Experimental and theoretical studies on SiH_n²⁺ dications (n = 1—5)

Ab initio molecular orbital calculations, executed at the MP4/6-31 G^∗∗//6-31 G^∗ level, were performed for SiH_n²⁺ 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 Q_min 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.