The geometric structure of pure SF6 and mixed Ar/SF6 clusters investigated by core level photoelectron spectroscopy

The S 2p core level photoelectron spectra of Sulphurhexafluoride clusters have been investigated together with heterogeneous Ar/SF₆ clusters, created by doping Ar host clusters (with a mean size of 3600 atoms) with the molecule. Surface and bulk features are resolved both in the argon 2p and the sulphur 2p core level photoelectron spectra. For the latter level such features were only observed in the pure cluster case; a single feature characterizes the S 2p core level spectra of SF₆ doped argon clusters. From the chemical shifts, investigated with respect to SF₆ doping pressure. It can be concluded that the host clusters get smaller with increasing doping pressures and that the SF₆ molecules predominantly stay below the cluster surface, whereas the Argon core stays intact. We have neither observed features corresponding to SF₆ on the cluster surface, nor features corresponding to molecules deep inside the bulk in any of the spectra from the pick-up experiments.     

Collision-induced simultaneous transitions in H2+CF4 and H2+SF6 mixtures

The simultaneous transitions of the v₃ fundamental vibrations of CF₄ and SF₆ with the fundamental Q branch and S(1) line of H₂ have been studied for various H₂+CF₄ and H₂+SF₆ mixtures at total pressures up to 185 bars. The integrated intensities are found to be proportional to the partial densities of the gas mixture components. The agreement between experimental and calculated intensities is generally better for the Kihara potential than for the Lennard-Jones potential.     

Low-temperature absorption contour of the ν3 band of SF6

A band contour analysis is carried out for the ν₃ absorption in SF₆. Values of ΔB = ? (1.0 ? 1.5) × 10^?4cm^?1, ζ₃ = 0.701, and ν₀ = 948.2cm^?1 are found. Tentative assignments are given for the SF₆ rotational states which are pumped by the P(14) through P(22) lines of the CO₂ laser.     

Continuous-wave CO2 laser spectroscopy of SF6, WF6, and UF6

The linear absorption of CO₂ laser radiation in SF₆, WF₆, and UF₆ has been measured by using optoacoustic detection techniques. Absolute absorption coefficients per Torr as low as 1 × 10^?7 cm^?1 Torr^?1 in a 2-cm active path length could be measured by taking advantage of calibration measurements performed with SF₆.     

Predictions of multiphoton resonances in SF6 and SiF4

The frequency dependence of the multiphoton resonances to the rotation-vibrational levels of the first two ν₃ overtones are calculated for SF₆ and SiF₄. From these calculations we can identify most of the features seen in the high intensity SF₆ absorption data and predict those features that would be seen in similar SiF₄ data.     

The forbidden far-infrared ν6 band of SF6

The far infrared spectra of SF₆ in the 33-μm region in the gas phase at different pressures and in the liquid phase have been studied. A small band situated at 351 cm^?1 on the high-frequency side of two difference bands situated at 304.5 cm^?1 has been observed in the gas phase. Since the integrated intensity of the 351-cm^?1 band varies linearly with the density, it cannot be collision induced. It appears that it is the forbidden ν₆ band that becomes active by Coriolis interaction. This band is seen in the liquid at about the same frequency when it is deconvoluted from the neighboring broadened and split difference bands.     

SF6 ground-based infrared solar absorption measurements: long-term trend, pollution events, and a search for SF5CF3 absorption

Infrared solar spectra recorded with the Fourier transform spectrometer in the McMath solar telescope complex on Kitt Peak (31.9°N latitude, 111.6°W, altitude), southwest of Tucson, Arizona, have been analyzed to retrieve average SF₆ tropospheric mixing ratios over a two-decade time span. The analysis is based primarily on spectral fits to absorption by the intense, unresolved ν₃ band Q branch at . A best fit to measurements recorded with SF₆ near typical background concentrations yields a SF₆ increase in the average tropospheric mixing ratio from (10⁻¹² per unit volume) in March 1982 to in March 2002. The long-term increase by a factor of 3.34 over the time span is consistent with the rapid growth of surface mixing ratios measured in situ at Northern Hemisphere remote stations, though the infrared measurements show a large scatter. Average tropospheric mixing ratio enhancements above background by 2-3 orders of magnitude have been identified in spectra recorded on 5 days between November 1988 and April 1997. These spectra were individually analyzed in an attempt to detect the strongest 8- band of SF₅CF₃, a molecule recently identified with an atmospheric growth that has closely paralleled the rise in SF₆ during the past three decades. Absorption by the strongest SF₅CF₃ band was predicted to be above the noise level in the Kitt Peak spectrum with the highest average mean tropospheric SF₆ mixing ratio, assuming the reported atmospheric SF₅CF₃/SF₆ ratio and a room temperature absorption cross sections reported for the SF₅CF₃ 903-cm⁻¹ band. An upper limit of 8×10¹⁵ for the SF₅CF₃ total column was estimated for this case. We hypothesize that the highly elevated SF₆ levels above Kitt Peak resulted from a local release experiment rather than production via electrochemical fluoridation of intermediate products, the proposed source of atmospheric SF₅CF₃. The absence of the SF₅CF₃ feature in the spectra with elevated SF₆ is consistent with the absence of SF₅CF₃ reported in a pure SF₆ sample.     

