LASER PHOTOFRAGMENTATION AND COLLISION-INDUCED REACTIONS OF SIF22+ AND SIF32+

摘要

Laser photofragmentation processes of the doubly charged ions SiF22+ and SiF32+ are studied at 1064, 532, and 355 nm. The observed photoproducts are compared with those of the reaction pathways induced by the collisions of these dications with the rare gases. Photodissociation, via absorption of a single photon occurs readily for both dications, and the photoion yields are dominated by the products of neutral-loss pathways: SiF2+ from SiF22+ and SiF22+ from SiF32+. A minor contribution from photoinduced charge separation is also observed. For SiF22+, a neutral-loss pathway forming the atomic dication Si2+ is also detected at higher photon energies. The excitation energy required for this fragmentation process is determined to be 2.2 +/- 0.2 eV. Collisions of SiF22+ and SiF32+ with He and Ne are also dominated by neutral-loss reactions, With the heavier rare gases, charge transfer is exothermic and dominates the product ion yield, although neutral-loss reactions still occur. Interpretation of the charge transfer results using Landau-Zener theory suggests that two electronic states of SiF22+, lying at 31.0 +/- 0.5 eV and 35.5 +/- 0.5 eV above the ground state of the neutral molecule, are present in the dication beam. Ab initio calculations reveal SiF32+ adopts an equilibrium geometry of C-2v symmetry with one elongated bond. Hence, electron capture by SiF32+ results in the formation of SiF3+ in an energetically unfavorable conformation. By taking account of these geometry differences in the Landau-Zener algorithm, a satisfactory rationalization of the ion yields from the charge transfer reactions of SiF32+ is achieved. (C) 1997 American Institute of Physics. [References: 58]