Showing papers on "Buffer gas published in 1999"

Decelerating Neutral Dipolar Molecules

Abstract: Slowing down atoms using laser cooling, together with techniques like magnetic trapping and evaporative cooling, has led to numerous exciting results over the last decade [1], with the realization of Bose-Einstein condensation as the most spectacular achievement [2]. Laser-cooling schemes are applicable to atoms since closed multilevel systems can be realized and many consecutive absorptionemission cycles can be induced. This leads to a significant momentum transfer from the photons of the laser to the atom, resulting in a change of the velocity of the atom. There currently is a great interest in producing cold molecules for the study of cold molecule-molecule collisions, molecular quantum collective effects, Doppler-free spectroscopy, frequency standards, as well as many other applications already realized for atoms [3]. Slowing down of neutral molecules, a prerequisite for being able to trap them, has proven to be considerably more difficult to be achieved, however. The above-mentioned laser-cooling scheme cannot be applied to slow down molecules as no simple closed-level systems can be found; the unavoidable off-resonant radiative decay of molecules from the excited state to other vibrational levels in the electronic ground state hampers efficient momentum transfer [4]. Once molecules are present at sufficiently low kinetic energies, below a fraction of a cm21, they can be trapped in various ways. Trapping of neutral molecules in inhomogeneous magnetic [3] and electric fields [5] and trapping based on the use of evanescent fields close to the dielectricvacuum interface have been proposed [6]. It has also been suggested that extremely high laser powers might be used to induce a dipole moment in polarizable molecules, leading to trapping of these molecules in the laser beam [7]. Other methods to produce slow molecules have been put forward and have recently been successfully applied to produce the first samples of trapped molecules. By injecting molecules in a cold buffer gas, sufficiently low kinetic energies of the injected molecules can be obtained after thermalization that they can subsequently be trapped in an inhomogeneous magnetic field. This has recently been demonstrated for CaH injected in 3He buffer gas at temperatures of around 0.3 K [8]. Associative collisions between ultracold trapped atoms [9] have proven to be

Application of a hexapole collision and reaction cell in ICP-MS Part I: Instrumental aspects and operational optimization

Abstract: The application of an ion-guiding buffer gas-filled hexapole collision and reaction cell in ICP-MS has been studied in order to give a preliminary performance characterization of a new instrument providing this feature for increasing the ion yield and decreasing contributions from Ar induced interfering molecular ions. As buffer gas He was used while H2 served as reaction gas. Addition of the latter can be an effective means for reduction of typical argon induced polyatomic ions (Ar+, ArO+, Ar2+) by orders of magnitude owing to gas phase reactions. Molecular interferences generated in the cell can be suppressed by a retarding electric field established by a dc hexapole bias potential of –2 V.

Buffer-gas loaded magnetic traps for atoms and molecules: A primer

Abstract: Over the past three years we have developed the technique of buer-gas cooling and loading of atoms and molecules into magnetic traps. Buer-gas cooling relies solely on elastic collisions (thermal- ization) of the species-to-be-trapped with a cryogenically cooled helium gas and so is independent of any particular energy level pattern. This makes the cooling technique general and potentially applicable to any species trappable at the temperature of the buer gas (as low as 240 mK). Using buer-gas loading, paramagnetic atoms (europium and chromium) as well as a molecule (calcium monohydride) were trapped at temperatures around 300 mK. The numbers of the trapped atoms and molecules were respectively about 10 12 and 10 8 . The atoms and molecules were produced by laser ablation of suitable solid precursors. In conjunction with evaporative cooling, buer-gas loaded magnetic traps oer the means to further lower the temperature and increase the density of the trapped ensemble to study a large variety of both static (spectra) and dynamic (collisional cross-sections) properties of many atoms and molecules at ultra-low tem- peratures. In this article we survey our main results obtained on Cr, Eu, and CaH and outline prospects for future work.

