Laboratory Optical Spectroscopy of Vibronic Transitions of the Thiophenoxy Radical

The thiophenoxy radical (C₆H₅S) is a species of possible astrophysical interest due to an electronic transition in a 5000 Å region. The $B^2{A}_2\leftarrow X^2{B}_1$ electronic transition of this radical in the discharge of thiophenol was measured using a cavity ring-down spectrometer. The optical absorption spectrum of this transition was obtained in the range covering from the origin band (0–0) to a frequency of 1750 cm⁻¹. The vibronic bands in the 400–1700 cm⁻¹ region are stronger than the origin band, suggesting structural difference between the ground and excited electronic states. The prominent progression was assigned to the 6a symmetric in-plane C–C–C bending mode starting from the $6b_0^1$ forbidden band. Band origins of individual bands were determined by analysis of the rotational profiles. Although these vibronic bands were not found in optical spectra of diffuse clouds, the upper limits of the column densities for the thiophenoxy radical in the diffuse clouds toward HD 183143 and HD 204827 were evaluated to be ∼4 × 10¹³ cm⁻².