Ab initio molecular orbital calculations, executed at the MP4/6-31 G**//6-31 G* level, were performed for SiHn2+ dications ( n = 1-5). In line with charge-stripping mass spectroscopic experiments both SiH2+ and SiH,2+ are stable ions. Depending on the chemical precursor [i.e. Si(CH,), or SiH,] either the singlet ground state or the lowest triplet state of SiH+ are involved in the charge-stripping process. Moreover, electron impact ionization of SiH, directly generates t w o stable states of SiH2'. For the system SiH2+-SiH2+, however, there is no experimental evidence for the involvement of excited states in the charge-stripping process. Si H: + is also an experimentally accessible dication; however, the experimentally derived Qmi, value and the theoretically predicted vertical ionization energy differ significantly for S i H t + , while they are in good to excellent agreement for SiH2+ and SiH,Z+, respectively. Owing to the lack of suitable cationic precursors no experimental data are available for SiH,2+ and SiH,Z+. The calculations predict these species to be weakly bound complexes of SiH,Z+ and atomic and molecular hydrogen respectively.
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