First principles investigation of19F* nuclear quadrupole interaction in fourth group tetrafluorides

Using the Hartree-Fock-Roothaan procedure the nuclear quadrupole interactions of the¹⁹F (spin 5/2) nucleus in CF₄, SiF₄ and GeF₄ are studied. The theoretical results explain within 10% the observed experimental measurements by the time-differential perturbed angular distribution technique, including the important feature of sharp decrease of the¹⁹F^* nuclear quadrupole coupling constant in going from CF₄ to SiF₄, followed by an increase in going to GeF₄, while a simple consideration of the ionic characters of the C-F, Si-F and Ge-F bonds together with Townes and Dailey theory would lead to a continuous decrease from CF₄ to GeF₄. The dependence of the results on the choice of basis sets and the role of many-body effects is discussed.     

Molecular constants of SiF4, GeF4, and RuO4: An improvement of the analysis of the ir-band contours of ν3(F2) by low temperature measurements and by using isotopically pure compounds

The ir-band contour of ν₃(F₂) of gaseous SiF₄, GeF₄, and RuO₄ have been analyzed making use of isotope substitution techniques, low temperature measurements, and matrix isolation spectroscopy. In the case of RuO₄ values for the hot band progression ν₃ + nν₄ ? nν₄ (X₃₄ = ?1.1 ± 0.1 cm^?1 for ⁹⁹RuO₄ and ¹⁰⁴RuO₄) have been obtained and F₂ block force constants have been calculated using isotope shifts Δω₃ and ζ₃ constants as additional data.     

The photoelectron spectra of the tetrafluoro and tetramethyl compounds of the group IV elements

The photoelectron spectra of CF₄, SiF₄, GeF₄, C(CH₃)₄, Si(CH₃)₄, Ge(CH₃)₄, Sn(CH₃)₄, and Pb(CH₃)₄ h     

Structures of vibrational absorption bands of the SiF4 molecule in a low-temperature nitrogen matrix

We have studied the IR absorption spectra of diluted mixtures SiF₄/M = 1/6000–1/10 000 in an N₂ matrix at 11 K (for comparison, the spectra of SiF₄ in Ar and Xe matrices have also been studied). It has been shown that, in solid nitrogen, the appearance of doublets is observed both in the range of the ν₃ band of the SiF₄ (²⁸SiF, ²⁹SiF₄, and ³⁰SiF₄) isotopologues of the SiF₄ molecule and in the range of the ν₁ + ν₄, ν₂ + ν₃, and ν₁ + ν₃ bands of ²⁸SiF₄, whereas, in the range of the 2ν₃ band of ²⁸SiF, a triplet appears. In order to analyze the influence of the matrix on the spectrum of free SiF₄ molecules, we have used a model that makes it possible to successively calculate (i) the spectrum of SiF₄ in terms of the model of local modes, (ii) the structure of a matrix composed by 864 N₂ molecules + a rigid SiF₄ molecule using the Monte Carlo method, and (iii) the interaction of matrix particles with local dipole moments in the approximation of dipole-induced dipole and dipole-quadrupole interactions. The model describes satisfactorily the low-frequency shift of bands in the nitrogen matrix. All obtained experimental and theoretical results are consistent with the assumption that two kinds of stable trapping centers of SiF₄ molecules obeying the T _d symmetry are realized in the nitrogen matrix.     

Theoretical total ionization cross-sections of CH x,CF x,SiH x,SiF x ( ) and CCl 4 targets by electron impact

Total ionization cross-sections of electron impact are calculated for the molecular targets CH_x, CF_x, SiH_x, SiF_x (x = 1-4) and CCl₄ at incident energies 20-3 000 eV. The calculation is based on Complex Scattering Potential approach, as developed by us recently. This leads to total inelastic cross-sections, from which the total ionization cross-sections are extracted by reasonable physical arguments. Extensive comparisons are made here with the previous theoretical and experimental data. The present results are satisfactory except for the CF_x and SiF_x (x = 1-3) radicals, for which the experimental data are lower than most of the theories by more than 50%. Received 23 May 2002 / Received in final form 24 October 2002 Published online 21 January 2003 RID="a" ID="a"e-mail: knjoshipura@yahoo.com     

The ν4 fundamental of three isotopic species of CF4

The vibration-rotation spectra of the ν₄ fundamental of ¹²CF₄, ¹³CF₄, and ¹⁴CF₄ have been observed with 0.06 cm^?1 resolution. A least squares fit of the data has been used to evaluate the pertinent molecular constants. The band centers are 631.199, 629.285, and 627.348 cm^?1 for ¹²CF₄, ¹³CF₄, and ¹⁴CF₄, respectively. The Coriolis constant ζ₄ has been estimated for the three isotopes.     

