Ab initio Study of Ground-State CS Photodissociation Via Highly Excited
  Electronic States
  Photodissociation by ultraviolet radiation is the key destruction pathway for
CS in photon-dominated regions, such as diffuse clouds. However, the large
uncertainties of photodissociation cross sections and rates of CS, resulting
from a lack of both laboratory experiments and theoretical calculations, limit
the accuracy of calculated abundances of S-bearing molecules by modern
astrochemical models. Here we show a detailed \textit{ab initio} study of CS
photodissociation. Accurate potential energy curves of CS electronic states
were obtained by choosing an active space CAS(8,10) in MRCI+Q/aug-cc-pV(5+d)Z
calculation with additional diffuse functions, with a focus on the \(B\) and
\(C\,^1\Sigma^+\) states. Cross sections for both direct photodissociation and
predissociation from the vibronic ground state were calculated by applying the
coupled-channel method. We found that the \(C-X\) \((0-0)\) transition has
extremely strong absorption due to a large transition dipole moment in the
Franck-Condon region and the upper state is resonant with several triplet
states via spin-orbit couplings, resulting in predissociation to the main
atomic products C \((^3P)\) and S \((^1D)\). Our new calculations show the
photodissociation rate under the standard interstellar radiation field is
\(2.9\ee{-9}\)\,s\(^{-1}\), with a 57\% contribution from \(C-X\) \((0-0)\)
transition. This value is larger than that adopted by the Leiden
photodissociation and photoionization database by a factor of 3.0. Our accurate
\textit{ab initio} calculations will allow more secure determination of
S-bearing molecules in astrochemical models.