Effect of plasma shielding on laser ablation rate of pure metals at reduced pressure

Abstract: The ablation rate expressed as the amount of removed material per laser shot was calculated for pure metal samples under different experimental conditions: laser fluence (1.3‐16.7 J cm 2 ), buffer gas (air, He and Ar) and gas pressure (10 3 ‐10 5 mbar). Fluence values covered the range between the plasma threshold (~1‐2 J cm 2 for most elements) and 16.7 J cm 2 . The 581 nm output of an excimer-pumped dye laser was used. Results pointed out a strong dependence of ablation rate on experimental parameters. At high fluence, the ablated material efficiently attenuates the incoming laser radiation (plasma shielding) and reduces the ablation rate. The extent of this shielding effect depend also on the experimental variables (buffer gas, pressure) and sample nature. These studies are useful to determine the amount of ablated material as a function of experimental parameters, to understand the extension of the shielding process and to establish the conditions under which it may be avoided. Copyright ” 1999 John Wiley & Sons, Ltd.

Apparatus for depositing thin films on semiconductor wafer by continuous gas injection

Abstract: An apparatus for depositing thin films of a semiconductor device. The thin film deposition apparatus includes: a reactor maintained at a constant pressure; at least two reaction gas supply portions for supplying reaction gases to the reactor; an exhaust pump for discharging the gases out of the reaction gas supply portions and/or the reactor; first flow control valves installed between each reaction gas supply portion and the reactor, for controlling the amount of gases flowing between the reaction gas supply portions and the reactor; second flow control valves installed between each reaction gas supply portion and the exhaust pump, for controlling the amount of gases flowing between the reaction gas supply portions and the exhaust pump; an inert gas supply portion for supplying an inert gas into the reactor; reaction gas pipe lines, wherein the reaction gases provided from the reaction gas supply portions flow through the reaction gas pipe lines to the reactor and/or the exhaust pump; an inert gas pipe line, wherein the inert gas provided from the inert gas supply portion flows through the inert gas pipe line to the reactor; and a plurality of valves installed in the reaction gas pipe lines and/or the inert gas pipe lines, for controlling the amount of reaction gases and inert gas flowing into the reactor and/or the exhaust pump.

Molecular fluorine (F2) laser with narrow spectral linewidth

Abstract: A molecular fluorine (F 2 ) laser is provided wherein the gas mixture comprises molecular fluorine for generating a spectral emission including two or three closely spaced lines around 157 nm. An etalon provides line selection such that the output beam only includes one of these lines. The etalon may also serve to outcouple the beam and/or narrow the selected line. Alternatively, a prism provides the line selection and the etalon narrows the selected line. The etalon may be a resonator reflector which also selects a line, while another element outcouples the beam. The etalon plates, preferably uncoated, comprise a material that is transparent at 157 nm, such as CaF 2 , MgF 2 , LiF 2 , BaF 2 , SrF 2 , quartz and fluorine doped quartz. The etalon plates are separated by spacers comprising a material having a low thermal expansion constant, such as invar, zerodur®, ultra low expansion glass, and quartz. A gas such as helium or a solid such as CaF 2 that does not absorb radiation 157 nm fills the gap between the etalon plates, or the gap is evacuated. The gas mixture preferably further includes neon as a buffer gas. Another etalon may be used for line narrowing. One or more of a highly reflective mirror, a high finesse etalon or a prism with a highly reflective back surface may serve as a highly reflective resonator reflector. Any of one or more etalons, a prism, a brewster plate and a highly or partially reflective mirror may seal the laser discharge chamber.

