Inner-shell and valence-shell electronic excitation of SF6, SeF6 and TeF6 by high energy electron impact: An investigation of potential barrier effects

Inner- and valence-shell electron energy loss spectra of gaseous SF₆, SeF₆ and TeF₆ have been measured at high impact energy (2.0–3.7 keV) and zero degree scattering angle. The resulting inner-shell excitation spectra include F 1s, S 2s and 2p in SF₆: F 1s, Se 3s, 3p and 3d in SeF₆ and F 1s, Te 4s, 4p, 4d and 3d in TeF₆. The results for each of these hexafluorides are interpreted, in the framework of the potential barrier model, as excitations to resonances involving a common manifold of virtual valence (inner-well) orbitals and also to Rydberg (outer-well) orbitals. The below-edge shape resonances in corresponding inner-shell excitation spectra of these hexafluorides show very similar variations in intensities in accord with expectations based on electric-dipole selection rules. This is an indication that these below-edge resonances in SeF₆ and TeF₆ are of the same symmetry as those in SF₆ (i.e. a_1g and t_1u). The continuum (above-edge) shape resonances in SeF₆ also show similar spectral behavior to those of SF₆ and can be understood by including Se 4d and S 3d orbitals in the basis for the respective MO schemes. However, in TeF₆ completely different spectral behavior is observed for the continuum resonances. In particular, there is a dramatic series of intense resonances observed above the Te 3d and 4d edges. The TeF₆ spectrum can only be understood by extending the MO basis set to include the Te 4f orbitals which are even lower lying than the Te 5d orbitals. Therefore, these continuum resonances which are also seen in the F 1 s and Te 3p spectra of TeF₆ are assigned to l=3 (f-type) continuum shape resonances due to involvement of the 4f orbitals rather than the l=2 (d-type) continuum resonances observed in SF₆ and SeF₆. This is the first reported observation of such f-type continuum shape resonances and such considerations will likely prove to be important in the understanding of near-edge spectra of heavy atom containing species. The VSEELS spectra which are very similar for the three hexafluorides also show significant continuum shape resonances. A consideration of both the ISEELS and the VSEELS spectra indicates that there is a weakening of the potential barrier in going through the series from SF₆, SeF₆ to TeF₆.     

Non-linear infrared spectroscopy of SF6 at 30 K

Calculations of non-linear absorption by SF₆ at 30 K from a tunable infrared laser are presented and compared with a recent experiment of Alimpiev and co-worker. Theory and experiment agree qualitatively and there is reasonable quantitative agreement on the positions and strengths of two-photon Q-branch reasonances which excite the 2 v₃ A_1g and E_g substates.     

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.     

Energy transfer among excited vibrational states of SF6

Studies of the risetime of laser induced 16μ fluorescence in SF₆ show the rapid equilibration (1.5 μsec torr) among the excited vibrational states. The subsequent decay of fluorescence consists of 2 exponentials. The faster of these is due to vibration-translation energy transfer (160μsec torr) and the slower is due to thermal cooling of translationally hot gas.     

Chemical consequences of electron scavenging by SF6 in radiolysis experiments

Contrary to the usual assumption that it is unreactive, SF₆^? formed by electron attachment is found to undergo rapid bimolecular reactions with molecules (HX) possessing acidic hydrogen, including HCl, HBr, HCO₂H, CH₃CO₂H, H₂S, and HCN. It is clear for many systems that meaningful mechanistic inferences about radiolytic processes occurring in the presence of SF₆ should consider a wide variety of non-scavenging reactions. Appropriate caution should be exercised in any experiment in which SF₆ is used to scavenge and detect low energy electrons. The reaction sequences observed are interesting in that they provide convenient bimolecular pathways for the formation of a variety of anionic dimers of the type XHY^?.     

Inner shell excitation of SF6 by 2.5 keV electron impact

The S 2p, S 2s and F 1s inner shell excitation spectra of SF₆ have been examined by small angle, inelastic scattering of 2.5 keV electrons. While the main features of the spectra are in agreement with previous photoabsorption spectra, a number of new features have been observed. A detailed investigation of the S 2p spectrum below the ionization limit has been performed. The weak Rydberg structure in this region has been interpreted on the basis of a comparison of the present electron energy loss results with previous photoabsorption studies. Part of the Rydberg structure in the electron impact spectrum has been identified as arising from electric quadrupole transitions.     

