Energy and pressure dependence of the CO2 laser induced dissociation of sulfur hexafluoride

Low pressure SF₆ with its isotopes in natural abundance was irradiated by a pulsed CO₂ laser operated on theP20 line (10.6 μm band). Dissociation yields of³²SF₆ and³⁴SF₆ were measured separately. If the radiation is focussed into the cell, the dissociation yield is proportional to the 3/2 power of the laser energy, as was derived under general conditions and confirmed experimentally. The reaction probabilityP(Φ), the fraction of molecules dissociated by an energy flux Φ, was measured using parallel light. For both isotopes,P(Φ) saturates at high energy flux close toP=1. At a lower flux (2 J cm⁻²), the dissociation probability of³²SF₆ displays a threshold, whereas the dissociation probability of³⁴SF₆ is a very steep function of Φ over the whole range of fluxes.P(Φ) at the higher energy flux was measured in a cavity absorption cell, in which up to 80% of the molecules were dissociated by a single pulse. Below 0.2 mbar SF₆ the dissociation yields for both isotopes are pressure independent. Above 2 mbar the isotopic selectivity is completely lost. Addition of hydrogen always decreases the dissociation yields.     

Selective multiphoton IR dissociation of SF6 molecules under nonequilibrium conditions of a pulsed gasodynamically cooled molecular flow interacting with a solid surface

The process of the isotope-selective multiphoton IR dissociation of SF₆ molecules under the non-equilibrium conditions of a pulsed gasodynamically cooled molecular flow interacting with a solid surface was experimentally studied. The SF₆ molecules dissociate as a result of excitation in a shock wave generated in the flow, in the flow incident onto the sold surface, and in an unperturbed flow (in the absence of the solid). The experiment was based on detecting the luminescence from HF^* molecules (λ ≈ 2.5) μm) accompanying the SF₆ dissociation in the presence of H₂ or CH₄, the emission intensity being a measure of the SF₆ dissociation yield. The molecular beam parameters were studied. The time-of-flight spectra of SF₆ in the flow interacting with the surface were measured under various experimental conditions. The spectral and energy characteristics of the SF₆ dissociation process were determined in the flow interacting with the solid surface and in the unperturbed flow. The dissociation product (SF₄) yield was measured and the coefficient of its enrichment with the ³⁴S isotope was determined. It is demonstrated that, using the shock wave formation, it is possible to increase the efficiency of the isotope-selective dissociation of SF₆ molecules. An explanation of the observed results is proposed. The gas density and temperature in the incident flow and in the shock wave were estimated. The results are analyzed and compared to the other published data on the SF₆ dissociation in a molecular beam.     

IR multiphoton absorption of SF6 in flow with Ar at moderate energy fluences

6 in flow with Ar (SF₆: Ar=1:100) in conditions of a large vibrational/rotational temperature difference (T_V≃230 K, T_R≃60 K) was studied at moderate energy fluences from ≃0.1 to ≃100 mJ/cm², which are of interest for isotope selective two-step dissociation of molecules. A 50 cm Laval-type slit nozzle for the flow cooling, and a TEA CO₂-laser for excitation of molecules were used in the experiments. The laser energy fluence dependences of the SF₆ MPA were studied for several CO₂-laser lines which are in a good resonance with the linear absorption spectrum of the ν₃ vibration of SF₆ at low temperature. The effect of the laser pulse duration (intensity) on MPA of flow cooled SF₆ with Ar was also studied. The results are compared with those obtained in earlier studies. Received: 4 September 1995/Revised version: 15 February 1996     

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.     