Vacuum versus gas environment for the synthesis of nanocomposite films by pulsed laser deposition

Abstract: Nanocomposite films formed by Cu nanocrystals (NCs) with sizes <10 nm embedded in an amorphous Al2O3 host have been grown by alternate pulsed-laser deposition both in vacuum and in a buffer gas (Ar) up to pressures of 0.1 Torr. The dimensions, dimension distributions, and shape of the NC produced in vacuum and in Ar up to pressures of 5×10-3 Torr follow a similar trend as a function of the Cu areal density. This allows us to conclude that the nucleation and growth of the NC are dominated by processes occurring at the substrate surface rather than in the gas phase. For Ar pressures ≥5×10-2 Torr, the anisotropy of the NC is enhanced, the deposition rate decreases abruptly and a significant amount of the buffer gas is incorporated into the host, thus leading to the formation of a porous material.

F2 (157nm) laser employing neon as the buffer gas

Abstract: An F 2 -laser has a discharge chamber containing a laser gas mixture including fluorine as a laser active component and neon as a buffer gas. The gas mixture is surrounded by a resonator and supplied with a pulsed discharge by a pair of electrodes connected to a power supply circuit. The concentration of neon within the gas mixture is preferably higher than any other constituent gas, and is more preferably the only gas accompanying the laser active molecular fluorine. In addition, the gas mixture is preferably maintained at an elevated temperature such as near, yet below, a temperature at which outgassing occurs within the discharge chamber.

Plasma pinch high energy with debris collector

Abstract: A high energy photon source. A pair of plasma pinch electrodes are located in a vacuum chamber. A working gas which includes a noble buffer gas and an active gas chosen to provide a desired spectral line. A pulse power source provides electrical pulses at voltages high enough to create electrical discharge between the electrodes to produce very high temperature, high density plasma pinch in the working gas providing radiation at the spectral line of the active gas. An external reflection radiation collector-director collects radiation produced in the plasma pinches and directs the radiation in a desired direction. In a preferred embodiment the active gas is lithium and the buffer gas is helium and the radiation collector-director is coated with the material used for the electrodes. A good choice for the material is tungsten. In a second preferred embodiment the buffer gas is argon and lithium gas is produced by vaporization of solid or liquid lithium located in a hole along the axis of the central electrode of a coaxial electrode configuration. Other preferred embodiments utilize a conical nested debris collector upstream of the radiation collector-director.

Light shift of coherent population trapping resonances

Abstract: We have measured the spectral position of the absorption minimum in a coherent population trapping resonance in thermal cesium vapor as a function of light intensity. The dependence of position on intensity is found to be almost linear. We have furthermore studied the dependence of this light shift on neon buffer gas pressure and find a strong reduction for higher pressures. So the addition of a buffer gas not only reduces the linewidth of the resonance but also a very important systematic effect for precision measurements.

Production of single-wall nanotubes by high-temperature pulsed arc discharge : mechanisms of their production

Abstract: Single-wall carbon nanotubes (SWNTs) have been produced for the first time using the high-temperature pulsed arc-discharge technique, which has been developed recently in this laboratory, as reported in the conventional steady arc discharge method. Scanning electron microscopic (SEM) and transmission electron microscopic (TEM) observations reveal significant production of bundles of SWNTs in soot. The pulsed arc production of SWNTs favors a high-temperature (≥1000°C), long pulses (≥1 ms) and a heavy rare gas such as Ar or Kr as a buffer gas.

Apparatus and method for detecting particles in reactive and toxic gases

Abstract: An apparatus and a method are disclosed for detecting particles in a particle-containing gas at a pressure greater than about 0 psig. The apparatus includes a gas distribution line containing a pressurized gas having a pressure greater than about 0 psig and a condensation nucleus counter in fluid communication with the pressurized gas in the gas distribution line. The condensation nucleus counter is adapted to receive a stream of the pressurized gas at a pressure substantially equal to the pressure of the pressurized gas in the gas distribution line. The condensation nucleus counter is constructed of materials resistant to corrosion and to reaction with the pressurized gas, which may be one or more reactive or toxic gases, such as those used in microchip processing, or an inert gas.

