Thiazyl chloride: an experimental and theoretical study of the valence shell HeI photoelectron spectrum

High level (CASSCF-MRCI) ab initio calculations are used to investigate the structural, electronic and vibrational properties of the electronic ground state of thiazyl chloride (NSCI) and of the low-lying electronic states of NSCl+. A new high resolution HeI photoelectron spectrum of NSCI has been recorded in the 10-16 eV energy region. From the results of the calculations, the first band is assigned to the (1)(2)A' state of NSCl+. The second one corresponds to the (2)(2) A' and (1)(2)A" states which are quasi-degenerate. Despite the high resolution, the two first bands show no vibrational fine structure. For the first one, Franck-Condon analysis shows that it is due to the overlapping of two vibrational progressions involving the S-Cl stretching and the NSCl bending modes. In the case of the second band, it is explained by the highly repulsive character of the potential energy surfaces of two states of NSCl+ in the Franck-Condon region of the neutral molecule. For the third band, which shows vibrational peaks, the calculation of Franck-Condon factors permits the determination of the adiabatic ionisation energy of the (3)(2)A' electronic state of NSCl+ at 13.798 eV. Finally, the fourth band, which is due to three different ionic states with vibrational progressions, is too complicated to be assigned quantitatively. (C) 2003 Published by Elsevier Science B.V.