The formation of an intense secondary pulsed molecular beam and obtaining accelerated molecules and radicals in it

A method for obtaining an intense secondary pulsed molecular beam is described. The kinetic energy of molecules in the beam can be controlled by vibrational excitation of the molecules in the source under high-power IR laser radiation. A compression shock (shock wave) is used as a source of secondary beams. The shock wave is formed in interaction between an intense pulsed supersonic molecular beam (or flow) and a solid surface. The characteristics of the secondary beam were studied. Its intensity and the degree of gas cooling in it were comparable with the corresponding characteristics of the unperturbed primary beam. Vibrational excitation of molecules in the shock wave and subsequent vibrational-translational relaxation, which occurs when a gas is expanded in a vacuum, allow the kinetic energy of molecules in the secondary beam to be substantially increased. Intense [≥10²⁰ molecules/(sr s)] beams of SF₆ and CF₃I molecules with kinetic energies approximately equal to 1.5 and 1.2 eV, respectively, were generated in the absence of carrier gases, and SF₆ molecular beams with kinetic energies approximately equal to 2.5 and 2.7 eV with He (SF₆/He=1/10) and H₂ (SF₆/H₂=1/10) as carrier gases, respectively, were obtained. The spectral and energy characteristics of acceleration of SF₆ molecules in the secondary beams were studied. The optimal conditions were found for obtaining high-energy molecules. The possibility of accelerating radicals in secondary molecular beams was demonstrated.     

Detection of SF<Subscript>6</Subscript> molecules sublimating from the surface of (CO<Subscript>2</Subscript>)<Subscript><Emphasis Type="Italic">N</Emphasis></Subscript> nanoparticles in a cluster beam by the infrared multiphoton excitation method

It has been found that SF₆ molecules captured by large van der Waals clusters (CO₂) _N (where N ≥ 10² is the number of monomers in a cluster) in intersecting molecular and cluster beams sublimate from the surface of clusters after a certain time and carry information on the velocity and temperature (internal energy) of clusters. Experiments have been carried out for detecting these molecules by means of a pyroelectric detector and the infrared multiphoton excitation method. The multiphoton absorption spectra of molecules sublimating from the surface of clusters have been obtained. The temperature of the (CO₂) _N nanoparticles in the cluster beam has been estimated using these spectra and comparing them with the infrared multiphoton absorption spectra of SF₆ in the initial molecular beam.     

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.     

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.     

Diagnostics of pulsed molecular beams and free jets with pyroelectric detectors and TEA CO2 lasers

A pyroelectric detector with a time resolution of 3–5 s and a TEA CO₂ laser have been used in diagnostics of a pulsed molecular beam (a free jet). The kinetic energy distribution of molecules was determined by using time-of-flight measurements both with a laser and without it. A combination of the laser with the pyroelectric detector makes it possible to determine the kinetic energy distribution of molecules in a selected internal state and to measure the energy absorted by the molecules of the beam from a laser pulse. The results obtained for pure SF₆ and the SF₆ seeded in He have been presented and analyzed. The advantages and the disadvantages of the method are being discussed in comparison with other available methods of diagnostics of molecular beams and free jets.     

Pressure-shock-controlled pulsed molecular beams

A method of controlling the duration of pulses of intense molecular beams is suggested. The idea of the method is the shortening of an initial molecular beam pulse by producing a pressure shock in front of a solid surface through which the beam passes. Experiments on shortening H₂, He, SF₆, SF₆/H₂(1/10), and SF₆/He(1/10) molecular beam pulses are reported. The parameters of the beams incident on, and transmitted through, the surface are studied. The gas density in the initial beam and in the pressure shock before the surface is estimated. The intensity and duration of shortened molecular pulses are found as a function of the initial intensity, angle of incidence, and the diameter of a hole on the surface through which the beam passes. It is established that the duration of the shortened beam decreases greatly with increasing incident intensity and decreasing hole diameter. It is shown that intense pulsed H₂, He, SF₆, SF₆/H₂(1/10), and SF₆/He(1/10) molecular beams with a pulse duration of ≤10–15 μs and an extent of ≤1–2 cm can be generated with the method suggested.     

On the depolarization of ultracold neutrons in traps

The method of pulse duration control is proposed for intense molecular beams. The method is based on the shortening of a primary molecular-beam pulse through the formation of a pressure shock ahead of a solid surface through which the beam is passed. The method was used to obtain intense SF₆, H₂, He, SF₆/H₂ (1/10), and SF₆/He (1/10) molecular beams with a pulse duration of ≤10?15 μs and a spatial length of ≤1?2 cm.     

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.     

Generation of high-energy secondary pulsed molecular beams

A method is suggested for generating high-intensity secondary pulsed molecular beams in which the kinetic energy of molecules can be controlled by an intense laser IR radiation through the vibrational excitation of molecules in the source. High-intensity [≥10²⁰ molecule/(sr s)] SF₆ molecular beams with a kinetic energy of ?1.0 eV without carrier gas and of ?1.9 and ?2.4 eV with carrier He (SF₆/He=1/10) and H₂ (SF₆/H₂=1/10) gases, respectively, were obtained.     