High resolution study of anion formation in low-energy electron attachment to SF<Subscript>6</Subscript> molecules in a seeded supersonic beam

Using two variants of the Laser Photoelectron Attachment (LPA) method involving a differentially-pumped, seeded supersonic beam (0.05% and 12.5% of SF₆ molecules in helium carrier gas, nozzle temperatures T₀= 300–600 K, stagnation pressures p₀= 1–5 bar) and mass spectrometric ion detection, we have investigated the energy dependence of anion formation in low-energy electron collisions with SF₆ molecules at high energy resolution. Using the standard LPA method, the yield for SF₆^- as well as SF₅^- and F^- anions was studied with an energy width around 1 meV over the electron energy range 0–200 meV. In addition, a variant of the LPA method with extended energy range (denoted as EXLPA) was developed and applied to measure the yield for SF₆^- and SF₅^- formation over the energy range 0–1.5 eV with an energy width of about 20 meV. The cross-section for formation of SF₆^- decreases by five orders of magnitude over the range 1–500 meV and is only weakly dependent on nozzle temperature. The yield for SF₅^- formation shows — apart from a weak zero energy peak which grows strongly with rising temperature — a broad maximum (located around 0.6 eV for T₀= 300 K and shifting to lower energies with rising T₀) and a monotonical decrease towards higher energies. SF₅^- attachment spectra taken at elevated temperatures exhibit changes with rising stagnation pressure which directly reflect rovibrational cooling of the SF₆ molecules with rising pressure. The SF₅^-/SF₆^- intensity ratio at near-zero energy and the low-energy shape of the broad peak in the SF₅^- spectra are used as thermometers for the internal temperature of the SF₆ molecules in the seeded supersonic beam which (at p₀= 1 bar) are found to be 50–100 K lower than the nozzle temperature. The energy dependence of the yield for F^- formation is similar to that for SF₆^-, but the F^- signals are three to four orders of magnitude lower than those for SF₆^-; in view of the rather high endothermicity of F^- formation the origin of the F^- signals is discussed in some detail.     

Isotope effects in the vibrational spectrum of the SF<Subscript>6</Subscript> molecule

The IR absorption spectra of solutions of SF₆ in liquid argon are studied at a temperature of 93 K in the concentration interval 10^?5–10^?7 mole fractions. A sample with a natural abundance of isotopes and a monoisotope ³⁴SF₆ sample are studied. The frequencies, half-widths, and relative intensities of bands in the vibrational spectrum of all isotopomers of the molecule are determined. For the ³⁴SF₆ molecule, the ratio of integral absorption coefficients of fundamental bands A(ν₄)/A(ν₃)=0.07(6) is larger than ³²SF₆:A(ν₄)/A(ν₃)=0.66(4) for the ³²SF₆ molecule, which corresponds to the same signs of P′₃ and P′₄. The change in the intensity of the ν₂+ν₆ and ν₅+ν₆ bands upon isotopic substitution is explained by the change in the resonance contributions of due to the isotope shift of the ν₃ band.     

Diode-laser spectroscopy of supersonic free jets

A supersonic-free-jet infrared spectrometer has been constructed for investigation of molecular vibrational spectra at low rotational and vibrational temperatures. The sensitivity of measurement in a pulsed jet is increased by employing a phase-sensitive detection method synchronized with the pulse frequency. The performance of the spectrometer is examined for the absorption lines of the NH₃ v ₂ band. A rotational temperature as low as 16K is attained when seeded in He. Cold-jet spectra are demonstrated for thev ₃ bands of PF₅,³⁴SF₆, and¹⁸²WF₆.     

Observation of simultaneous ν1(SF6) + ν3(NF3) and ν2(SF6) + ν3(NF3) transitions enhanced by the resonance dipole-dipole interaction with the ν1 + ν3 and ν2 + ν3 states of the SF6 molecule

IR spectra of the solution of SF₆ molecules in liquid NF₃ at 84 K have been recorded. In a solvent transmission window of 1500–1750 cm⁻¹, two wide absorption bands with pronounced peaks in the high-frequency part are observed. The profile of these bands is explained by the influence of the resonance dipole-dipole (RDD) interaction of the states of the simultaneous transition ν₁(SF₆) + ν₃(NF₃) and ν₂(SF₆) + ν₃(NF₃) with the states (ν₁ + ν₃) and (ν₂ + ν₃) of the SF₆ molecules, respectively. The use of three isotopic modifications ³²SF₆, ³³SF₆, and ³⁴SF₆ has allowed us to vary the resonance detuning and thus to change the strength of the RDD interaction. With the liquid near the melting point being represented as a close-packed cubic crystal, the profile was calculated and its spectral characteristics were determined. The frequencies of the main peaks coincide with the experimental values accurate to the error.     

