Single site double core level ionisation of OCS

Single site O1s, C1s and S2p double ionisation of the OCS molecule has been investigated using a magnetic bottle multi-electron coincidence time-of-flight spectrometer. Photon energies of 1300, 750 and 520 eV, respectively, were used for the ionisation, and spectra were obtained from which the double core ionisation energies could be determined. The energies measured for 1s double ionisation are 1172 eV (O1s^-2) and 659 eV (C1s^-2). For the S2p double ionisation three dicationic states are expected, ³P, ¹D and ¹S. The ionisation energies obtained for these states are 373 eV (³P), 380 eV (¹D) and 388 eV ( ¹S). The ratio between the double and single core ionisation energies are in all cases equal or close to 2.20. Auger spectra of OCS, associated with the O1s^-2, C1s^-2 and S2p^-2 dicationic states, were also recorded incorporating both electrons emitted as a result of the filling of the two core vacancies. As for other small molecules, the spectra show an atomic-like character with Auger bands located in the range 480-560 eV for oxygen, 235-295 eV for carbon and 100-160 eV for sulphur. The interpretation of the spectra is supported by CASSCF and CASCI calculations. The cross section ratio between double and single core hole creation was estimated as 3.7 × 10^-4 for oxygen at 1300 eV, 3.7 × 10^-4 for carbon at 750 eV and as 2.2 × 10^-3 for sulphur at 520 eV.