Generation of high-energy-resolved NH_3 molecular beam by a Stark decelerator with 179 stages

We demonstrate the production of cold, slow NH_3 molecules from a supersonic NH_3 molecular beam using our electrostatic Stark decelerator consisting of 179 slowing stages. By using this long Stark decelerator, a supersonic NH_3 molecular beam can be easily decelerated to trappable velocities. Here we present two modes for operating the Stark decelerator to slow the supersonic NH_3 molecules. The first is the normal mode, where all 179 stages are used to decelerate molecules, and it allows decelerating the NH_3 molecular beam from 333 m/s to 18 m/s, with a final temperature of 29.2 mK.The second is the deceleration-bunch mode, which allows us to decelerate the supersonic NH_3 beam from 333 m/s to 24 m/s,with a final temperature of 2.9 m K. It is clear that the second mode promises to produce colder(high-energy-resolution)molecular samples than the normal mode. Three-dimensional Monte Carlo simulations are also performed for the experiments and they show a good agreement with the observed results. The deceleration-bunch operation mode presented here can find applications in the fields of cold collisions, high-resolution spectroscopy, and precision measurements.     

Gasdynamic parameters of a supersonic molecular beam seeded by fullerene molecules

Gasdynamic parameters of a molecular beam formed by passing a pulsed beam of a neutral gasthrough a Knudsen cell filled with a fullerene vapor are studied experimentally. It is shown that this method canbe used to advantage in generating supersonic molecular beams of virtually any substances, which is of specialimportance for depositing epitaxial films and films of low-volatility materials.     

Formation of mixed clusters in a pulsed supersonic helium-oxygen-isoprene jet

An experimental study of the formation of mixed van der Waals oxygen-isoprene complexes, generated in an expanding supersonic helium-oxygen-isoprene jet at various stagnation pressures and at diverse oxygen and isoprene concentrations, has been performed. To measure the composition and distribution of the partial densities of the individual components, molecular beam mass spectrometry was adapted to pulsed modes of gas source operation. The particularities of applying mass spectrometry to studying clustered isoprene streams in a pulsed mode have been discussed. The composition of small clusters generated in a free supersonic jet has been checked for dependencies upon the initial mixture composition and stagnation pressure. The mechanism of nucleation has been identified for different partial concentrations of impurities in the helium stream. It has been shown that, even at a 0.3% concentration of isoprene in the mixture, nucleation starts with the formation of hydrocarbon complexes. The specific features of the dissociative ionization of van der Waals complexes, consisting of pure isoprene and mixed complexes, have been discussed. The conditions needed for the formation of binary oxygen-isoprene van der Waals complexes have been identified.     

Features of molecular-beam mass spectrometry registration of clusters in underexpanded supersonic jets

The structure of clustered supersonic underexpanded jets of molecular nitrogen and argon was measured by the method of molecular beam mass-spectrometry. Peculiarities of application of the molecular beam methods for recording the supersonic rarefied gas jets under the conditions of weak and developed condensation (i.e., in the presence of small and large clusters in jets) have been discovered, identified, and studied. An unusual shape of longitudinal and transverse cross sections of the clustered supersonic jets was revealed and explained when scanning with a molecular-beam system. It has been determined that small clusters and monomers are the sources of double-ionized monomers available near the flow axis, and dimer ions at the early stages of condensation, whereas another mechanism of such ion formation dominates, when large clusters area available in the flow. A marker for fixing the stage of formation of small clusters in a supersonic flow is proposed.     

Effect of diluent gas on silicon film deposition from a free jet of monosilane-diluent mixture activated by electron-beam plasma

Thin silicon films were synthesized by the gas-jet electron beam plasma chemical vapor deposition method from monosilane-argon, monosilane-argon-helium, and monosilane-argon-hydrogen mixtures. Addition of argon to the argon-silane mixture increased the deposition rate of silicon films, whereas addition of helium and hydrogen to the same mixture decreased the growth rate. It is shown that the process of silicon film deposition by this method from argon-monosilane mixture is primarily governed by fast secondary electrons, and argon dilution of mixture leads to increasing concentration of fast secondary electrons and increasing deposition rate of silicon films. Dilution of the initial mixture with helium or hydrogen causes a decrease in the deposition rate either due to gas-dynamic behavior of the supersonic jet of the mixture of light and heavy gases, or due to the etching effect of metastable helium atoms or hydrogen atoms on the surface of the growing silicon film.     

HL-2A托卡马克超声分子束注入深度和加料效果研究

超声分子束注入深度与加料效率是分子束加料研究中的基本课题.在近期开展的超声分子束注入实验中，发现分子束注入深度与等离子体电子温度和密度、分子束源的气压和温度有直接关系，获得了分子束注入深度的定标律.在低温气体源(液氮冷却)的分子束注入实验中，发现分子束流中形成了团簇，其注入深度超过30 cm，分析了在低温气源分子束注入实验中的团簇现象. 关键词： 超声分子束注入 注入深度 加料效率 团簇     

Electron impact ionization of Ar n , (H2O) n , Ar n (H2O) m clusters

Signals from ions forming in a supersonic molecular beam consisting of an argon-water vapor mixture are measured as functions of the exciting electron energy in the range to 120 eV. The thresholds of electron impact excitation of (H₂O) _{n − 1}H⁺ and Ar _n (H₂O _m ⁺ clusters are determined for the first time. It is found that the proton-hydroxyl group binding energy decreases considerably both in the case of water molecule clustering and when mixed Ar _n (H₂O) _m clusters arise.     

