Showing papers on "Buffer gas published in 1995"

Buffer-gas loading of atoms and molecules into a magnetic trap

Abstract: We describe a method for loading paramagnetic atoms or molecules into a magnetic trap. A $^{3}\mathrm{He}$ buffer gas is employed to thermalize atoms or molecules to a temperature of approximately 240 mK, lower than the depth of the trap. A model is described that indicates an initial loading density of approximately ${10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. Once loading has taken place the buffer gas is removed by cryopumping. Evaporative cooling can then be applied to further lower the temperature and increase the density of the trapped sample.

Thermal-grating contributions to degenerate four-wave mixing in nitric oxide

Abstract: We report investigations of degenerate four-wave mixing (DFWM) line intensities in the A2∑+ ← X2Π electronic transitions of nitric oxide. Contributions from population gratings (spatially varying perturbations in the level populations of absorbing species) and thermal gratings (spatially varying perturbations in the overall density) were distinguished and compared by several experimental and analytical techniques. For small quantities of nitric oxide in a strongly quenching buffer gas (carbon dioxide), we found that thermal-grating contributions dominated at room temperature for gas pressures of ≈0.5 atm and higher. In a nearly nonquenching buffer (nitrogen) the population-grating mechanism dominated at pressures of ≈1.0 atm and lower. At higher temperatures in an atmospheric-pressure methane/air flame, population gratings of nitric oxide also dominated. We propose a simple model for the ratio of thermal- to population-grating scattering intensities that varies as P4T−4.4. Preliminary investigations of the temperature dependence and detailed studies of the pressure dependence are in agreement with this model. Measurements of the temporal evolution and the peak intensity of isolated thermal-grating signals are in detailed agreement with calculations based on a linearized hydrodynamic model [ J. Opt. Soc. Am. B12, 384 ( 1995)].

Reaction of molybdenum clusters with molecular nitrogen

Abstract: Reactions of molybdenum clusters with molecular nitrogen have been investigated by using a fast‐flow reactor equipped with a laser vaporization cluster source and laser ionization time‐of‐flight mass spectrometer for detection of clusters. Absolute rate coefficients are reported for reaction of clusters in the range 3–35 metal atoms, at temperatures 279, 300, and 372 K in He buffer gas at 1 and 2 Torr pressure. Rate coefficients generally decrease with increasing temperature, as expected for a mechanism involving formation of a weakly bound precursor complex. By comparison with surface chemistry studies of nitrogen adsorption on molybdenum and tungsten, the reactivity trend with cluster size is interpreted as reflecting the geometric configurations of metal atoms on the surfaces of the clusters. It is suggested that structural transitions occur in molybdenum clusters in the size range 13–20 metal atoms. Evidence is described that suggests that cluster reaction products have nitrogen bound in both atomic a...

Laser photolysis studies of hydrazine vapor: 193 and 222-nm H-atom primary quantum yields at 296 K, and the kinetics of H + N2H4 reaction over the temperature range 222–657 K

Abstract: The primary quantum yield of H-atom production in the pulsed-laser photolysis of hydrazine vapor, N2H4 + hν H + N2H3, was measured to be (1.01 ± 0.12) at 193 nm relative to HBr photolysis, and (1.06 ± 0.16) at 222 nm relative to 248-nm N2H4 photolysis, in excess He buffer gas at 296 K. The H-atoms were directly monitored in the photolysis by cw-resonance fluorescence detection of H(2S) at 121.6 nm. The high H-atom yield observed in the photolysis is consistent with the continuous ultraviolet absorption spectrum of N2H4 involving unit dissociation of the diamine from repulsive excited singlet state(s). The laser photodissociation of N2H4 was thus used as a ‘clean’ source of H-atoms in excess N2H4 and He buffer gas to study the gas-phase reaction, H + N2H4 products; (k1), in a thermostated photolysis reactor made of quartz or Pyrex. The pseudo-first-order temporal profiles of [H] decay immediately after photolysis were determined for a range of different hydrazine concentrations employed in the experiments to calculate the absolute second-order reaction rate coefficient, k1. The Arrhenius expression was determined to be k1 = (11.7 ± 0.7) × 10−12 exp[−(1260 ± 20)/T] cm3 molec−1 s−1 in the temperature range 222–657 K. The rate coefficient at room temperature was, within experimental errors, independent of the He buffer gas pressure in the range 24.5–603 torr. The above temperature dependence of k1 is in excellent agreement to that we determine in our discharge flow-tube apparatus in the temperature range 372–252 K and in 9.5 torr of He pressure. The Arrhenius parameters we report are consistent with a metathesis reaction mechanism involving the abstraction of hydrogen from N2H4 by the H-atom. © 1995 John Wiley & Sons, Inc.