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.     

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.     

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.     

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.     

SF6-sensitized dissociation of UF6 with a pulsed CO2 laser

The dissociation of UF₆ sensitized by SF₆ excited with a pulsed CO₂ laser in the presence of H₂ and CO as scavengers has been investigated. In the SF₆-UF₆-H₂ system the dissociation yields have been determined as a function of the laser frequency, the fluence, and H₂ partial pressure. A maximum dissociation yield has been found at a laser frequency of 935 cm^?1. No obvious dissociation of UF₆ was observed in the UF₆-SF₆ system without F-atom scavengers.     

Quantised rotation of HBr in liquid SF6

Absorption data on HBr in liquid SF₆ at 296 K are presented in the frequency range 20–200 cm^?1. The broadened J → J + 1 transitions are clearly resolved, more so at the higher frequencies. Although noisy, the data are good enough to discriminate between two models of liquid phase rotational dynamics. It is found that the modified M and J diffusion model of Frenkel and Wegdam rests on sounder hypotheses than a formalism, based on a truncated Mori expansion, that does not predict preferential broadening of the lower frequency J → J + 1 lines.     

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.     

Vibrational excitation of SF6 scattering from graphite

We have observed vibrationally excited sulfur hexafluoride molecules in direct inelastic scattering from hot graphite surfaces. The vibrational temperature for the scattered flux has been determined by probing the effect of internal temperature on electron-induced fragmentation observed in mass spectra. The vibrational excitation depends on incident translational energy, E_tr, and a maximum temperature increase of 50 K is reached in direct scattering at E_tr = 2.5 eV. No effect of surface temperature has been observed at 950–1400 K. Inelastic angular distributions are reproduced by a collision complex model, and the experimental results are related to existing models for vibrational excitation.     

Sensitized multiphoton dissociation of UF6 IN SF6-UF6 mixtures by a tea CO2 laser

UF₆ undergoes decomposition in the presence of SF₆ when mixtures of both are irradiated with a TEA CO₂ laser. The mechanism for UF₆ decomposition may involve vibrational energy transfer from excited SF₆ and laser absorption from the same laser pulse by excited UF₆ in its vibrational quasi-continuum     

Ab initio prediction of MnOH2/MnOH2 electron transfer reactivity at electron correlation level

An ab initio calculation of electron transfer reactivity of Mn²⁺OH₂/Mn³⁺OH₂ redox system, in gas and in solution, has been made in this paper. The detailed geometry optimization and the scanning of the potential energy surfaces have been carried out at UMP2 (full)/6-311+G^* level. The relevant energy quantities (such as the activation energy and the binding energy) are calculated at different levels of theory (HF, MP2, MP3, MP4 and QCISD and corresponding spin-projection PUHF, PMP2 and PMP3) with the same basis set (6-311+G^*). Both all-electrons and valence electrons have been correlated in energy calculations. The electronic transmission coefficient is calculated using the ab initio potential energy surface slopes and the coupling matrix element determined from the two-state model and the Slater-type d-electron wave functions. The pair distribution function is obtained using both a classical sphere approximation scheme and a novel scheme. The relevant kinetic parameters are obtained at different ab initio levels in terms of the new model. The contact-distance dependence of these parameters and the applicability of the presented models are also discussed.     

Vibrational relaxation of highly excited molecules: VV transfer from SF6 to N2O

Vibrational energy transfer from SF₆ to N₂O was studied as a function of SF₆ vibrational energy. The intensity, rise time and decay time of N₂O fluorescence increased monotonically with the level of donor excitation. The observations are consistent with a mechanism that is not mode specific, with donor VT relaxation faster than intermolecular VV transfer.     

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.     

Semiclassical calculation of energy transfer in polyatomic molecules. X. Energy transfer in Ar + SF6 at 1 eV

Rotational and vibrational energy transfer in Ar + SF₆ collisions at 1 eV is calculated as a function of scattering angle. Differential cross sections for the two most excited vibrational modes ν₆ and ν₅ are also reported.     

Molecular motions of SF6 dissolved in cryogenic liquids

Infrared and Raman spectra of SF₆ dissolved in liquid oxygen and krypton were recorded. Angular momentum correlation times have been determined with the help of the extended J-diffusion model and used to obtain an estimate of the vibrational dephasing time in the ν₃ mode. Raman profiles are interpreted in terms of the rough hard sphere model.