Ab Initio Potential Energy Surfaces and Quantum Dynamics of Rotational   Inelastic Processes in the H+ Collision with CS
  Rate coefficient for state-to-state rotational transitions in H+ collision with CS has been obtained using accurate quantum dynamical close-coupling calculations to interpret microwave astronomical observations. Accurate three dimensional ab initio potential energy surfaces have been computed for the ground state and low-lying excited states of H+ - CS system using internally contracted MRCI method and aug-cc-pVQZ basis sets. Rotational excitation and deexcitation integral cross sections are computed at low and ultra low collision energies, respectively. Resonances have been observed at very low energies typically below 50 cm-1. Among all the transitions, Deltaj=+1 and Deltaj=-1 are found to be predominant for excitation and deexcitation, respectively. Deexcitation cross section in the ultracold region is found to obey Wigner's threshold law. The magnitude of state-to-state excitation rate obtained is maximum for j'=1 in the temperature range 2-240 K while minimum for deexcitation in ultracold region. The rotational excitation cross-section obtained using vibrationally averaged potential show rotational rainbow maximum for j'=2 state. From simple unimolecular kinetics, the mean lifetime of rotationally excited CS trap is estimated to be 550 ns due to the H+ collision at microkelvin temperature enabling precise spectroscopic measurement and studying molecular properties near quantum degeneracy. 
Rajwant Kaur and T. J. Dhilip Kumar
physics.chem-ph