Infrared multiphotonic excitation and the cell geometry problem: case of CH3Br and SF6

Gas samples of CH₃Br and SF₆ were irradiated by a focused CO₂ laser beam. Three cells with different sizes were used. With CH₃Br, fluorescence and decomposition were observed which depended upon the cell's dimensions. With SF₆ we always obtained decomposition and enrichment in ³⁴S by irradiation with the P(16) line.     

Synthesis of SF5benzene (SF5C6H5) by the SF5halide method

Several significant and useful syntheses of pentafluorothiobenzene (SF₅C₆H₅) from SF₅halides (SF₅X, X=Cl, Br) and cyclohexene or derivatives of cyclohexene are presented.     

S 2p photoabsorption of the SF5CF3 molecule: Experiment,theory and comparison with SF6

The S 2p core excitation spectrum of the SF₅CF₃ molecule has been measured in the total ion yield mode. It resembles a lot the analogous spectrum of SF₆, also recorded in this study, displaying intense transitions to the empty molecular orbitals both below and above the S 2p ionization potential (IP) and weak transitions to the Rydberg orbitals. The S 2p photoabsorption spectra of SF₆ and SF₅CF₃ have been calculated using time-dependent density functional theory, whereby the spin–orbit coupling was included for the transitions below the S 2p IP. The agreement between experiment and theory is good for both molecules, which allows us to assign the main S 2p absorption features in SF₅CF₃.     

On the absolute absorption cross-section of SF5CF3

The absolute absorption cross-sections of a recently discovered atmospheric gas, SF₅CF₃, have been measured at He I (21.22 eV) and Ne I (16.64 and 16.82 eV) photon energies using a VUV discharge lamp and a double ion chamber method. Absorption cross-sections of (9.52 ± 0.95) × 10⁻¹⁷ cm² (He I) and (8.79 ± 0.88) × 10⁻¹⁷ cm² (Ne I) were obtained and compared with data from other studies. The consequences for the cross-section at the hydrogen Lyman- energy (10.20 eV) are discussed.     

SF6气体的富集与解吸方法

介绍SF6的低温富集及高温解吸方法.测定了SF6在Porapak Q和PorapakT两种吸附剂上的动态吸附系数(kd).结果显示,Porapak Q对SF6的吸附性能明显优于Porapak T,温度是影响kd值的主要因素之一,温度和kd的关系符合Arrhenius方程,实验范围内的样品气流量对kd的影响较小.SF6解吸前需预抽真空至30 kPa,然后放入热水浴中恒温30 min,用N2洗提,逐段收集洗提气体,洗提流量为2mL/min.当解吸体积为7 mL时,回收率可达到90％以上,富集系数可达到102量级.     

Novel sulfur-nitrogen-fluorine compounds : synthesis and properties of SF5NHF,SF5NClF and SF4NF

Using reactions developed for the synthesis of CF₂NF, CF₃NClF and CF₃NHF from ClCN, F₃SN can now be converted in high yield to the analogous sulfur compounds, SF₄NF, SF₅NClF and SF₅NHF. N-fluorotetrafluorosulfurimine is the simplest sulfur(VI) imine and is a rigid molecule. Details on the synthesis and properties of these novel compounds will be presented.     

Formation of SF5 in electron attachment to SF6; swarm and beam results reconciled

Based on the results of recent swarm experiments, it has been proposed that the increase in the cross section for SF₅⁻ formation observed at an electron energy, E_e of about 0.3 eV in electron beam studies of electron attachment to SF₆ is due to the combined (opposing) effects of the vibrational heating of the molecule by the attached electron, which enhances the dissociation of the nascent (SF₆⁻)^* ion, and the reduction of the cross section for capture (s-wave) of the electron by SF₆ with increasing E_e. Further, it has been shown that the dissociation reaction is endothermic by 0.12 eV, and that, contrary to previous suggestions, there is no potential barrier to this dissociation reaction. Now we have carried out electron beam studies of the SF₆ attachment reaction in Berlin at gas temperatures, T_g, over the range 300 to 920 K and in Innsbruck at T_g below 300 K. These studies have provided support for the above proposals concerning the appearance of the SF₅⁻ peak and for a reaction endothermicity of 0.12 eV. Thus these studies have clarified the doubts about the products of the SF₆ attachment reaction at low electron energy.     

Geometry and electronic structure of CBr3. Comparison with CF3 and CCl3

Ab initio UHF geometry investigation of the CBr₃ radical and the SW X calculations for its ionization potential, electron affinity and electronegativity are reported. A comparison is made with our previous results for the CCl₃ and CF₃ radicals.     

Isotopic selectivity in the sensitized dissociation of SF5Cl and CF3I by multiphoton excitation of SF6

The dissociation of SF₅Cl and CF₃I sensitized by multiphoton excitation of SF₆ by a pulsed CO₂ laser has been studied versus pressure, laser fluence, inert gas and optical frequency. Isotopic effects have been observed between ³²SF₅Cl/³³SF₅Cl and between ¹³CF₃I/¹²CF₃I and selectivity factors as high as α^?1_{$\text{33}\text{32}$} = 1.57 or α_{$\text{13}\text{12}$} = 1.23 obtained.     

The unusual chemical equilibria F3SSF ⇌ 2 SF2 and CF3SF2SCF3 ⇌ 2CF3SF

In order to understand more about the instability of sulfur difluoride, we have investigated the chemical interrelations between each of the monomers SF₂ and CF₃SF and the corresponding dimers F₃SSF and CF₃SF₂SCF₃. Thus we have found that SF₂ and CF₃SF exist in chemical equilibria with their dimers. These equilibria are unusual because they involve two different bonds (SF and SS). The equilibrium constants and dissociatíon enthalpies have been determined by i.r. and mass spectroscopic measurements.

