational states.

Photodissociation of S2 (X3Σg-, a1Δg, and b1Σg+) in the 320-205 nm Region

Photodissociation of vibrationally and electronically excited sulfur dimer molecules (S₂) has been studied in a combined experimental and computational quantum chemistry study in order to characterize bound-continuum transitions. Ab initio quantum chemistry calculations are carried out to predict the potential energy curves, spin–orbit coupling, transition moments, and bound-continuum spectra of S₂ for comparison with the experimental data. The experiment uses velocity map imaging to measure S-atom production following S₂ photoexcitation in the ultraviolet region (320–205 nm). A pulsed electric discharge in H₂S produces ground-state S₂ X³Σ_g^–(v = 0–15) as well as electronically excited singlet sulfur and b¹Σ_g⁺(v = 0, 1), and evidence is presented for the production and photodissociation of S₂ a¹Δ_g. In a previous paper, we reported threshold photodissociation of S₂X³Σ_g^–(v = 0) in the 282–266 nm region. In the present study, S(³P_J) fine structure branching and angular distributions for photodissociation of S₂ (X³Σ_g^–(v = 0), a¹Δ_g and b¹Σ_g⁺) via the B″³Π_u, B³Σ_u^– and 1¹Π_u excited states are reported. In addition, photodissociation of the X³Σ_g^–(v = 0) state of S₂ to the second dissociation limit producing S(³P₂) + S(¹D) is characterized. The present results on S₂ photodynamics are compared to those of the well-studied electronically isovalent O₂ molecule.