Sulfur isotope enrichment in SF6 by high intensity CO2 laser radiation

Enrichment of ³⁴SF₆ following irradiation of SF₆?H₂ mixtures by the focused output of a pulsed TEA CO₂ laser has been studied as a function of the number of laser pulses, excitation wavelength, total pressure, and laser energy.     

Doppler-free two-photon spectroscopy of the 2ν3 band of SF6

The assignments of most of the observed Doppler-free two-photon spectra of the 2ν₃ band of SF₆ near the P(14)–P(20) lines of a 10-μm CO₂ waveguide laser are reported. The rovibrational and anharmonic constants for this band were obtained, along with ground state constants determined from observed “forbidden” transitions.     

Influence of collisions and pulse intensity on multiple photon absorption in SF6

Using a pyroelectric detector, the multiple photon absorption (MPA) of the SF₆ molecule in a wide range of pressures (10^-3 -1 torr) has been studied. The significant role of collisions in MPA has been shown. The fraction of molecules excited under essentially collisionless conditions has been defined. It is shown that under collisionless excitation of SF₆ (p < 10^-2 torr) at energy fluences E < 10^-1 J/cm² the intensity of the laser pulse plays the essential role, while in presence of collisions MPA is determined mainly by the energy fluence in the pulse.     

Multiple photon excitation of the v2 + v6 combination band of SF6 in a bulk and in a pulsed free jet

Multiple photon excitation of the v₂ + v₆ combination band of SF₆ in a bulk at T ≈ 295 K and cooled in a pulsed free jet up to T_V ≈ 160 K and T_R ≈ 40 K by a pulsed TEA CO₂ laser has been investigated. Obtained results are compared with the data on the v₃ vibration excitation. At exciting energy fluences ø = 0.1?2.5 J cm^-2 the levels in the region of the discrete vibrational states (v=3?5) are found mainly to be excited. Multiphoton absorption spectra at room temperature have a sharp resonant structure. The fraction of interacting molecules is considerably (3–7 times) less compared than that for the case of v₃ vibration excitation. Multiphoton absorption of the v₂ + v₆ and v₃ vibrations of SF₆ is shown to be proportional to the dipole moments of the corresponding transitions.     

Threshold behavior of multiple photon dissociation of SF6

Multiple photon dissociation of SF₆ by a short pulse of a CO₂ laser has been investigated by simultaneous measurements of the average number of photons absorbed per molecule 〈n〉 and chemiluminescence intensities which result from the dissociated F atoms. A criterion for the dissociation threshold which is independent of laser wavelength is found to be 〈n〉 = 16 ± 3 photons per molecule. A thermal distribution of the excited molecules is shown to be inconsistent with the behavior just above threshold.     

Some aspects of CO2 laser induced fluorescence in SF6−H2 mixtures

Measurements in SF₆?H₂ mixtures of HF¹ fluorescence at 2.8 μm induced by pulsed CO₂ laser radiation are reported. The dependence of fluorescence intensity on laser fluence is found to be strongly affected by the laser beam geometry in the interaction region. Our results show that the technique of HF¹ fluorescence intensity detection can be a sensitive and reliable single-shot measure of multiple-photon dissociation of SF₆ in a collisionless regime on condition that the laser fluence is uniform along the interaction region which is monitored.     

High-resolution Raman spectroscopy of the ν1 region and Raman-Raman double resonance spectroscopy of the 2ν1−ν1 band of SF6 and SF6. Determination of the equilibrium bond length of sulfur hexafluoride

The ν₁ region of ³²SF₆ and ³⁴SF₆ has been studied by stimulated Raman spectroscopy. For both isotopomers, a detailed analysis has been performed. Several hot bands (ν₁+ν₆−ν₆, ν₁+2ν₆−2ν₆, ν₁+ν₅−ν₅) have been taken into account to calculate synthetic spectra that satisfactorily reproduce the experimental data. These results, together with the previous studies of the other fundamental bands have allowed us to determine the equilibrium bond length of sulfur hexafluoride as r_e=1.5560(1) Å, in very good agreement with recent ab initio calculations. The 2ν₁−ν₁ band has also been studied for both isotopomers by Raman-Raman double resonance spectroscopy and the resulting spectra have been analyzed. In this case, a striking difference is observed between the two isotopomers, since the 2ν₁−ν₁ band of ³⁴SF₆ appears to have a very narrow structure that could not be rotationally resolved under the present experimental conditions. All analyses have been performed thanks to the HTDS program suite (http://www.u-bourgogne.fr/LPUB/hTDS.html) dedicated to octahedral XY₆ molecules.     

