Si2 Molecule Research Articles

High-Resolution Laser Spectroscopic Survey of the H3Σu--X3Σg- Electronic Transition of Si2.

Rotationally resolved spectra of the H3Σu--X3Σg- electronic transition bands of Si2 have been experimentally studied using laser-induced fluorescence in the 380-520 nm range. Si2 molecules are produced in a supersonically expanding planar plasma by discharging a silane/argon gas mixture. In total, 44 bands belonging to the H3Σu--X3Σg- electronic transition system of the most abundant isotopologue 28Si2 are experimentally recorded. With a spectral resolution of ∼0.04 cm-1, the triplet spin-splitting structures in individual rotational transition lines are fully resolved. Detailed analyses on the high-resolution spectra have yielded an accurate determination of spectroscopic constants for both X3Σg- and H3Σu- states. The spin-spin interaction constants for the two triplet states are found to be comparable (λ ≈1.5 cm-1), which may originate from the 3p atomic orbital interaction in the triplet Si2 molecule. The measured isotopologue spectra of 29Si28Si and 30Si28Si indicate that the H3Σu--X3Σg- transition system of 29S28S and 30S28S can be reasonably reproduced by the isotope mass-scaling rule. Spectroscopic parameters, including the Franck-Condon factors, the Einstein coefficients, and the oscillator strengths, are also determined from the experimental results and the Rydberg-Klein-Rees (RKR) calculations. The agreement between the experimentally measured and calculated dispersed fluorescence spectra indicates that the RKR calculations with the molecular constants determined in this work can accurately reproduce the diatomic potentials of both states. These molecular data provide a benchmark in high-level theoretical studies on Si2 and likely other small silicon clusters.

Atomization enthalpy and enthalpy of formation of gaseous Si2 and Si3 from mass spectrometric equilibrium measurements

Recent experimental and theoretical studies have allowed us to calculate new thermal functions for the Si2 and Si3 molecules. We have determined the atomization enthalpies and enthalpies of formation of Si2 and Si3 from mass spectrometric data and have combined them with reevaluated previously published mass spectrometric data using a consistent set of ionization cross sections and thermal functions. The selected atomization enthalpy and enthalpy of formation for the Si2 molecule are ΔHa,0=319±7 and ΔHf,0=584±7 kJ mol−1, the corresponding values for Si3 are 705±16 and 649±16 kJ mol−1. Reevaluated values for Si4 are 1151±22 and 654±22 kJ mol−1. Experimental literature values (Neumark and co-workers) for the electron affinities of Si2–Si4 have been combined with the present results to obtain enthalpies of formation and binding energies for the anions Si−2–Si−4. The experimental atomization enthalpies are compared to recent theoretical studies.

Magneto-infrared spectra of the Si2, Ge2, and Sn2 molecules in rare-gas matrices

Absorption spectra of Si2, Ge2, and Sn2, observed between 50 and 5000 cm−1 in neon and argon matrices at 4 K, were assigned to the lowest 3Πu←X 3Σg− electronic transition. In all three cases, the Franck–Condon envelope of vibrational bands allowed T00, ωe′, ωe′xe′ to be determined. In the Si2 molecule [near Case (a)] T00=313 cm−1 and each vibrational band was split into Ω triplets (0,1,2) in the upper state. A Zeeman effect on these lines was detected in magnetic fields up to 4 T. As the spin–orbit coupling becomes larger in Ge2 and Sn2, the 1u←X0+g transition [case (c)] was observed, and the corresponding parameters in the upper state determined. For Sn2 the magnetic-dipole 1g←X0+g transition was also detected and identified by the splitting and broadening of the band in magnetic fields up to 4 T. The results are discussed in the context of the ab initio calculations and other experimental results on these molecules.

Observation of Si2 in a chemical vapor deposition reactor by laser excited fluorescence

For the first time an intermediate chemical species, Si2, has been observed in the gas phase during the chemical vapor deposition of silicon from silane. This observation is inconsistent with previous theoretical models for silicon deposition, which assume that either thermodynamic equilibrium or surface kinetics determines the silicon deposition rate. It is consistent with the predictions of a new model for chemical vapor deposition by Coltrin et al., in which gas phase chemical kinetics play an important role in the deposition mechanism. The Si2 molecule could be a precursor to the nucleation of silicon particulates in the gas phase.

Potential energy curves and transition moments for the low-lying electronic states of the Si2 molecule

The potential energy curves for the low-lying electronic singlet, triplet and quintet valence-shell states in Si2 are determined by large-scale ab initio MRD CI calculations and intensities are calculated for a few important transitions. Comparison with experimental data shows that the analogues to the prominent bands in C2 lie at longer wavelengths (lower energies) in Si2 than have been investigated in spectroscopic measurements so far. The observed bands are largely assigned; the calculated data should give reliable guidelines for the further spectroscopic study of low-lying Si2 triplet bands as well as transitions within the singlet manifold which have not been observed so far.

Dissociation energy of Si2 molecule

The true potential energy curves have been constructed for the different electronic states of the Si2 molecule by the methods ofLakshman andRao as well asMorse. The dissociation energy for the ground state of the Si2 molecules has been estimated to be 3.28 eV by the method of curve fitting using the three parameterLippincott potential function. The estimatedDe is in good agreement with the mass spectrometric value 3.25±0.22 eV as well asGaydon’s value 3.35±0.2 eV.

The accuracy of first-order density matrices obtained by electronic structure methods involving the valence electrons only

First-order density matrices are obtained from all-electron unrestricted-Hartree–Fock (UHF) calculations and from effective-core-potential valence-electron UHF calculations for the silicon and argon atoms, and for the Si2 molecule. The density matrices are compared by means of their derived X-ray scattering factors, directional Compton profiles, and the charge density, at selected points in k and r space.

New Absorption Systems of the Si2 Molecule in the Vacuum Ultraviolet Region

Two new band systems attributed to the Si2 molecule have been obtained in absorption in flash discharges in SiH4. They are situated in the region 1 840-1 900 Å. They are here designated O 3Σu- - X 3Σg- and P 3Πg - D 3Πu. Rotational and vibrational constants have been derived for the O 3Σu- state. The 1-0 band of the N 3Σu--X 3Σg- system at 2118 Å has been reinvestigated.

THE SPECTRUM OF THE Si2 MOLECULE

Two band systems attributed to the Si2 molecule have been excited in a discharge tube. The first of these is a 3Π – 3Π system of red-shaded bands with heads at 3489, 3556, 3626, and 3696 Å. The second is an extensive system of 3Σ–3Σ bands in the visible and near ultraviolet region. High resolution spectra have been analyzed and the rotational and vibrational constants have been determined.