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     

Measurement of the vibrational energy-transfer rates in mixtures of polyatomic molecules

The pressure coefficient of the vibrational deexcitation of methanol molecules caused by collisions with NH₃, CO₂, SF₆ molecules and He atoms is measured by infrared-far-infrared double-resonance spectroscopy. Large coefficients are measured for those buffer gases which increase the output power of far-infrared lasers. For other buffer gases such as NH₃ and CO₂ the measured coefficients are zero within the limits of experimental error. The relevance to measurements on the photoacoustic trace-gas analysis is discussed.     

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.     

The influence of vibrational excitation on the burning voltage of a pulsed electric discharge in HF laser working media

The burning voltages of an intermediate pressure self-sustained volume discharge (SSVD) in SF₆ and SF₆-C₂H₆ mixtures irradiated by a 10.6 μm pulse TEA CO₂ laser, have been measured on varying the laser fluences over a wide range. The delay between the voltage application and the laser pulse onset is 4 μs, and the laser pulse lasts ∼3 μs. The considerable rise observed in the discharge voltages with increasing absorbed specific laser radiation energy, is due to electron attachment to vibrationally excited molecules of SF₆. Different processes of relaxation of the vibrational energy stored in SF₆ molecules are analyzed and the relevant characteristic times are numerically assessed. The gas heating process owing to vibration-translation energy exchange is qualitatively described in terms of the “thermal explosion”. The relation between the “explosion” and delay times determines the peculiarities of electron attachment to vibrationally excited SF₆ molecules. The burning voltages of a submicrosecond non-irradiated SSVD in the above-mentioned media versus the specific electric energy deposited are also measured. They are compared to those of a laser-illuminated SSVD at commensurable specific laser energy depositions. It is concluded that electron attachment to the discharge-produced vibrationally excited SF₆ molecules is not capable of noticeably affecting the discharge voltages of a submicrosecond non-irradiated SSVD. PACS 42.55 Ks; 52.80     

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.     

Messung und Deutung des Leitwertabklingens zylindrischer Bögen

The decay of the electrical conductance of the column of a cascaded arc (100 A) after current interruption is measured for the gases N₂, SF₆, CO₂ and Ar. Numerical computations of temperature and conductance decay are in good agreement with experiment for Ar but for N₂ and SF₆ there are drastic deviations. An interpretation of the decay processes is given by the aid of a simple model for the energy transfer including nonequilibrium processes with regard to recombination in the wall near arc regions. Introducing the model by a formal manner in the computer program leads to good agreement with experiment.     

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.     

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.     

Absorption of CO2 laser pulses at different wavelengths by ground-state and vibrationally heated SF6

The absorption of CO₂ laser pulses by low pressure SF₆ gas has been investigated over a wide range of energy fluxes. For laser energy fluxes of 0.01–1 J cm^-2 the effective absorption cross section varies between 0.2 and 2 × 10^-18 cm². For each laser line an individual dependence on the energy is found and in some cases minor changes in the absorption behaviour seem to occur around 0.1 J cm^-2. SF₆ excited with an average vibrational energy content of up to 20 photons/molecule does not absorb measurable amounts of 9.4 μm laser light. The influence of various SF₆ and Ar pressures on the temporal shape of the transmitted pulses has been investigated.     

Spectroscopic studies of UV production from ARC sources in iodine laser gas mixtures

Time-integrated spectral measurements of the light from multiple arc sources in iodine laser gas mixtures (He:SF₆:i-C₃F₇I and Ar:SF₆:i-C₃F₇I) are described. Copious soft-UV output, fully sufficient as a pump source, is observed with no adverse decrease in UV production for i-C₃F₇I concentrations as high as 3% by volume. Conversion efficiencies of 3–4% for stored electrical energy into UV radiation within the iodide compound band were determined.     

悬浮型微波共振探针在电负性容性耦合等离子体中电子密度的测量

本文首先利用悬浮型微波共振探针测量了Ar等离子体的电子密度,并与朗缪尔双探针的测量结果进行了比较,表明了微波共振探针在低密度等离子体测量的可行性.对40.68 MHz单射频容性耦合Ar/SF₆和SF₆/O₂等离子体的测量结果表明:电负性气体SF₆掺入Ar等离子体显著降低了等离子体电子密度,但随着增加SF₆的流量,电子密度表现为缓慢下降;而O₂掺入SF₆等离子体中,电子密度则随着O₂流量的增加表现为持续的下降.另外,40.68 MHz/13.56 MHz双频激发的SF₆/O₂容性耦合离子体的电子密度并不随低频功率的变化而变化.本文对上述的实验现象进行了初步的解释.     