Collision-induced vibrational polarizability and mixed second refractivity virial coefficients B ab R: an experimental study of the SF6—rare gas mixtures

Mixed second refractivity virial coefficients B ^ab _R (ω) are determined in the infrared spectral range from the analysis of the buffer gas-induced intensity redistribution in the ν₃ absorption band at ω ? 930 cm^?1 of ³⁴SF₆ perturbed by the rare gases Ar, Kr, and Xe at relative densities up to 140 Amagat. The values of the refractivity virial coefficients are found to be several orders of magnitude greater than those observed in the spectral regions far removed from single-photon resonances. The undamped dynamic dipole—induced-dipole (DID) model is shown to qualitatively correctly reproduce the observed unusual dispersion of the B ^ab _R (ω) functions. A new estimate of the integrated band intensity S(ν₃) of SF₆ is also obtained.     

ESR study on the structure of the ethylene radical cation stabilized in frozen SF6 and some fluoroalkanes: An evidence for a twisted structure

ESR spectra of the ethylene radical cation were detected at cryogenic temperatures in SF₆, C₂F₆ and C₃F₈. From the unusually small hyperfine couplings estimated for¹H and¹³C, it has been shown that the ethylene radical cation has a non-planar structure with a torsional angle in the range of 8°–23°. Upon annealing the sample at a temperature above 93 K, the ethylene radical cation in SF₆ changed into a monofluoroethyl radical through charge recombination with fluoride anion or SF ₆ ^? .     

Temperature determination of (CF<Subscript>3</Subscript>I)<Subscript><Emphasis Type="Italic">N</Emphasis></Subscript> clusters in a beam by time-of-flight measurements of IR-laser excited SF<Subscript>6</Subscript> molecules sublimating from the surface of clusters

The method is described and the experimental results are presented on the temperature determination of the (CF₃I) _N clusters in a beam (N ⩽ 10² is a number of monomers in a cluster) using SF₆ molecules from intersecting molecular beam as probe thermometers. The SF₆ molecules are captured by clusters in the crossed cluster and molecular beams and, after a certain time, sublimate from the surface of clusters carrying information on the velocity and temperature (internal energy) of clusters. Using time-of-flight (TOF) method the kinetic energy (velocity) of sublimated SF₆ molecules was measured and the temperature of clusters was determined to be T _cl = (88 ± 15) K.     

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.     

Measurement of nanoparticle temperature in a (CO2) N cluster beam using SF6 molecules as tiny probe thermometers

A temperature measurement technique using SF₆ molecules as tiny probe thermometers is described, and results are presented, for large (CO₂) _N van der Waals clusters (with N ≥ 10²) in a cluster beam. The SF₆ molecules captured by (CO₂) _N clusters in crossed cluster and molecular beams sublimate (evaporate) after a certain time, carrying information about the cluster velocity and internal temperature. Experiments are performed using detection of these molecules with an uncooled pyroelectric detector and infrared multiphoton excitation. The multiphoton absorption spectra of molecules sublimating from clusters are compared with the IR multiphoton absorption spectra of SF₆ in the incoming beam. As a result, the nanoparticle temperature in the (CO₂) _N cluster beam is estimated as T _cl < 150 K. Time-of-flight measurements using a pyroelectric detector and a pulsed CO₂ laser are performed to determine the velocity (kinetic energy) of SF₆ molecules sublimating from clusters, and the cluster temperature is found to be T _cl = 105 ± 15 K. The effects of various factors on the results of nanoparticle temperature measurements are analyzed. The potential use of the proposed technique for vibrational cooling of molecules to low temperatures is discussed.     

Strong dependence of ir multiphoton absorption and dissociation yield of SF6 molecule on intensity (duration) of exciting laser pulse

The role of played by the intensity of the exciting TEA CO₂ laser pulses in the processes of ir multiphoton absorption (MPA) of SF₆ molecules cooled toT _R?40 K andT _v?160 K in a pulsed supersonic jet and the dissociation of SF₆ in a bulk atT?300 K under essentially collisionless conditions have been investigated. A strong dependence of MPA and the dissociation yield on pulse intensity were observed. The frequency dependences of the intensity effects were studied.     

