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 A region. The B <-- X
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 CCC bending mode starting from the 6b10
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 x 10^13 cm-2.