Optically Pumped Electron Spin Filter

Abstract: This paper reports the first experimental demonstration of an optically pumped electron spin filter. Unpolarized electrons produced in a cold-cathode discharge drift through a mixture of spin-polarized Rb and a nitrogen or helium buffer gas. Through spin-exchange collisions with the Rb, the drifting electrons become polarized along the optical pumping axis. We study the role of the buffer gas in both the optical pumping and the spin transfer to the free electrons. This spin filter produces electron beams with currents and polarizations comparable to first-generation GaAs polarized electron sources.

Quantum-classical molecular dynamics simulation of femtosecond spectroscopy on I2 in inert gases: Mechanisms for the decay of pump–probe signals

Abstract: A mixed quantum–classical method is proposed to describe the dynamical behavior of a diatomic molecule in a gas environment. The vibrational coordinate is treated quantum-mechanically and all other degrees of freedom classically. Within the present approach the classical equations for the rotational motion have no singularities. A symplectic, energy conserving and time-reversible algorithm is used for the propagation. As an application we treat the dynamics of I2 molecules excited by femtosecond laser pulses moving in collision-free and high pressure rare-gas environments. For freely rotating I2 molecules, the thermal average over rotational states leads to the decay of the pump–probe signal. For I2 in inert gases, we show that dephasing by collisions with the buffer gas is a weak effect in comparison with the decay of the signal due to the anharmonicity of the potential energy curve. Therefore the oscillating structure of the pump–probe signals depends weakly on the mass and the pressure of the solvent, ...

Excimer lamp pumped by a triggered longitudinal discharge

Abstract: The ultraviolet (UV) radiation performance of an XeCl excimer lamp is measured as a function of the gas mixture composition (with and without neon as buffer gas) and pumping condition (longitudinal discharge with and without trigger). Both spatial distribution and spectral content of the emitted UV radiations are carefully determined including the contribution of the XeCl (B-X), XeCl (C-A), and Xe continuum emissions. A peak power density of up to 3 kW/cm/sup 2/ and an average power density of up to 0.2 W/cm/sup 2/ can be achieved when using a simple cylindrical aluminum reflector.

Heating of ions moving in a gas under the influence of a uniform and constant electric field

Abstract: A simple model for estimation of the internal temperature of ions, moving in a monoatomic gas under the influence of an electric field, is considered. The basic assumption of the model is formation of ion-atom complexes in a quasi-equilibrium state for some of the ion-atom collisions, and other collisions are considered as elastic in which no energy is transferred to the internal energy of the ion. For Langevin collision cross sections, the model allows coinciding equations to be obtained for internal and translational or effective temperatures of ions, in cases where the polarization energy of the atom in the local electric field of the ion is not taken into account. The influence of this polarization energy leads, in the context of the model considered here, to an increase of internal ion temperature by a factor of 1.3‐1.5 compared to its effective temperature (which coincides with the buffer gas temperature for zero external electric field). Using this result the significant discrepancies in activation energies for dissociation of protonated leucine enkephalin, measured by different methods, are qualitatively explained. Copyright # 1999 John Wiley & Sons, Ltd.

Optical magnetic-resonance imaging of laser-polarized Cs atoms

Abstract: Optical magnetic-resonance imaging (MRI) is performed to observe a density distribution of the laser-polarized Cs atoms that diffuse in helium buffer gas at roughly room temperature. Spatial resolution of optical MRI and sensitivity of optical detection are discussed for gaseous atoms in a weak magnetic field. We propose a fast method of three-dimensional optical MRI with parallel processing of signals from a photodetector array, and we discuss its application to the spin-polarized noble gas.