The equilibrium between F₃SSF and SF₂ is disturbed by a decomposition reaction of these compounds yielding SF₄ and SSF₂. In both systems (1) and (2) the achievement of the equilibrium is comparatively slow at ?30 to 30 °C. The rates for dissociation and decomposition are strongly surface-dependent and the kinetics of the two processes have been studied separately. Under favorable conditions the half-lives at 298 K for the dissociation of F₃SSF and CF₃SF₂SCF₃ are found to be ca. 8.h and ca. 2 h, respectively, and for the decomposition of SF₂ to SF₄ and SSF₂ and CF₃SF to CF₃SF₃ and CF₃SSCF₃ (p?13 mbar) the values are ca. 10 h and 1 year respectively.     

Vibrational energy distribution of CsF produced by reactive beam scattering of Cs by SF6 and SF4

The molecular beam electric resonance method has been used to measure the vibrational energy distribution of CsF produced in the chemical reactions Cs + SF₆ and SF₄. The reaction Cs + SF₆ yields a perfect Boltzmann distribution for the products CsF. For the reaction Cs + SF₄ a more complicated distribution has been found which, according to the molecular structure of SF₄, can be explained as a superposition of two Boltzmann distributions of different vibrational temperatures.     

Laser isotope separation in SF6

Laser induced isotope separation in SF₆ and SF₆ mixtures has been investigated in a collisionally dominated pressure regime. Experimental results with SF₆/rare gas mixtures point out the importance of collision induced dissociation following the initial collisionless dissociation. Rotational relaxation induced by rare gases and H₂ is shown to play a signficant role in the dissociation of both isotopic species. Total vibrational relaxation (V-T/R) induced by H₂ as a collision partner is shown to dominate the dissociation efficiency of SF₆/H₂ mixtures.     

Ground state geometry of cinso

The ground state geometry for ClNSO is investigated using ab initio calculations in a moderately large basis set. Variation of the dihedral angle yields a planar syn geometry. The geometry is discussed on the basis of a population analysis.     

Composition of gas hydrates of CHClF2, CCl2F2 and SF6

Gas hydrates are forms of ice stabilized by the presence of molecules of gas occupying cavities in the solid water lattice. There are two common forms: structure I and structure II. The mean free diameters of the two types of cavities in structure I are about 5.0 and 5.8 angstroms. Very small gas molecules such as Xe or H₂S can occupy both. In the past it has been considered that gas molecules of larger diameter than 5.0 angstroms could not occupy the smaller cavities. It has now been shown through measurement of hydration numbers of CHClF₂ (diameter about 5.4 angstroms) under various pressures at 0°, that some of the small cavities are filled. This state of affairs also exists for CH₃Br. In structure II, the mean diameters of the two types of cavities are about 5.0 and 6.6 angstroms. Hydration numbers of SF₆ (diameter about 5.8 angstroms) and CCl₂F₂ (about 6.2 angstromsm) show that nearly all of the large cavities but essentially none of the small cavities are occupied.     

Multiphoton excitation and dissociative ionization of SF6

A simple model of multiphoton excitation and dissociative ionization process of SF₆ is proposed. The model is compared with experimental results of Brunner et al. It was found that a three-photon absorption law reproduced the experimental results with good agreement. In addition the relative enhancement ratios of dissociative ionization of excited SF₆ were evaluated.     

The electronic structure of the BrF3 and BrF5 molecules

Ionization potentials of the BrF₃ and BrF₅ molecules have been calculated by the SCF DV Xα method. The calculations hale been carried out in numerical Hartree-Fock basis sets, or, to be more specific, in bases that are extensions of these bases since “virtual” bromine 4d-functions are added to the numerical HF bases. The results are used for the interpretation of the experimental PDS of these compounds.     

Laser induced predissociation of SF6 clusters

Dimer predissociation is sensitively detected as attenuation of the SF⁺₅ ion signal from a SF₆ molecular beam crossed by a line tunable cw CO₂ laser. The dimer spectra show a double peak structure which is assigned to the symmetric and anti-symmetric combination of the v₃ vibration. The minimum observed linewidth is 3 cm^?1 fwhm.     

Thermal effects in collision-free infrared multiphoton absorption by SF6 and CF3Br

An opto-thermal molecular beam study has been carried out to investigate the multiple-photon laser excitation of SF₆ and CF₃Br. The molecular beam was produced by means of a supersonic expansion through a nozzle at variable temperature. The opto-thermal signal was measured by means of a high-sensitivity superconducting bolometer. The multiple-photon excitation of SF₆ has been measured as a function of the initial ro-vibrational population of the molecule. The experimental results have been compared with both previously published data of molecular beam and gas cell experiments and theoretical calculations. A satisfactory agreement has been found between some of our experimental results and the theoretical spectra obtained by means of the heat-bath feed-back model.     

Bond function parameter transferability in SO2, SF2 and SO2F2

Ab initio MO studies of the use of gaussian bond functions in the molecules SO₂,SF₂ are presented. It is shown that optimization of bond function exponent and position parameters in the small molecules is a good approximation to the optimum parameter set for SO₂F₂,     

Electronic properties of the SF6 molecule: Ab-initio calculation for the ground state

We report an ab-initio computation for the ground-state wavefunction of sulphur hexafluoride where 55 Slater-type orbitals were used for a basis-set expansion with 3d-functions included on the central atom.