Structure and properties of the anions MF4−, MCl4− and MBr4− (M = C,Si, Ge)

Density functional theory (DFT), Møller–Plesset (MP2) and coupled cluster with single and double substitutions including non-iterative triple excitations (CCSD(T)) calculations on the anions MX₄^?, with M = C, Si, Ge and X = F, Cl, Br, show that GeF₄^?, SiCl₄^?, GeCl₄^? and SiBr₄^? prefer a C_2v conformation, but CCl₄^? is an elongated C_3v structure. CBr₄^? has T_d symmetry in MP2, but is slightly more stable in elongated C_3v form with DFT and CCSD(T). GeBr₄^? has T_d symmetry. CF₄^? and SiF₄^? are unstable with respect to loss of an electron. Vertical electron affinities (EAs) are negative also for CCl₄ and SiCl₄, and close to zero for GeF₄ and SiBr₄. Adiabatic EAs range from 0.47 eV for SiCl₄ to 1.78 eV for GeBr₄. The lowest excited states at T_d symmetry are ²T₂ resonances with energies of 2.1–3.5 eV, resulting from excitation of the a₁ singly occupied molecular orbital to vacant t₂ orbitals. Vertical excitation energies (VEEs) and vibrational frequencies are given for the most stable anionic geometries. Comparison with experimental VEEs for CCl₄^? is made. From dissociation energies of MX₄, MX₄^?, MX₃ and MX₃^?, appearance energies of X^?, MX₃^?, X₂^? and MX₂^? were calculated. Most were found to be in reasonable agreement with experimental values. Theoretical spin densities and g-factors have been compared with experimental results available for CCl₄^?, SiCl₄^? and GeCl₄^?.     

Cs2XF6 (X=Si,Ge) compounds: Common and different features as uncovered by the first-principles calculations

Results of ab initio calculations of the electronic, optical and elastic constants for Cs₂GeF₆ and Cs₂SiF₆ are reported for the first time. Both compounds are the direct band gap dielectrics, with the calculated band gaps 6.926 eV (Cs₂SiF₆) and 6.417 eV (Cs₂GeF₆). Analysis of the calculated elastic constants and Cauchy condition shows both crystals as slightly covalent compounds. However, with increased pressure chemical bonds in Cs₂GeF₆ turn to be more ionic, whereas in Cs₂SiF₆ the covalent character of chemical bonds is enhanced. Pressure dependence of the chemical bond lengths and lattice constants was determined and represented as the second power functions of external pressure. Different trends in the values of the Mulliken charges for both compounds were found. The obtained results are applicable to the analysis of the luminescence properties of impurity ions at varying pressure or to the microscopic studies of crystal field effects in these crystals.     

E.S.R. spectra of SiF3 radicals produced in a single crystal of SiF4

E.S.R. spectra of a γ-irradiated single crystal of SiF₄ were investigated. The spectra observed were attributed to SiF₃ radicals having ²⁸Si (I=0) and ²⁹Si (I=1/2) atoms. From the angular dependence of the spectral lines on rotation of the single crystal, hyperfine tensors were determined for three fluorine atoms and the ²⁹Si atom of the SiF₃ radical. The three fluorine atoms in the radical are equivalent, whereas the directions of their hyperfine tensors are different from one another owing to the pyramidal structure of the radical. In addition to the hyperfine analysis, the analysis of the superhyperfine structure due to neighbouring fluorine atoms gave information on the orientation of the radicals in the crystal and the mechanism of radical formation. The structure of the radical is discussed in comparison with that of the CF₃ radical.     