Apparatus for depositing thin films

Abstract: The thin film deposition apparatus includes: a reactor (200) maintained at a constant pressure; at least two reaction gas supply portions (1110,1120) for supplying reaction gases to the reactor; an exhaust pump (310) for discharging the gases out of the reaction gas supply portions and/or the reactor; first flow control valves (112a) installed between each reaction gas supply portion and the reactor, for controlling the amount of gases flowing between the reaction gas supply portions and the reactor; second flow control valves (114a) installed between each reaction gas supply portion and the exhaust pump, for controlling the amount of gases flowing between the reaction gas supply portions and the exhaust pump; an inert gas supply portion (1130) for supplying an inert gas into the reactor; reaction gas pipe lines, wherein the reaction gases provided from the reaction gas supply portions flow through the reaction gas pipe lines to the reactor and/or the exhaust pump; an inert gas pipe line, wherein the inert gas provided from the inert gas supply portion flows through the inert gas pipe line to the reactor; and a plurality of valves installed in the reaction gas pipe lines and/or the inert gas pipe lines, for controlling the amount of reaction gases and inert gas flowing into the reactor and/or the exhaust pump.

Mercury-free metal halide lamp with a fill containing halides of hafnium or zirconium

Abstract: A mercury-free metal halide lamp with a light efficiency of at least 70 Im/W and a color rendering index of at least 80 has a ceramic discharge vessel, into which electrodes are introduced in a vacuum-tight manner. The fill comprises the following components: an inert gas which acts as buffer gas, a compound of a halogen X with at least one of the metals hafnium and/or zirconium (referred to below as metal halide HZM), this halide being referred to below as HZH for short, HZH simultaneously performing tasks of voltage gradient formation and of promoting the cycle, a light generator comprising at least a further metal halide, at least one further metal halide MY n which vaporizes readily and is used as voltage gradient generator, the specific molar content of HZH being greater than or equal to 3 μmol/cm 3 . In addition, the following relationship applies: 5≦(X+Y)/HZM≦15, resulting in a lamp service life of more than 5000 hours.

Laser ablation with copper vapour lasers

Abstract: The use of copper vapour lasers for laser ablation in laser materials processing applications is studied. To this purpose, the generation of near diffraction-limited beam quality output from a single medium-scale oscillator is demonstrated via matching the total buffer gas pressure to the specific electrical input power loading and the degree of insulation of the plasma tube. The design and characterisation of a Master-Oscillator Power-Amplifier system based on a smallbore oscillator is also described, focusing on pulse stretching techniques to provide efficient seeding required for producing 20-50 W high beam-quality output for laser materials processing purposes. Various experimental studies on the fundamental processes of laser ablation of metals are presented. The effect of the background gas properties on shock-wave formation in laser generated plasmas is studied using a ballistic pendulum. The experimental findings are found to be accurately described by a modified Sedov-Taylor-Von Neumann theory which accounts for the effect of the piston-mass. The theory is applied to characterise the fluorination process in the shock-wave, in view of oxygen isotope analysis in geochemistry. Atomic emission spectroscopy is shown to provide some measure of the electron temperature and electron density at the plasma core. The experimental results are discussed in view of existing models to describe the extreme Stark-broadening and self-absorption in dense, cool plasmas. A comparative study of the use of femtosecond and nanosecond pulsed lasers for laser ablation of metals is presented to assess the relative importance of thermal diffusion. Measurement of the recoil momentum due to ejection of molten particulates during ablation in vacuum provides insight into the effect of material properties. Diffusion-limited surface texturing of metals via direct transfer of an optical interference patterns is demonstrated.

Development of a discharge in pulsed metal-vapour lasers

Abstract: An analysis is made of the electric characteristics of the exciting discharge in pulsed metal vapour lasers. The initial stages of the current and voltage pulses are given most attention. Our estimates are based on a series of recent experimental results and of calculations, obtained for typical operating conditions of present-day metal vapour lasers. The initial development of the discharge is shown to depend only slightly on the conductivity of the hot part of the discharge tube, where a metal vapour coexists with a buffer gas. The effects of the electrode properties and of the processes in cold near-electrode regions on laser operation are discussed.