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.     

Total cross-sections for positron scattering by a series of molecules

The additivity rule is employed to obtain the total (elastic+inelastic) cross-sections for positron scattering from molecules including a number of diatomic, polyatomic molecules (H₂, N₂, HCl, CO₂, NH₃, SF₆, CH₄, C₂H₄ and C₃H₈) over an incident energy range of 10-1000 eV. The total cross-sections (TCS) of the constituent atoms of molecules are obtained by employing a complex optical model potential (composed of static, polarization and absorption potential). The present results are compared with experimental data and other theoretical calculations, good agreement is obtained in intermediate- and high-energy region. Received: 11 November 1997 / Revised: 23 March 1998 / Accepted: 16 June 1998     

Deposition and Etching of Thin Films by Means of Beam Plasma Discharge with Tube-Like Electron Beam

The efficiency of energy dissipation caused by the mechanism of turbulent plasma heating from a tube-like electron beam in a electronegative gas is measured calorimetrically. Amorphous Si-layers are deposited from a SiH₄/Ar discharge both on conducting and nonconducting substrates and are analyzed by means of Auger-spectrometry. The velocity and specific energy of etching of SiO₂-layers in a CF₄-plasma are determined. Measured etching velocities of tungsten surfaces biased differently against the space potential provide information on the mechanism of plasma etching in a SF₆-plasma. Simultaneously performed Langmuir-probe measurements permit predictions to be made on the properties of the etching plasma.     

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.     

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.     

羟基碳纳米管吸附SF6放电分解组分的DFT计算

本文根据密度泛函理论(density functional theory , DFT), 采用MS分子动力学仿真软件对羟基修饰的单壁碳纳米管(SWNT-OH) 吸附SF₆局部放电分解的四种主要组分SOF₂, SO₂F₂, SO₂和CF₄进行了详细的理论计算, 通过分析气体分子和SWNT-OH的前线轨道, 吸附过程中吸附能、电荷转移量和电子态密度的情况, 以及吸附前后SWNT-OH能隙的变化, 评判了SWNT-OH对气体分子的敏感性和选择性, 给出了SWNT-OH是否可以制备气体传感器检测SF₆局部放电分解组分的理论依据.     

±100 kV气体轨道开关击穿特性

 研制了一种用于快Marx发生器的三电极场畸变结构的气体轨道开关,针对3种不同的绝缘介质N₂、干燥空气和SF₆/Ar混合气体开展了轨道开关的自击穿特性和触发特性研究。结果表明:轨道开关的自击穿电压与气压基本呈现线性关系,并且在相同实验条件下,相对于N₂和干燥空气,当绝缘介质为SF₆/Ar混合气体时,轨道开关具有工作电压范围宽、工作系数低、触发击穿延时短、抖动小的优点。采用数码相机拍摄了开关导通过程中形成的放电通道积分图像,初步研究了绝缘介质、电流大小等因素对开关形成多通道的影响。结果表明:提高触发电压、充电电压和放电电流及采用SF₆/Ar混合气体均能够有效增加放电通道数目。     

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.     

Growth of van der Waals clusters in steady supersonic Laval nozzle flow: change in number density of SF6 monomers undergoing clustering

A mixture of SF₆ (4% molar fraction) and inert Ar gas was continuously fed into a plane-symmetric guided Laval nozzle. The clustering of SF₆ was observed using an FT-IR spectrometer in the nozzle flow (optical pathlength: 10 mm). The density and vibrational temperature of the SF₆ monomers were experimentally determined from the absorption peaks of the monomers. We theoretically calculated the density of the SF₆ monomers as a function of distance from the throat of the nozzle using a cluster formation model based on the RRK (Rice–Ramsperger–Kassel) theory to represent the dynamics of dissociation of the vibrationally excited clusters. We found that the model adequately simulated the decrease in SF₆ monomers and 10 collisions of the vibrationally excited SF₆ clusters with Ar atoms were necessary for the stabilization of the clusters under our experimental conditions. Received: 20 June 2000 / Revised version: 26 October 2000 / Published online: 21 February 2001