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.     

The laser sensitized reaction SF26 + NO + O3

A pulsed CO₂ laser was used to irradiate a rapidly flowing mixture of NO, O₃, and SF₆. When the laser was tuned to an SF₆ absorption line, an increase in the visible NO^*₂ emission was observed. The laser-induced signal has two unusual features. First, the rise time is much longer than is observed when O₃ is excited directly, and, second, the signal decays to a value above the original baseline. The rise rate is attributed to VV energy transfer from SF²₆ to O₃, while the baseline shift is attributed to a temperature jump resulting from rapid non-resonant VV relaxation within the SF₆ molecule. Both the size of the T-jump and the fraction of vibrationally excited ozone molecules vary inversely with NO pressure.     

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.     

Fluorination reactions of UF6

Uranium hexafluoride is known to be an oxidative fluorinating agent, frequently breaking CC bonds and oxidizing many elements to a higher oxidation state. This work will compare the behavior of UF₆ to that of two other fluorinating agents, WF₆ and SF₄, which are for the most part non-oxidative.The reactions of UF₆ with a number of quite simple organic compounds have been studied; alcohols, aldehydes, ketones, acids, acid halides, ethers, olefins, and alkanes are included. The reactions of WF₆ and SF₄ with these compounds were investigated also when no data existed in the literature. The primary tool was ¹⁹F NMR, assisted by ¹H NMR, infrared, powder diffraction, thermogravimetry and elemental analysis when needed.The differences in behavior of the three agents with respect to the same compounds will be emphasized.     

Relaxation in pure and seeded supersonic jets of SF6

Nozzle expansion of pure SF₆ and SF₆ seeded in carrier gases e.g. Ar, He and N₂, is used to study relaxation of the various degrees of freedom. The diode laser absorption spectrum of the Q branch of the ν₃ mode of SF₆ is used to derive the rotational temperature (T_rot). Time-of-flight studies determine the translational temperature (T_tg) while the vibrational temperature (T_vib) is estimated from the energy balance equation. Rotational and translational temperatures follow each other closely while the vibrational temperature lags behind considerably. The effective specific heat ratio (γ) and the collisional effectiveness parameter (e) for SF₆ are determined from these measurements. Seeded-beam data prove argon to be a better refrigerant than He or N₂. Cooling in seeded beams at moderate stagnation pressures is not substantially enhanced even at very high dilution.     

Laser induced dielectric breakdown studies of the reaction UF6 + H2

Laser induced dielectric breakdown (LIDB) has been documented in UF₆ at pressures ranging from 8–100 torr. A high power, line selectable TEA CO₂ laser has been used as the source to induce the dielectric breakdown (DB). Reactions of the fragmented UF₆ with H₂ have been studied at various pressure ratios. In all cases a rapid and large pressure drop and heavy deposits of suspended particulates were observed and attributed to the LIDB driven reaction 2UF₆ + H₂ → 2UF₆ + H₂ → 2UF₅ + 2HF.     

Rapid interspecies energy flow between vibrationally excited SF6 and the asymmetric stretch of N2O

Rapid, selective collision-dependent excitation of N₂O following pumping of SF₆ with a CO₂ laser is reported. The N₂O fluorescence rise depends on the pressure of each component and is dominated by the SF₆-dependent contribution of 2290 ms^?1 Torr^?1. The subsequent fall is governed by V→V processes among SF₆ vibrational modes.     

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.     

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.     

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.     

CO2 laser-induced interaction between sulfur hexafluoride and carbon disulfide

Interaction between carbon disulfide and sulfur hexafluoride is excited by cw CO₂ laser radiation and has been investigated for different ratios over a pressure range from 3.1 to 34.6 kPa. The reaction yields sulfur tetrafluoride, sulfur, carbon, thiocarbonyl fluoride, tetrafluoromethane and hexafluoroethane, the ratio of these latter products is dependent on the partial pressure of sulfur hexafluoride in the initial CS₂-SF₆ mixture. Interaction is considered to include both the SF₆-sensitized decomposition of carbon disulfide and reaction between sulfur hexafluoride and carbon disulfide.     