Production of fullerenes by high-temperature pulsed arc discharge

Abstract: We have developed a high-temperature pulsed arc discharge apparatus, which can operate in a buffer gas heated up to 1000 °C, and have succeeded in producing fullerenes for the first time with this method. We have quantitatively analyzed the products, using high-performance liquid chromatography (HPLC), to estimate the concentration of fullerenes in soot. The results show that fullerenes are produced not at room temperature but at much higher temperatures such as 1000 °C for Ar. The concentration of fullerenes C70 and higher increases as the pulse width of the discharge increases. In the pulsed arc discharge, the negative electrode is consumed by the sputtering of buffer gas ions.

High-pressure, high-efficiency operation of a chemical oxygen-iodine laser

Abstract: High-pressure subsonic mode operation of a chemical oxygen-iodine laser (COIL) was demonstrated. Singlet delta oxygen generated by a liquid-jet type singlet oxygen generator (SOG) was directly utilized in the laser cavity, without supersonic expansion. The operating pressure in the laser cavity was 0.80 kPa (6.0 Torr). Cooled nitrogen gas was added to the singlet oxygen flow to enhance the output power. An output power of 448 W was obtained for a chlorine input rate of 19.7 mmol/s. This is equivalent to a chemical efficiency of 25.0%, and it is comparable to the highest reported chemical efficiency of COIL using nitrogen as the buffer gas. The obtained value of specific energy of 3.5 J/liter, is more than fivefold higher than that for our supersonic COIL device.

Experimental study of rotational relaxation processes by pulsed photoacoustic technique

Abstract: The IR pulsed photoacoustic method was used to investigate rotational relaxation processes in MPA. We have examined the bulk rotational relaxation times and the bulk rotational relaxation cross sections , related to R-R and R-T relaxation mechanisms. The saturation intensities for gas molecules have been measured in the gas mixtures with (buffer) molecules, at constant gas pressure (0.47 mbar) and variable gas pressure (1-140 mbar), using the 10P(16) laser line in the fluences range 0.1-0.7 J . It has been found that for proper investigation of the rotational relaxation processes, the buffer gas pressure range must be divided into two parts: the low pressure range (0-20 mbar) and the high pressure range (20-140 mbar), according to the behaviour of the saturation intensities versus buffer gas pressure. Consequently, two different sets of values for rotational relaxation parameters are obtained. The found step-function behaviour for shows that the rotational hole-filling effect during MPA only dominates at low buffer gas pressure ( mbar). At higher buffer gas pressures (between 20-140 mbar) collisional deactivation takes place. We also derive the enhanced absorption cross sections for absorbing gas molecules from the measured saturation intensities data (at zero buffer gas pressure) confirming the expectation that the enhanced absorbing cross sections are not only a function of laser fluence and buffer gas pressure, but are very strongly dependent on laser beam parameters, too.

Method and apparatus for forming material layers from atomic gasses

Abstract: A method of forming material layers on a substrate using atomic gas is provided. A substrate is heated to an elevated temperature and is exposed to an atomic gas. The atomic gas reacts at a surface of the substrate to form a material layer thereon. The source of atomic gas preferably comprises a molecular gas source operatively coupled to a remote microwave plasma system that dissociates the molecular gas into highly reactive atomic gas. Gate quality silicon dioxide, oxynitride and silicon nitride may be formed by the dissociation of O 2 , O 2 and N 2 or NH 3 , and N 2 or NH 3 , respectively, at reduced temperatures (e.g., about 600-650° C.). Because of the reduced formation temperatures, a uniform heating mechanism such as a ceramic heater may be employed for substrate heating so that a more uniformly-thick material layer results. To reduce recombination of gas atoms into molecular gas, the path length between the atomic gas source and the substrate is reduced, or an inert gas may be used to dilute the atomic gas so as to spatially separate gas atoms. A portion of the path between the atomic gas source and the substrate also may be coated with a protective coating to prevent gas atom recombination.