Gaseous Si Research Articles

Detection of SiN in IRC + 10216

The first extraterrestrial silicon-nitrogen molecule, SiN, has been detected in the outer circumstellar envelope IRC + 10216, by means of five transitions in the 1 and 3 mm spectral regions. The column density is N(SiN) = 3.8 x 10 exp 13/sq cm, and, together with recent redetermination of the SiC2 abundance, means that SiN/SiC2 is about equal to 0.05 NH3/C2H2, assuming the latter ratio is similar in the outer envelope to the value found by IR observations in the inner core. This result is probably consistent with predictions of the 1989 ion-molecule model of Herbst et al., with Si(+) the progenitor of both SiN and SiC2. The source of Si(+) is proposed to be SiH4, the carrier of gaseous Si to the outer envelope. SiH4 photodissociates first to silylene (SiH2), then to Si(+) and Si. The fraction of all silicon lost to the interstellar medium in gaseous form is between 1 x 10 exp -4 and 2 x 10 exp -3. Thus, the traditional idea that the interstellar gas-to-dust ratio of about 100 represents the condensation of all refractory elements in mass-losing evolved stars needs no revision.

Electron Impact Induced Excitation Processes Involving the Sulfur Clusters S2 to S8

AbstractIonization efficiency curves of the gaseous sulfur species Si (i = 2–8) in equilibrium with solid red HgS were obtained by electron impact ionization. Analysis of fine structure of the curves yields energy levels of ground and excited ion states. — For S2 and S8 the experimental values are in good agreement with independent photoionization and photoelectron spectroscopic data from the literature. In the case of S3 to S7 numerous appearance potentials were determined which reflect processes such as direct ionization, autoionization, fragmentation, and predissociation. Extended Hückel calculations were performed for verification of the experimental energies and for spectroscopic assignment of states. — By the second and third law methods the heats of the gas phase reactions Si(g) = i/2 S2(g) (i = 3–8) and average bond energies of the sulfur species were determined.

Bonding of fluorine in amorphous hydrogenated silicon

The infrared spectra of amorphous fluorinated silicon samples ($a$-Si: F,H) have been measured. The following lines were identified (in addition to the hydrogen-induced bands at 2090, 1985, 890, 840, and 630 ${\mathrm{cm}}^{\ensuremath{-}1}$): 1010 ${\mathrm{cm}}^{\ensuremath{-}1}$ (Si-${\mathrm{F}}_{4}$ stretching); 930 ${\mathrm{cm}}^{\ensuremath{-}1}$ (Si-${\mathrm{F}}_{2}$, Si-${\mathrm{F}}_{3}$ stretching); 828 ${\mathrm{cm}}^{\ensuremath{-}1}$ (Si-F stretching); 510 ${\mathrm{cm}}^{\ensuremath{-}1}$ (Si TO mode induced by F); 380 ${\mathrm{cm}}^{\ensuremath{-}1}$ (Si${\mathrm{F}}_{4}$ bond bending); 300 ${\mathrm{cm}}^{\ensuremath{-}1}$ (Si-F, Si-${\mathrm{F}}_{2}$ wagging). The intensities of these bands were measured as a function of preparation condition and annealing temperature. In addition to the Si${\mathrm{F}}_{4}$ molecules trapped in the films during preparation, annealing temperatures above 300 \ifmmode^\circ\else\textdegree\fi{}C produced a transformation of some Si---F groups into Si${\mathrm{F}}_{4}$ molecules. Gas-evolution experiments show that fluorine-containing molecules (F, HF, Si${\mathrm{F}}_{4}$) evolve all in one sharp peak around 680\ifmmode^\circ\else\textdegree\fi{}C, near the crystallization point. An average cross section $\overline{\ensuremath{\Gamma}}$ for Si-F vibrations of 11.2 ${\mathrm{cm}}^{2}$/mmol and bond was obtained in good agreement with similar values in gaseous and solid Si${\mathrm{F}}_{4}$.

Characterization of Charge States of Energetic Ions in Solids from AssociatedKX-Ray Production

The strong projectile---charge-state dependence of $K$ x-ray production in gases establishes a charge-state scale for ions penetrating solids. Si $K$ x-ray cross sections in gaseous Si${\mathrm{H}}_{4}$ were compared to those in solid Si for 40-MeV O (6+ to 8+) and 86-MeV Ar (6+ to 16+) projectiles. For Ar an effective charge of 11 \ifmmode\pm\else\textpm\fi{} 1 is found compared to an emergent charge of 14.8 \ifmmode\pm\else\textpm\fi{} 0.5. The result is discussed in terms of alternate models for steady-state excitation in solids.

The Reaction of Oxides with Water at High Pressures and Temperatures

AbstractThe reactions of the oxides SiO2, MoO3, and WO3 with steam at temperatures between 400 and 700 °C and pressures of 5 to 500 atm were studied by means of the transfer method. The results were evaluated by a new method. In the system Sio2/H2O the formation of gaseous Si(OH)4, Si2O(OH)6, and [SiO2(OH)2]x was found to occur in three different water‐density ranges. In the systems MoO3/H2O and WO3/H2O the gaseous compounds MoO2(OH)2 and WO2(OH)2, which were already known, exist up to densities of approximately 0.05g/cm3. At higher densities, at which the supercritical phase begins to show the properties of a liquid to an ever increasing extent, isopolymolybdic or isopolytungstic acids appear.

Thermodynamic Study of SiC Utilizing a Mass Spectrometer

The sublimation of hexagonal SiC has been studied under equilibrium conditions. The predominant gaseous species above SiC are Si, SiC2, and Si2C. By combining the heat of formation of gaseous Si from solid SiC and the known standard heat of formation of SiC, a value of 113 kcal/g atom is obtained for the heat of sublimation of Si at 298°K. From the measured partial pressures, using estimated free energy functions, dissociation energies for Si2, Si3, SiC, SiC2, Si2C, Si2C2, Si2C3, and Si3C are calculated and compared with previously known dissociation energies for group IV B molecules.