32SF6 dissociation rate during irradiation by a tea CO2 laser

A new experimental method is presented for measuring the multiple-photon dissociation rate of SF₆. It appears that there is a reverse process which associates the photofragments into SF₆. The measured dependence of the dissociation probability versus laser flux seems to agree qualitatively with theoretical models.     

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 excitation of SF6 in collisions with He atoms

Vibrational excitation of SF₆ molecules in collisions with He atoms is studied using a vibrational close-coupling, rotational infinite-order-sudden method of calculation. Integral and differential cross sections for excitation of the triply degenerate ν₆ and ν₅ vibrational modes of SF₆ are reported for thermal collisional energies. Excitation of the ν₆ mode is found to be particularly efficieny. The cross sections are much larger than those calculated previously for the excitation of the bending mode in the He + CO₂ system. The differential cross sections are backward peaked.     

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.     

Spectroscopy of the polyatomic molecules,excited by an intense IR laser field

Using double IR resonance technique the transformation of the SF₆ linear spectrum while excited by fn intense laser field has been investigated. The saturation intensities for the transitions between excited vibrational states of the SF₆ have been measured. It has been found that the multiplephoton laser excitation is forming two molecular ensembles, one of these two is strongly excited along the ν₃ mode. It is shown that between excited vibrational states a fast collisional relaxation is taking place.     

Electron impact ionization of CCl4 and SF6 embedded in superfluid helium droplets

Electron impact ionization of helium nano-droplets containing several 10⁴ He atoms and doped with CCl₄ or SF₆ molecules is studied with high-mass resolution. The mass spectra show significant clustering of CCl₄ molecules, less so for SF₆ under our experimental conditions. Positive ion efficiency curves as a function of electron energy indicate complete immersion of the molecules inside the helium droplets in both cases. For CCl₄ we observe the molecular parent cation CCl₄⁺ that preferentially is formed via Penning ionization upon collisions with He*. In contrast, no parent cation SF₆⁺ is seen for He droplets doped with SF₆. The fragmentation patterns for both molecules embedded in He are compared with gas phase studies. Ionization via electron transfer to He⁺ forms highly excited ions that cannot be stabilized by the surrounding He droplet. Besides the atomic fragments F⁺ and Cl⁺ several molecular fragment cations are observed with He atoms attached.     

Collisionless intramolecular energy transfer in vibrationally excited SF6

Infared-infrared double resonance measurements of the ν₃ absorption band of SF₆ using a CO₂ TEA pump laser (0.3 J/cm² maximum excitation) and a cw CO₂ probe laser have been carried out. These measurements probe both the coherent and quasi-continuum phases of the multiphoton dissociation process in SF₆. The spectral and temporal dependences of the induced absorption are obtained. The results give the first direct measurement of collisionless intramolecular vibrational energy redistribution times in a vibrationally excited complex molecule; for an excess energy of three quanta deposited in the ν₃ mode of SF₆, the ν₃ mode comes into equilibrium with all the remaining vibrational degrees of freedoom with a 3 ± 1 μs time constant.     

Various aspects of the reactivity of the xenon(VI) oxyfluoride: XeOF4

In the liquid state XeOF₄ exhibits an amphoteric behaviour : its ability to form complexes with the strong Lewis base CsF and the strong Lewis acid SbF₅ has been established earlier. Its relative stability toward reduction is discussed.In the gas phase, the sensitization of its dissociation giving Xenon tetrafluoride and oxygen is performed using SF₆ excited by a pulsed CO₂ laser. This experiment shows an efficiency 60 times greater than the multiphoton dissociation for equal energies. These last results have been explained by a theory of the vibrational intermolecular transfer for molecules in their quasi continuum.