Combustion of a supersonic flowing propane-air mixture within a DC longitudinal-transverse discharge

The findings of experimental investigations on the combustion of a supersonic flowing propane-air mixture with initiation by a dc discharge are given. It was found that the pattern of combustion depended on a number of initial conditions associated with the generation of plasma and with the external parameters of the supersonic flow.     

Reactor laser using a mixture of xenon with uranium hexafluoride

The possibility is considered of developing a reactor laser based on a mixture of xenon, uranium hexafluoride, and a buffer gas. The kinetic scheme of populating the xenon monofluoride exciplex is analyzed. The conditions for the possibility of less stringent requirements on the nuclear pumping intensity are investigated. The conditions that ensure the necessary heat dissipation under various reactor-laser operating conditions are considered. It is shown that in principle supersonic flow of the working gas can ensure energy output in the form of laser radiation.Translated from Trudy Fizicheskogo Instituta im. P. N. Lebedeva, Vol. 120, pp. 43–50, 1980.     

Dissociative attachment of low-energy electrons to vibrationally excited molecules using a photoelectron source

The process of dissociative attachment (DA) of low-energy electrons ) to vibrationally excited sodium dimer molecules is studied with high electron energy resolution () in a supersonic molecular beam. A novel photoelectron source, based on two-step photoionization of the sodium atoms in the beam, may deliver a current of up to 1 nA and has been used with a current of typically 0.2 nA in this experiment. The energy dependence of the rate of sodium anion formation is determined by ion detection based on a time-of-flight analysis. The molecules are selectively excited to levels using the technique of coherent population transfer by delayed pulses (STIRAP). The comparison of the experimental data with recent resonance model calculations based on improved potential curves reveals generally good agreement for levels v ”>12. For some distinct differences between theoretical and experimental results persist. Received: 21 November 1998 / Received in final form: 7 April 1999     

Kinetic mechanisms for the initiation of supersonic combustion of a hydrogen-air mixture behind a shock wave under the excitation of molecular vibrations in initial reagents

Mechanisms for the initiation of autoignition in hydrogen-air mixtures in a supersonic flow behind a shock at temperatures ≤700 K when the H₂ and N₂ molecule vibration is selectively excited are considered. By exciting molecular vibration in the gases, one can initiate detonation combustion behind the shock front even at weak shocks at gas temperatures ≤500 K. It is established that even a small (<0.15%) amount of vibrationally excited ozone present in the reacting mixture may considerably shrink the induction zone.     

Organic thin-film transistors of pentacene films fabricated from a supersonic molecular beam source

Top-contact organic thin-film transistors (OTFTs) of pentacene have been fabricated on bare SiO₂ and SiO₂ modified with hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (OTS). The pentacene films were deposited from a supersonic molecular beam source with kinetic energy of incident molecules ranging from 1.5 to 6.7 eV. The field-effect mobility of OTFTs was found to increase systematically with increasing kinetic energy of the molecular beam. The improvements are more important on HMDS- and OTS-treated surfaces than on bare SiO₂. Tapping mode atomic force microscopy images reveal that pentacene thin films deposited at high kinetic energy form with significantly larger grains—independent of surface treatment—than films deposited using low-energy beams.     

Removal of 10-nm contaminant particles from Si wafers using argon bullet particles

Removal of nanometer-sized contaminant particles (CPs) from substrates is essential to successful fabrication of nano scale devices. But the cleaning limit of various current technologies stay around 50 nm. Cryogenic aerosol beam has long been successfully employed to remove CPs down to 50 nm, and supersonic particle beam using particles smaller than 100 nm lowered the limit of cleaning down to 20 nm size. In this study, the particle beam technique that uses nanometer-sized bullet particles moving at supersonic velocity was improved, and successfully employed to remove contaminant particles as small as 10 nm. Ar nano-bullets of about 20–50 nm were generated by gas-phase nucleation, and growth in a supersonic nozzle: appropriate size and velocity of the nano-bullets were obtained by optimizing the Ar/He mixture fraction and nozzle contours. Cleaning efficiency >95% was attained. Nano-bullet velocity was found to be the most important parameter affecting removal of contaminant particles in the 10-nm size range.     

