Experimental and theoretical studies on SiH n 2+ dications (n= 1–5) 1

Ab initio molecular orbital calculations, executed at the MP4/6–31G**//6–31G* level, were performed for SiHn2+ dications (n= 1–5). In line with charge-stripping mass spectroscopic experiments both SiH2+ and SiH22+ are stable ions. Depending on the chemical precursor [i.e. Si(CH3)4 or SiH4] either the singlet ground state or the lowest triplet state of SiH+ are involved in the charge-stripping process. Moreover, electron impact ionization of SiH4 directly generates two stable states of SiH2+. For the system SiH2+–SiH22+, however, there is no experimental evidence for the involvement of excited states in the charge-stripping process. SiH32+ is also an experimentally accessible dication; however, the experimentally derived Qmin value and the theoretically predicted vertical ionization energy differ significantly for SiH32+, while they are in good to excellent agreement for SiH2+ and SiH22+, respectively. Owing to the lack of suitable cationic precursors no experimental data are available for SiH42+ and SiH52+. The calculations predict these species to be weakly bound complexes of SiH22+ and atomic and molecular hydrogen respectively.