Marco Savoca

Vibrational Spectra and Structures of Neutral SimCn Clusters (m + n = 6): Sequential Doping of Silicon Clusters with Carbon Atoms

Vibrational spectra of mixed silicon carbide clusters Si(m)C(n) with m + n = 6 in the gas phase are obtained by resonant infrared-vacuum-ultraviolet two-color ionization (IR-UV2CI for n ≤ 2) and density functional theory (DFT) calculations. Si(m)C(n) clusters are produced in a laser vaporization source, in which the silicon plasma reacts with methane. Subsequently, they are irradiated with tunable IR light from an IR free electron laser before they are ionized with UV photons from an F(2) laser. Resonant absorption of one or more IR photons leads to an enhanced ionization efficiency for Si(m)C(n) and provides the size-specific IR spectra. IR spectra measured for Si(6), Si(5)C, and Si(4)C(2) are assigned to their most stable isomers by comparison with calculated linear absorption spectra. The preferred Si(m)C(n) structures with m + n = 6 illustrate the systematic transition from chain-like geometries for bare C(6) to three-dimensional structures for bare Si(6). In contrast to bulk SiC, carbon atom segregation is observed already for the smallest n (n = 2).

Infrared Spectrum of the Si3H8+ Cation: Evidence for a Bridged Isomer with an Asymmetric Three‐Center Two‐Electron SiHSi Bond

The IR spectrum of Si3H8(+) ions produced in a supersonic plasma molecular beam expansion of SiH4, He, and Ar is inferred from photodissociation of cold Si3H8(+)-Ar complexes. Vibrational analysis of the spectrum is consistent with a Si3H8(+) structure (2(+)) obtained by a barrierless addition reaction of SiH4 to the disilene ion (H2Si=SiH2(+)) in the silane plasma. In this structure, one of the electronegative H atoms of SiH4 donates electron density into the partially filled electrophilic π orbital of the disilene cation. The resulting asymmetric Si-H-Si bridge of the 2(+) isomer with a bond energy of approximately 60 kJ mol(-1) is characteristic for a weak three-center two-electron bond, which is identified by its strongly IR active asymmetric Si-H-Si stretching fundamental at about 1765 cm(-1). The observed 2(+) isomer is calculated to be only a few kJ mol(-1) less stable than the global minimum structure of Si3H8(+) (1(+)), which is derived from vertical ionization of trisilane. Although more stable, 1(+) is not detected in the measured IR spectrum of Si3H8(+)-Ar, and its lower abundance in the supersonic plasma is rationalized by the production mechanism of Si3H8(+) in the silane plasma, in which a high barrier between 2(+) and 1(+) prevents the efficient formation of 1(+). The potential energy surface of Si3H8(+) is characterized in some detail by quantum chemical calculations. The structural, vibrational, electronic and energetic properties as well as the chemical bonding mechanism are investigated for a variety of low-energy Si3H8(+) isomers and their fragments. The weak intermolecular bonds of the Ar ligands in the Si3H8(+)-Ar isomers arise from dispersion and induction forces and induce only a minor perturbation of the bare Si3H8(+) ions. Comparison with the potential energy surface of C3H8(+) reveals the differences between the silicon and carbon species.

Vibrational spectra and structures of bare and Xe-tagged cationic SinOm+ clusters

Vibrational spectra of Xe-tagged cationic silicon oxide clusters Si(n)O(m)(+) with n = 3-5 and m = n, n ± 1 in the gas phase are obtained by resonant infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory calculations. The Si(n)O(m)(+) clusters are produced in a laser vaporization ion source and Xe complexes are formed after thermalization to 100 K. The clusters are subsequently irradiated with tunable light from an IR free electron laser and changes in the mass distribution yield size-specific IR spectra. The measured IRMPD spectra are compared to calculated linear IR absorption spectra leading to structural assignments. For several clusters, Xe complexation alters the energetic order of the Si(n)O(m)(+) isomers. Common structural motifs include the Si2O2 rhombus, the Si3O2 pentagon, and the Si3O3 hexagon.

IR Spectrum and Structure of Protonated Monosilanol: Dative Bonding between Water and the Silylium Ion

We report the spectroscopic characterization of protonated monosilanol (SiH3 OH2+ ) isolated in the gas phase, thus providing the first experimental determination of the structure and bonding of a member of the elusive silanol family. The SiH3 OH2+ ion is generated in a silane/water plasma expansion, and its structure is derived from the IR photodissociation (IRPD) spectrum of its Ar cluster measured in a tandem mass spectrometer. The chemical bonding in SiH3 OH2+ is analyzed by density functional theory (DFT) calculations, providing detailed insight into the nature of the dative H3 Si+ -OH2 bond. Comparison with protonated methanol illustrates the differences in bonding between carbon and silicon, which are mainly related to their different electronegativity and the different energy of the vacant valence pz orbital of SiH3+ and CH3+ .