Chemical sputtering,a discussion of mechanisms

Sputtering can be defined as the process whereby particles leave the surface as a direct consequence of the presence of incident radiation. When particles leave the surface as a result of receiving momentum from the collision cascade induced by the incident radiation, the process is called “physical sputtering”. If the incoming radiation (ions, electrons, or photons) induces a chemical reaction which leads to the subsequent desorp-tion of particles, the process could be classified as “chemical sputtering”. There are a number of molecules such as CH₄, CF₄, CF₃H, CF₃CI, etc., whose binding energy to a large variety of surfaces is believed to be only a few kcal/mole. Therefore, these molecules will not remain absorbed at room temperature. Consequently, if they are generated from surface atoms by radiation-induced processes, they will almost immediately desorb into the gas phase. This process is one type of chemical sputtering. Recent data obtained in plasma environments suggest that this type of reaction is a widely occurring phenomena: however, few systematic quantitative investigations of the subject have been completed. In this paper we will review the evidence for chemical sputtering and discuss mechanisms based on experimental information obtained for the chemical sputtering of silicon and SiO₂ under argon ion bombardment in the presence of a molecular beam of XeF2. Under these conditions, 25 or more silicon atoms can leave the surface per incident argon ion. About 75% of the silicon is emitted as SiF₄ (gas) and the rest leaves as silicon atoms or SiFx radicals. The total yield (silicon plus fluorine) is greater than 100 atoms/ion. The measured yields are a strong function of XeF₂ flux and a much weaker function of ion energy in the range 500-5000 eV. The chemical-sputtering yield for SiO₂ is smaller than that of silicon by about an order of magnitude, but it is still larger than the physical-sputtering yield. Moreover, SiO₂ is also sputtered by electrons. These results indicate that the incident radiation induces a chemical reaction between silicon and adsorbed fluorine which produces SiF₄, and the SiF4 is subsequently desorbed into the gas phase. We define this process as chemical sputtering. The large yields are probably a consequence of weak binding between the surface and the SiF₄ molecule.     

Medium effects on the fluorine chemical shifts of non-polar molecules in liquids

The gas-to-infinite dilution fluorine chemical shifts of CF₄, SiF₄, SF₆, C₆F₆, p-fluorotoluene and p-difluorobenzene have been measured for a series of non-polar solvents. Downfield displacements ranging from 3–16 p.p.m. have been observed. Comparisons with proton solvent shifts make it evident that the London dispersion forces are the principal agent in causing the shifts. However, unlike proton resonance where the local diamagnetic shielding is mostly affected, it is probably through the local paramagnetic shielding that solvents alter fluorine chemical shifts.     

High-resolution spectroscopy of the 16-μm bending fundamental of CF4

The 16-μm bending fundamentals (ν₄) of ¹²CF₄, ¹³CF₄, and ¹⁴CF₄ have been observed at Doppler-limited resolution using a tunable PbSnSe semiconductor diode laser. The tetrahedral splittings of the rotational manifolds have been observed in all three branches, and in particular the dense and partially overlapping transitions in the Q branches have been resolved and assigned. A least-squares fit of the Hamiltonian, including off-diagonal terms, yielded five scalar and three tensor spectroscopic constants for each of the three isotopes. From these constants the upper-state rotational constant B₄ and the Coriolis constant ζ₄ have been calculated, together with some of the other molecular constants. An absorption feature at about 0.18 cm^?1 to the red of the main Q branch of each isotopic species has been identified as the Q branch of (ν₂ + ν₄) ? ν₂, which is the transition that lases when CF₄ is pumped by a CO₂ laser at 9.4 μm (i.e., in ν₂ + ν₄).     

Dipole oscillator strengths,dipole properties and dispersion energies for SiF4

A recommended isotropic dipole oscillator strength distribution (DOSD) has been constructed for the silicon tetrafluoride (SiF₄) molecule through the use of quantum mechanical constraint techniques and experimental dipole oscillator strength data. The constraints are furnished by experimental molar refractivity data and the Thomas-Reiche-Kuhn sum rule. The DOSD is used to evaluate a variety of isotropic dipole oscillator strength sums, logarithmic dipole oscillator strength sums and mean excitation energies for the molecule. A pseudo-DOSD for SiF₄ is also presented which is used to obtain reliable results for the isotropic dipole-dipole dispersion energy coefficients C₆, for the interaction of SiF₄ with itself and with 43 other species and the triple-dipole dispersion energy coefficient C₉ for (SiF₄)₃.     

Influence of chemical bond length changes on the crystal field strength and “ligand-metal” charge transfer transitions in Cs2GeF6 doped with Mn and Os ions

Detailed study of dependence of the crystal field strength 10Dq and lowest charge transfer (CT) energies for different interionic distances in Cs₂GeF₆:Mn⁴⁺ and Cs₂GeF₆:Os⁴⁺crystals is presented. The calculations were performed using the first-principles discrete-variational Dirac-Slater (DV-DS) method. As a result, the functional dependencies of 10Dq and lowest CT energy on the metal-ligand distance R were obtained without any fitting or semiempirical parameters. It was shown that 10Dq depends on R as 1/Rⁿ, with n=4.0612 and 4.3874 for Cs₂GeF₆:Mn⁴⁺ and Cs₂GeF₆:Os⁴⁺, respectively. Two approximations (linear and quadratic) are obtained for the dependence of the lowest CT energy on R; CT energy decreases when R increases with dE(CT)/dR=−638 and −1080 cm⁻¹/pm for Cs₂GeF₆:Mn⁴⁺ and Cs₂GeF₆:Os⁴⁺, respectively, if the linear approximation is used. These values can be used for estimations of the lowest CT energies for Mn⁴⁺ and Os⁴⁺ ions in other hosts with fluorine ligands. Estimations of the electron-vibrational interaction (EVI) constants, Huang-Rhys parameters, and Stokes shifts for all the above-mentioned crystals were performed using the obtained 10Dq and E(CT) functions.     

