Resonance-enhanced multiple-photon ionisation spectroscopy of excited sulphur atoms

Sulfur atoms are produced in the excited 3s²3p⁴¹D₂ and 3s²3p⁴¹S₀ levels by two-photon photodissociation of CS₂ in the gas phase in the region 285–305 nm. These excited atoms are detected by three-photon (two to resonance) ionisation at selected laser wavelengths. Many new transitions have been observed which have not been reported previously. The new dark states that have been accessed by two-photon absorption have been characterised and the energy levels with respect to the ground ³P₂ state have been determined. Configuration interaction between 6p³P₁, 6p⁵P₁, and 4p¹P₁ states, and also between 6p³P₂, 6p⁵P₂, and 4p¹D₂ states has been observed. It is found that intermediate states reached from the ¹S₀ level through two-photon absorption lie above the first ionisation potential of S⁺(⁴S⁰). It is proposed that autoionisation into the ⁴S⁰+e^– continuum is a dominant mechanism in the decay of these levels, although ionisation by a further photon absorption is not ruled out.