Infrared multiple-photon dissociation of ion-molecules in a beam

First experiments on the infrared multiple-photon dissociation with ion molecules in beams have been carried out. A mass selected beam of SF⁺₅ parent ions is crossed with a focused pulsed CO₂ laser beam. Using mass spectroscopy for the detection, fragmentation into SF⁺₄ and SF⁺₃ has been observed. The relative yield and power dependence have been measured. Features of the MPD of ions in beams are discussed.     

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.     

Mass spectrometry of arcs in SF6 circuit breakers

Today, SF₆ is used to a great extent as insulating and arc-quenching medium in high-voltage gas-blast circuit breakers. The arcing in SF₆ during current interruption forms decomposition products. These can influence the arc-quenching properties of the circuit breaker. Furthermore, they can cause corrosion of the circuit breaker housing. In this comprehensive study we present results obtained for the first time from a direct mass spectrometric investigation of the exhaust gases of a high pressure SF₆ arc in a model circuit breaker. Our mass spectrometric system consists of a time-of-flight mass spectrometer (TOFMS) equipped with a molecular beam sampling systems. This device allows us to measure mass spectra of high pressure sources with a time resolution of up to 10,000 spectra per second. We have determined the formation rate of the most abundant decomposition products in a SF₆ arc at 1 bar. These products are SF₄, CF₄, WF₆, SOF₂, SO₂, CS₂ S₂F₂ and HF. The fast detection time inherent to our system permits also the determination of the formation of SF₄, which is 0.45–0.50 Vol. %/(kJ/1SF₆). In addition, we have studied the influence of water and oxygen impurities which are responsible for the production of highly corrosive HF. Finally, we have considered the influence of the thermal degradation of teflon (P.T.F.E.), which is used as nozzle and insulating material in circuit breakers. On this occasion we have demonstrated that CF₄, which exhibits dielectric properties similar to SF₆, is the main decomposition product formed from teflon. However, we have found that besides CF₄ also excess carbon is formed, which is deposited on insulators of the model circuit breaker.Our time-resolved mass spectra reveal that the CF₄ production from teflon is delayed by a few milliseconds with respect to the SF₆ dissociation in the arc. This delay can influence the interrupting process of the circuit breaker by changing the plasma composition during the arcing period. Although our experiments have been performed on a model circuit breaker we claim that the results presented in this study can be applied to real circuit breakers, since the arc current density and the energy dissipated per liter SF₆ are of the same order of magnitude in both devices.     

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.     

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.     

Spectral characteristics of IR-multiphoton excitation in supersonic molecular beams

The multiphoton absorption of SF₆ was investigated in supersonic molecular beams in dependence on the fluence and the wavelength of the CO₂ laser. The temperatures of the SF₆ molecules have been reduced using seeded beams of different concentrations. The experimental results are discussed on the basis of the known spectroscopic data of SF₆ and provide some novel information about the spectral characteristics of the ir-multiphoton excitation of strongly cooled molecules in the collision-free case.     

Vibrational predissociation of SF6 dimers and trimers

IR photo-dissociation spectra of SF₆ clusters have been studied. A He-seeded molecular beam has been attenuated by crossing it with a line tunable cw CO₂ laser of moderate power. — In the electron bombardment beam ionizer (E _el=100eV) small neutral clusters are found to fragment predominantly to the main monomer mass (SF ₅ ⁺ ). — Predissociation spectra have been calculated for clusters containing up to six SF₆-molecules invoking the dipole-dipole resonance force to lift the degeneracy of the molecule — excited molecule interaction. On the basis of these spectra, dimer and trimer concentrations have been determined quantitatively, for different molecular beam conditions.     

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.     

Generation of intense pulsed low-kinetic-energy molecular beams

A method for the generation of intense pulsed low-kinetic-energy molecular beams is described. The method is based on the formation of a cold (≈77 K) pressure shock as a result of interaction between an intense pulsed gas-dynamically cooled molecular beam with a solid surface. The pressure shock is used as a source of a secondary beam for generating low-energy molecules. The suggested method was used to obtain intense molecular beams of H₂, He, CH₄, N₂, and Kr with kinetic energies lower than or equal to 10 meV and H₂/Kr and He/Kr molecular beams with kinetic energies of H₂ and He molecules lower than 1 meV. The energy (velocity) of molecules in low-energy beams can be controlled by varying the intensity of the initial beam or temperature in the pressure shock.     

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.     

The role of partial saturation of the λ3 ladder in SF6 absorption under high power infrared laser excitation

A rate process model is developed to interpret the experimental results of Smith, Schmid, Tablas and Kompa on the time and intensity dependence of the infrared absorption of SF₆. In this model incoherent excitation of the λ₃ molecular mode by the processes of absorption and stimulated emission is followed by radiationless energy transfer from a maximum level to an inactive mode heatbath. This achieves control of the level population by an empirical transfer rate parameter. Good theoretical fits of the experiment absorption decays are obtained with values of approximately 0.1 ns.