Temperature-dependent studies of the reaction of W(a 5DJ, a 7S3) with O2

Abstract: The gas-phase reactivities of W(a 5DJ, a 7S3) with O2 in the temperature range 298–573 K are reported. Tungsten atoms were produced by the photodissociation of W(CO)6 and detected by laser-induced fluorescence. The disappearance rate constant of the s1d5 a 7S3 state is of the order of the gas kinetic rate constant. The removal rate constants for the s2d4 a 5DJ states are J dependent. The a 5DJ states are not as reactive as the s1d5 a 7S3 state and are found to be temperature dependent. The disappearance rates of all states in the presence of O2 are found to be pressure independent with buffer gas. Physical quenching rate constants for the excited states in the presence of Ar, He, N2 and SF6 are also reported. Results are discussed in terms of the different electron configurations of the states.

Robust high‐temperature sapphire cell for metal vapors

Abstract: We describe a simple vacuum cell that successfully contains high‐temperature and high‐density alkali‐metal and alkaline‐earth vapors. The transparent cell, made from sapphire parts joined by melted glass powders, operates without a buffer gas. Temperatures over 700 °C and metal‐vapor densities greater than 1017 cm−3 have been achieved. Such an optical cell is useful for many atomic physics experiments where collisions with buffer gas atoms are problematic or where a spatially uniform distribution of high‐density, corrosive atoms is desired.

Efficient dry etching of Si with vacuum ultraviolet light and XeF2 in a buffer gas

Abstract: Replicas of a mask are etched in Si wafers with a micrometer lateral resolution and typical depths of 200 nm by irradiation with filtered synchrotron radiation using cutoff wavelengths of 105, 122, and 150 nm. An excellent selectivity and anisotropy is obtained by suppressing the spontaneous etching of the XeF2 etch gas (typical 10−2 mbar) with O2 or Ar buffer gas (typical 1 mbar). The efficiency of etching increases by more than two orders of magnitude by reducing the wavelength from longer than 150 nm to the spectral range of 105–122 nm. The number of removed Si atoms per incident photon reaches a value above unity for the short wavelengths. This very high quantum efficiency, which exceeds that in the visible spectral range by more than four orders of magnitude, is attributed to selective electronic excitation of a thin fluorosilyl layer on top of the Si wafer. The low probability of absorption in this layer implies a reaction efficiency far above unity.

Atomic hydrogen-deuterium mixtures at 1 Kelvin : recombination rates, spin-exchange cross sections, and solvation energies

Abstract: A new technique has been applied to the study of atomic hydrogen and deuterium mixtures confined by liquid helium coated walls The method uses standard low-field hyperfine magnetic resonance of the H atoms at 1420 MHz, but takes advantage of the dramatically different spin-exchange broadening for H-H and H-D collisions to simultaneously monitor the H and D densities This provides a powerful means for studying the interaction of D with itself and with liquid helium, something otherwise difficult to achieve, and it also makes possible the study of spin-exchange and recombination interactions between H and D A wide range of experimental results are presented, including the rate constants for H-D and D-D recombination, the spin-exchange broadening cross sections for H-D and D-D collisions, the H-D spin-exchange frequency shift cross section and an improved value for the H-4He buffer gas shift Finally, a detailed study of the solvation of D into liquid4He has yielded an improved value for the salvation energy, a useful lower bound for the effective mass for D in liquid4He, and evidence for the reaction D + HD → D2 + H on the surface under the liquid4He film

Doppler broadening and collisional relaxation effects in a lasing-without-inversion experiment

Abstract: In the early experiments on optical pumping and coherent population trapping (``dark resonances'') increasing buffer gas pressure enhances the effect. In our recent lasing-without-inversion experiments, based on population trapping, we find the opposite behavior, namely, decreasing atomic coherence with increasing buffer gas pressure. We provide a theoretical explanation of this result in agreement with experiment.

Process for purifying halogen-containing gas

Abstract: There is disclosed a process for purifying a halogen-containing gas (halogen gas such as chlorine, fluorine alone or diluted with an inert gas) which comprises bringing the halogen-containing gas into contact with a purifying agent comprising a hydroxide of an alkaline earth metal such as strontium hydroxide and an iron oxide such as triiron tetraoxide to efficiently remove hydrogen halogenides such as hydrogen chloride and hydrogen fluoride along with moisture that are contained as impurities in the halogen-containing gas. The above process enables the formation of a non-corrosive halogen-containing gas having an extremely high purity and capable of being favorably used as etching gas for silicon films, aluminum alloy films, etc. in a semiconductor manufacturing process.

Removal of oxygen from inert gases

Abstract: A method and apparatus for removing oxygen from a crude inert gas containing oxygen as a contaminant is disclosed. The inert gas is passed through a reactor containing an oxidizable material. The reactor is then regenerated via a gaseous reducing agent such as hydrogen. The hydrogen can be generated on site via water or steam hydrolysis.