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.     

Effect of mode-locking on the multiple-photon reaction of SF6

The effect of mode locking on the CO₂ laser-induced reaction of SF₆ is reported. A mode-locked laser pulse enhances the SF₆ reaction yield by a small (8%) but significant amount. The mode-locked pulse also decreases the isotopic selectivity of the reaction. This small effect is greater for enrichment of ³³SF₆ than for ³⁴SF₆. These results suggest that absorption mechanisms that have high power dependence play a minor role in the multiple-photon reaction.     

Optical Q-branch pumping of the P- and R-branch transitions in the ν3 mode of SF6

One consequence of pumping the Q-branch of the ν₃ mode of SF₆ heretofore not examined is the production of substantial gain on certain low J R-branch transitions. In the analysis, the small-signal gain equation is derived for an individual symmetry species within a rotational manifold, since the nuclear spin statistical weigfhts play a significant role in the gain. The large value for the gain is a consequence of the selection rules governing the symmetry species and the way the spin statistical weights vary with the rotational quantum number J. It is shown that the transitions with gain, although only a fraction of a wave number removed from the pumped Q-branch, should be observable under appropriate conditions. For example, at a temperature of 135 K and a pump intensity of 10 kW/cm², the transitions with gain are not obscured by either hot-band absorption or by power broadening of the Q-branch. This induced gain may be of importance for the multiple photon dissociation of SF₆.     

From quantum chemistry to dissociation kinetics: what we need to know †

The relationship between rate constants for dissociation and the reverse association reactions and their potential energy surfaces is illustrated. The reaction systems e^? + SF₆ ? SF₆ ^? →SF₅ ^? + F, H + CH₃ ?CH₄, 2 CF₂ ? C₂F₄, H + O₂ →HO₂, HO + O ?HO₂ ? H + O₂, and C + HO →CHO are chosen as representative examples. The necessity to know precise thermochemical data is emphasised. The interplay between attractive and anisotropic components of the potentials influences the rate constants. Spin–orbit and electronic–rotational coupling in reactions between electronic open-shell radicals so far generally has been neglected, but is shown to have a marked influence on low temperature rate constants.     

Universal probe method for measuring the temperature of large clusters (nanoparticles) in a cluster beam

A universal probe method for measuring the temperature of large clusters (nanoparticles) in a cluster beam has been proposed and experimentally implemented. The temperature of large van der Waals clusters (nanoparticles) (CO₂) _N (where N ⩾ 10² is the number of monomers in a cluster) in the cluster beam is measured using this method with SF₆ molecules as miniature probe thermometers. The SF₆ molecules are captured by the (CO₂) _N clusters in intersecting cluster and molecular beams and sublimate from the surface of the clusters, carrying information on the velocity and temperature (internal energy) of the clusters. The velocity (kinetic energy) of SF₆ molecules sublimating from the surface of the clusters has been measured by the time-of-flight method and the temperature of the clusters has been determined as T _cl = (105 ± 15) K.     

Infrared Spectroscopy of the ν1 Band of Sulfur Tetrafluoride, SF4

Infrared absorption spectra of the ν₁ axial S-F stretching vibrational bands of sulfur tetrafluoride, SF₄, have been recorded and analyzed. A diode-laser jet spectrometer was used to record high-resolution spectra of ³²SF₄, with 87 vibration-rotation lines of the ν₁ fundamental being assigned. Least-squares fitting of the data yielded a precise vibrational band origin, as well as rotational parameters for the upper state. In addition, lower-resolution spectra of a static sample of SF₄ held at room temperature were recorded using a Fourier transform (FTIR) spectrometer. The progressions of prominent features observed in the room-temperature FTIR spectra are attributed to summation bands arising from ν₁ transitions from excited vibrational levels of the low-frequency ν₄ manifold of both ³²SF₄ and ³⁴SF₄.