Observation of the SCI radical by resonance-enhanced multiphoton ionisation spectroscopy

Abstract

We report the first observation of any excited electronic state of the SCI radical. SCI (X ²Π) radicals are produced by photolysis of SCl₂ vapour at near-UV wavelengths around 343 nm, and detected by multiphoton ionisation spectroscopy, resonance enhanced at the two-photon energy by a Rydberg state of ²Π electronic symmetry. We suggest that this state derives from an electronic configuration involving a 4pπ Rydberg electron built on the ³∑^– ground-state ion core. Analysis of the (partially) rotationally resolved resonance-enhanced multiphoton ionisation (REMPI) spectra seems most fully consistent with all transitions originating exclusively from X ²Π_3/2 levels of both the ³²S³⁵Cl and ³²S³⁷Cl isotopomers and yields the term value for the newly identified ²Π state (ν_o= 58 220 ± 20 cm^–1) measured relative to the lower (X ²Π_3/2) spin–orbit component of the ground state. The excited state is shown to have a bond length ca. 13% shorter than that of the ground-state neutral. Parallel REMPI–photoelectron spectroscopy (PES) studies yield values of 9.57 ± 0.01 eV for the first ionisation energy (E_i) of SCI (measured relative to the lower, ²Π_3/2, spin–orbit component of the ground state), and 685 ± 30 cm^–1 for the vibrational level separation in the ground-state ion (ca. 19% higher than that of the corresponding energy interval in the ground-state neutral).