Accurate millimetre and submillimetre rest frequencies for $cis$- and
  $trans$-dithioformic acid, HCSSH
  A better understanding of sulphur chemistry is needed to solve the
interstellar sulphur depletion problem. A way to achieve this goal is to study
new S-bearing molecules in the laboratory, obtaining accurate rest frequencies
for an astronomical search. We focus on dithioformic acid, HCSSH, which is the
sulphur analogue of formic acid. The aim of this study is to provide an
accurate line list of the two HCSSH $trans$ and $cis$ isomers in their
electronic ground state and a comprehensive centrifugal distortion analysis
with an extension of measurements in the millimetre and submillimetre range. We
studied the two isomers in the laboratory using an absorption spectrometer
employing the frequency-modulation technique. The molecules were produced
directly within a free-space cell by glow discharge of a gas mixture. We
measured lines belonging to the electronic ground state up to 478 GHz, with a
total number of 204 and 139 new rotational transitions, respectively, for
$trans$ and $cis$ isomers. The final dataset also includes lines in the
centimetre range available from literature. The extension of the measurements
in the mm and submm range lead to an accurate set of rotational and centrifugal
distortion parameters. This allows us to predict frequencies with estimated
uncertainties as low as 5 kHz at 1 mm wavelength. Hence, the new dataset
provided by this study can be used for astronomical search.