用H-S波前传感器测量穿过超声速流场的激光像差特性

 用Hartmann-Shack(H-S)波前传感器可以准确测量穿过超声速流场的激光波面及其变化过程,进一步计算可以获得光束质心漂移、远场分布等数据。给出了H-S方法测量穿过超声速流场激光波面的原理,采用模式法进行了波前重构,计算了在几种流场条件下的激光波面像差特性PV值,RMS值和Zernike像差系数、Strehl比和环围能量曲线等。结果表明,H-S法可以很好地反映流场建立、稳定和结束过程中Zernike像差的变化。比较无流场和给定参数的超声速流场,激光穿越后产生的最明显的像差变化为离焦和低阶像散的增大。在相同流场参数下,无模型时光束质量好于有模型时光束质量。     

State reduction in quantum mechanics: A calculational example

The question is posed as to the role of the state reduction hypothesis in the passage of a molecular beam through a Stern-Gerlach apparatus of the ordinary sort, and of the sort proposed by Wigner, in which the beam is rejoined at the end of the apparatus. The effect of the magnetic field is calculated, as is the effect of a “detector” atom placed asymmetrically in the apparatus, and sensitive to the passage of the molecular beam nearby. It is shown that in the absence of the detector atom the rejoined beam is represented by a pure case density matrix. However, when the detector is present in one arm of our Stern-Gerlach apparatus, the beam is found to be in a statistical mixture approximating “reduction”, even if we never “look” at the detector atom. It is also seen that the beam density matrix is reduced to diagonal form even though it is not physically affected by the detector.     

Initiation of a detonation wave by resonant laser radiation in a hydrogen-oxygen mixture flowing about a wedge

The formation of an oblique detonation wave in a supersonic hydrogen-oxygen flow about a planar wedge is considered. It is shown that the excitation of the electronic state b ¹Σ _g ⁺ in oxygen molecules by resonant laser radiation with a wavelength of 762 nm makes it possible to initiate detonation combustion at a distance of ≈1 m from the tip of the wedge at low temperatures (500–600 K). Notably, it suffices to irradiate the gas in the narrow (0.5–1.0 cm across) paraxial region of the flow near the tip of the wedge. It is found that the laser-induced excitation of molecular oxygen is several times more efficient than ordinary heating of the mixture to initiate a detonation wave.     

Discharge in the Atmosphere in a Gaussian Beam of Subthreshold Millimeter Waves

The propagation velocities of a subthreshold discharge excited in air at atmospheric pressure by a pulsed microwave beam with a Gaussian field distribution, a wavelength of 4 mm, and an intensity up to 30 kW/cm² have been measured by means of optical and microwave diagnostics. It has been shown that the motion of a discharge front along the path of the beam toward the region of an increasing microwave field is accompanied by an increase in the velocity from subsonic (~10^–4 cm/s) to supersonic (~(6–8) × 10⁴ cm/s). At the same time, motion toward the decreasing field region is accompanied by a decrease in the velocity from supersonic to subsonic. It has been found that the maximum temperature of the gas in the discharge at velocities of ~10⁴ cm/s reaches ~5.3 kK.     

Interaction of polar molecules with a resonant RF electric field: strong deflection of a NO molecular beam

Deflection of a cold supersonic NO beam seeded in He has been observed when these molecules interact with both static and a resonant oscillating electric field. The NO beam splits into two beams each one deflecting about 0.5° towards the positive and negative direction of the Stark field when the employed resonant frequency between the two Stark levels of the NO molecule is 1515 kHz. This deflection angle is about four orders of magnitude higher than the value one would expect from the NO dipole moment and the employed RF field gradient. This phenomenon suggests the possibility of a significant translational motion perpendicular to the beam axis, which is induced by the resonant RF electric field on the cold and high-density supersonic beam.     

The analysis of a time-of-flight neutron diffractometer for amorphous materials: the structure of a molecule in a liquid

A comparison is made of the conventional method of using a continuous monochromatic slow neutron beam method of measuring the structure factor of an amorphous material (e.g. a liquid or a glass) and the method using a pulsed beam of heterochromatic neutrons and measurement of their time-of-flight. The Placzek [1] method has been adapted to the analysis of the time-of-flight experiment. The corrections required to convert the measured counting rates to static structure factors are explicitly evaluated for various detector laws and for any incident spectrum. It is shown that there is no particular virtue in equal-path time-of-flight arrangements. There is however a particular interest in a ‘1/v ²’ detector.

The method of analysis is of particular value for the determination of the interference part of the structure factor at high momentum transfer. Some recent experiments on molecular liquids (Clarke and Dore [2 (a] are analysed and it is shown that certain features of the structure of a molecule in the liquid state can be determined. This is a novel result and is of particular importance for liquid water. It is suggested that the D₂O molecule in liquid heavy water at 20°C has a form closer to that in the vapour than that found in ice.     

Interaction of a supersonic beam of toluene with a resonant RF electric field

Deflection of a cold supersonic toluene beam seeded in He has been observed when these molecules interact with both a static and a resonant oscillating electric field. The toluene beam splits into two beams each one peaking at a deflection angle of 1 degree towards the positive and negative direction of the Stark field when the employed resonant frequency between the two Stark levels of the toluene molecule is 1411 kHz. This deflection angle is about four orders of magnitude higher than the value one would expect from the toluene dipole moment and the employed RF field gradient. Different hypothesis are suggested to explain the observed strong beam splitting including the possibility of transverse beam interferences induced by both the resonant RF field and the transverse uniform electric field. A theoretical model is presented based on molecular beam interferences induced by the resonant RF field which seems to account satisfactorily for the present observations.