Anomalous Broadening of CF4 Molecule Lines. Observation of Carbon Tetrafluoride Hydrates?

Spectroscopic studies of pure carbon tetrafluoride and carbon tetrafluoride in the presence of water vapor have been carried out. Studies have revealed changes in the absorption spectrum of the 1280 cm^‒1 band of CF₄, indicating the formation of new molecules, CF₄–H₂O hydrates. The bond between CF₄ and H₂O is not chemical in nature. The formation of these molecules can accelerate the removal of carbon tetrafluoride from the atmosphere with precipitation in the form of rain or snow.

     

Temperature Dependence of Integrated Intensities of Vibrational Bands in M2GeF6 IR-Absorption Spectra

The paper presents experimental results of studies on the temperature dependence of integrated intensities of vibrational bands corresponding to ₃(GeF ₆) in M₂GeF₆ IR-spectra. It is shown that unlike Na₂GeF₆, Rb₂GaF₆, and Cs₂GeF₆, where the dependence of intensity on temperature is monotonous, in the case of K₂GeF₆ there exist anomalous temperature ranges of 220-240 and 150-170 K, where the intensity of a band, corresponding to ₃(GeF^2- ₆) changes abruptly. The study of heat evaluation, heat capacity, unit cell parameters, and intensities of selective reflexes in K₂GeF₆ X-ray diffraction patterns indicates the presence in this compound, as distinguished from others in the seris considerod, of phase transitions of the first order. On the basis of IB, ¹⁹F NNR, X-rat and calorimetric data, a possible mechanism of polymorphic transformation is discussed.     

Resonance dipole-dipole coupling and Fermi resonance in CF4 dimers

The FTIR spectra of Ar(Ne)/CF₄ matrixes have been recorded at matrix concentrations varying in the range 10 000-600 cm⁻¹. The absorptions due to the CF₄ monomers and to the (CF₄)₂ resonance dimers in the spectra of the two matrixes have been identified. The intramolecular Fermi resonance in the spectra of the CF₄ monomers trapped in solid argon and neon has been analyzed. The spectra of the (CF₄)₂ dimers are determined by the strong resonance dipole-dipole interaction of the two interacting molecules and by strengthening or weakening of the intramolecular Fermi resonance interaction.     

The microwave spectrum,harmonic force field,and structure of difluorodichloromethane

Very large numbers of rotational transitions have been accurately measured for ¹²CF₂³⁵Cl₂, ¹²CF₂³⁵Cl³⁷Cl, and ¹³CF₂³⁵Cl₂, and have been analyzed for rotational constants and quartic centrifugal distortion constants. The distortion constants have been combined with vibrational wavenumbers (both from the literature and from the present work), and with ab initio force constants also evaluated in the present work, to give an approximate harmonic force field. The rotational constants and force field have been used to evaluate ground state effective, substitution, and ground state average structures for the molecule.     

Absorption spectrum of the (CF4) dimer in liquid argon solution

The concentration dependence of the shape of absorption bands in the spectrum of CF₄ in liquid argon is studied in the concentration range (0.01–17)×10^?3 molar fractions at 93 K. In all spectral regions related to ν₃, the shape of the spectral function is determined, along with the Fermi resonance 〈ν_i,ν₃+1,ν₄|≈〈νi,ν₃,ν ₄+2|, by the resonance dipole-dipole interaction. In the spectral region of the Fermi doublet ν ₁+ν₃≈ ν₁+2ν₄, the spectrum of the contact (CF₄)₂ dimer is identified. Agreement between this spectrum and the calculated spectrum is achieved by simultaneously taking intramolecular and intermolecular resonances into account. The distance R _C-C in the dimer is 4.85(15) Å. The calculations of the spectra of (¹²CF₄)₂ and (¹³CF₄?¹²CF₄) dimers with this value of R _C-C in the region ν ₃≈2ν₄ agree with the experiment.