Laser photofragmentation and collision-induced reactions of SiF22+ and SiF32+

Abstract

Laser photofragmentation processes of the doubly charged ions SiF 2 2ϩ and SiF 3 2ϩ 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: SiF 2ϩ from SiF 2 2ϩ and SiF 2 2ϩ from SiF 3 2ϩ . A minor contribution from photoinduced charge separation is also observed. For SiF 2 2ϩ , a neutral-loss pathway forming the atomic dication Si 2ϩ is also detected at higher photon energies. The excitation energy required for this fragmentation process is determined to be 2.2Ϯ0.2 eV. Collisions of SiF 2 2ϩ and SiF 3 2ϩ 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 SiF 2 2ϩ , lying at 31.0Ϯ0.5 eV and 35.5Ϯ0.5 eV above the ground state of the neutral molecule, are present in the dication beam. Ab initio calculations reveal that SiF 3 2ϩ adopts an equilibrium geometry of C 2v symmetry with one elongated bond. Hence, electron capture by SiF 3 2ϩ results in the formation of SiF 3 ϩ 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 SiF 3 2ϩ is achieved.

Silicon Compounds of Neon and Argon

“…The very low yield of ArSiF 2 2+ at larger collision energies also explains why previous studies at elevated collision energies failed to observe this channel. [23] The experimental findings are fully confirmed by a theoretical investigation of Reaction (1). A fluorine atom can be lost from the initially formed ArSiF 3 2+ intermediate in a process that is overall exothermic by 0.81 eV.…”

The SiF32+ Dication: Chemistry Counts!

“…Here, we investigate the chemical reactivity of the SiF 3 2 + dication, which had been already addressed theoretically [5] as well as experimentally [6] in the late 1990s because of its possible relevance in etching processes of silicon surfaces with fluorocarbons.…”

“…We note in passing that the previous experiments mentioned above were conducted at higher collision energies and therefore the bond-forming processes escaped observation. [6] The small signals at m/z 64 and m/z 83 are assigned to SiFOH + and SiF 2 OH + , respectively, as SiF x + cations are known to efficiently react with H 2 O present in the background to form SiF xÀ1 OH + products. [11,12] A DFT study of the reaction mechanism reveals that the formation of the complex between the reactants SiF 3 2 + and N 2 is exothermic by 3.14 eV (Figure 3).…”

Siliciumverbindungen von Neon und Argon

“…Die bei hohen Stoßenergien verschwindende Ausbeute an ArSiF 2 2+ erklärt auch, warum frü-here Studien diesen Reaktionskanal nicht auffinden konnten. [23] Diese experimentellen Befunde werden von der quantenchemischen Behandlung von Reaktion (1) [19] Der Einschub in (a) zeigt das Neutralzuwachsspektrum [20,21] der SiF 3 2+ -Isotope mit einer Massenverschiebung des zweiten Analysators von Dm = 10.5 amu, was dem bei U oct = 2 V gebildeten Produkt-Ion [8][9][10][11][12][13][14][15]22,29] sodass vergleichbare Strategien in der Zukunft zu weiteren neuen Edelgasverbindungen führen könnten. [30] Eingegangen am …”

“…Die bei hohen Stoßenergien verschwindende Ausbeute an ArSiF 2 2+ erklärt auch, warum frühere Studien diesen Reaktionskanal nicht auffinden konnten. [23] Diese experimentellen Befunde werden von der quantenchemischen Behandlung von Reaktion (1) gestützt. Ausgehend vom Begegnungskomplex ArSiF 3 2+ kann ein Fluoratom verloren werden, was insgesamt mit 0.81 eV exotherm ist.…”