Method and system for plasma processing

Abstract: PROBLEM TO BE SOLVED: To reduce plasma damage on an object to be processed by simply adding a gas to conventional processing gas. SOLUTION: Discharge He gas us fed between a pair of electrodes 12, 14 of an atmospheric plasma processing system through feeding means 42a, 44a to generate plasma under atmospheric pressure and an object 1 is processed. In this regard, an additive gas is fed through feeding means 42b or 44b. The additive gas is an inner gas selected tram argon Ar, krypton Kr or xenon Xe. Alternatively, a reaction gas selected from O2 , CF4 , N2 , CO2 , SF6 and CHF3 , a gas containing at least O, N, F, Cl, or the gas phase of a liquid containing them is employed as an additive gas. A gas for enhancing the wettability on the surface of object, and a reaction gas, e.g. ashing gas or etching gas, are also fed through feeding means 42c or 44c depending on the processing. When a reactive gas is employed as an additive gas, an additive gas different from the reaction gas for processing is employed.

Disappearance of Yb+ in Excited States from rf Trap by Background Gases

Abstract: Yb+ ions confined in a rf trap disappear due to some background gases when the Yb+ ions are in excited states. We examined O2, H2O, and H2 as the test background gas. We found that O2 gas enhanced the disappearance to a large degree, as did H2O gas to some degree, but H2 gas had little effect.

Efficient quenching of population trapping in excited Yb

Abstract: We report on a study to determine the efficiency of various buffer gases in quenching the population trapping states of laser-excited ytterbium ions. The ions were confined in an rf-dc hybrid trap, and their (2)S(.5) left and right arrow (2)P(3/2) transition was excited with 329-nm laser light. Buffer gases used include H2, N2, CO2, Ar, and He at several different pressure regimes. We have identified N2 to be highly efficient in quenching the population of the trapping states and thus be the most suitable buffer gas to use with ytterbium for the development of a trapped-ion microwave frequency standard.

Visible Emission Sepctra of o-Xylyl Radical

Abstract: The visible emission spectra of the o-xylyl radical in the gas phase have been obtained using a Fourier transform spectrometer coupled with a technique of supersonic expansion. The o-xylyl radical was generated in a jet by expansion with an inert buffer gas He from a high voltage dc discharge of the precursor o-xylene. The spectra were analyzed on the basis of the rotational contours of the vibronic bands as well as the known vibrational frequencies by a matrix isolation method.

Reduced angular dependence for degenerate four-wave mixing in potassium vapor by including nitrogen buffer gas

Abstract: Degenerate four‐wave mixing in alkali vapors exhibits a strong dependence on the angle between the forward pump and probe beams, due to atomic motion. We show that this angular dependence is dramatically decreased by including several hundred Torr of N2 buffer gas in a potassium vapor cell. The angle at which the signal has decreased by half is 250 mrad, an improvement of over 50 times the angle for potassium vapor without buffer gases.

Process for generating nitrogen for heat treatment

Abstract: A process for generating a nitrogen-based gas, comprising the steps of: (i) combining a secondary reducing gas including at least one reducing gas with a primary nitrogen gas including 0.5-5% by volume of oxygen, the combining of the secondary gas being performed according to an uncontrolled all-or-nothing method; (ii) reacting the primary gas and the secondary gas in a catalytic deoxygenation reactor so as to obtain a nitrogen-based reaction gas which includes water vapor; and (iii) removing at least a part of the water vapor present in the nitrogen-based reaction gas by cooling the reaction gas under pressure to induce formation of a liquid phase and then extracting the liquid phase from the reaction gas under pressure to obtain a purified reaction gas.

Hot gas cleaning of dust-laden flue gas from gas-fired power stations etc.

Abstract: A process for hot gas cleaning of flue gas laden with dust is disclosed. The flue gas or synthesis gas, which is under pressure and contaminated with ash and fluidised slag, is precleaned using an inertial separator, for example, a hot gas cyclone, which is fitted with high temp.-resistant, slag-resistant SiC/MoSi2 parts. The gas then passes into a SiC/MoSi2-based high temp. heat exchanger in tubular bundle form, which contains no metal/ceramic cpds. in the heat transfer regions, where fine dust is removed. In the heat exchanger a heat transfer medium, e.g. nitrogen and/or inert gas at a slightly greater pressure than the flue gas is heated.

Dynamics of laser beam switching in sodium vapor

Abstract: The interaction of left- and right-hand circularly polarized narrow-band cw laser beams that copropagate through sodium vapor and are tuned to the homogeneously broadened D1 transition can lead to the mutual extinction of both beams. Such beam switching is caused by an intensity- and polarization-dependent refractive index due to optical pumping even at low intensities. The dynamics of beam switching is investigated experimentally and explained qualitatively by the conventional J = 1/2 to J = 1/2 transition model. The accurate quantitative description, however, is only achieved by an extended model comprising hyperfine ground states in spite of homogeneous line broadening by argon buffer gas, which is larger than the hyperfine splitting.

System for production of a quenchant gas mixture

Abstract: The invention provides a process and apparatus for producing a quench gas mixture for increasing the cooling rate of an article. The quench gas mixture is produced by introducing helium gas into the bottom of a vessel containing a cryogenic liquid. Heat is transferred directly from the helium gas to the cryogenic liquid as the helium bubbles rise through the liquid to the surface. The resulting cryogenic vapor mixes with the helium gas in the ullage space at the top of the vessel, and the gas mixture is taken off from the ullage space and supplied to a cooling process.