The fluorescence and IR energy deposition from from CO2 laser irradiation of SF6−H2 mixtures

The multiple photon excitation and dissociation of SF₆ and hydrogen mixtures is measured by using simultaneously pulsed optoacoustic detection to monitor the energy deposition and time resolved HF fluorescence to monitor the production of vibrationally hot HF. From these studies we deduce that at least three mechanisms lead to production of vibrationally excited HF. One mechanism produces free F from the unimolecular laser-induced decomposition of SF₆. The second mechanism involves the reaction between two vibrationally hot SF₆ molecules to produce free F. In both of these cases the F atom subsequently react with H₂ to produce vibrationally hot HF. The third involves the reaction between a vibrationally hot SF₆ molecule and a hydrogen molecule producing vibrationally hot HF directly.     

Slow intermolecular redistribution of vibrational energy in SF6 gas excited by a tea CO2 laser

Infra-red fluorescence (IRF) spectra of SF₆ excited by the 944.2 cm^-1 line of a pulsed CO₂ laser were observed at various times after the time of the laser excitation. Each spectrum showed a strong IRF peak of the v₃ mode which was red shifted relative to the room temperature fundamental (948 cm^-1) by an amount which depended, apart from the level of excitation, on the different times employed. For a strong excitation with 〈n〉 ≈ 11 photons absorbed per molecule, a significant decrease of red shift versus time was observed, indicating mainly excitation losses by IRF emission. For weak excitation with 〈n〉 ≈ 1.4, almost an constant red shift versus time is observed. This result, and the previous finding that at weak excitation a nonthermal energy distribution in the ensemble of molecules exists, leads to the conclusion that intermolecular redistribution of vibrational energy in SF₆ is slow, and does not exceed the observed fluorescence duration (～1 ms).     

Collisionless dissociation and isotopic enrichment of SF6 using high-powered CO2 laser radiation

Dissociation of ³²SF₆ and the resultant isotopic enrichment of ³⁴SF₆ using high-powered CO₂ laser radiation has been studied with higher experimental sensitivity than previously reported. Enrichment factors have been measured as a function of laser pulse number, wavelength, energy and time duration. A geometry-independent dissociation cross section is introduced and measured values are presented. Threshold energy densities, below which no dissociation was observed, were also determined.     

Diode laser spectrum of the ν3 band of 34SF6

A diode laser was used to measure the absorption spectrum of the ν₃ band of ³⁴SF₆. This isotopic species, which is present in the natural sample (4.2%), was cooled in a molecular beam of pure SF₆. Subbranches up to J = 22 were recorded and identified. The molecular parameters, determined with a simple fitting procedure, are compared with those known of ³²SF₆ and ³³SF₆.     

A KrF fast discharge laser in mixtures containing NF3, N2F4 or SF6

A fast discharge KrF laser system (λ = 248.5 nm) has been operated at 25 mJ/pulse, 3.0 MW peak power in high pressure He: Kr: fluoride mixtures containing low concentrations of both krypton and the fluorine donors N₂F₄, NF₃ and SF₆. Lasing action is reported for the first time in N₂F₄ and SF₆ with optimum energy output at 750 and 160 mJ/l respectively.     

Coherent Stokes and anti-Stokes Raman spectra of the ν1 (A1) and ν2 (E) bands of SF6 and UF6

Coherent Stokes and anti-Stokes Raman scattering are used to study the ν₁ and ν₂ spectral band profiles of UF₆ and SF₆. Most of the observed SF₆ “hot” bands are assigned, leading to evaluations of the anharmonicity constants X_ij: X₁₂ = ?(2.80 ± 0.30) cm^?1, X₁₄ = ?(1.00 ± 0.15) cm^?1, X₁₅ = ?(1.00 ± 0.15) cm^?1. For UF₆, a tentative assignment of the “hot” bands is made: X₁₂ = ?(1.80 ± 0.30) cm^?1, X₁₃ = ?(1.60 ± 0.30) cm^?1, X₁₄ = ?(0.20 ± 0.10) cm^?1, X₁₅ = ?(0.25 ± 0.10) cm^?1, and X₁₆ = ?(0.10 ± 0.05) cm^?1. Parameters such as the vibration-rotation coupling constants are determined. For SF₆: α = (7 ± 2) × 10^?5 cm^?1 for the ν₂ band and α = ?(1.02 ± 0.01) 10^?4 cm^?1 for the ν₁ band. The calculated spectral profiles of the coherent Stokes or anti-Stokes spectra, which are in good agreement with experimental results, give values for the resonant and nonresonant parts of the susceptibility in both molecules. They also show, in some cases, the influence of neighboring combination bands.