High Pressure Rare Gas Research Articles

Quasi-stationary convection in a periodic-pulsed optical discharge in high pressure rare gas

Unusual convection flows were observed in stabilized pre-breakdown phase of the periodic-pulsed optical discharge (POD) called “quiet” POD. The discharge was a relatively weakly glowing plasma filament sustained by focused λ = 1.064 μm laser pulses with repetition rate of fr = 50÷100 kHz at the intensity several times below than that required for the optical breakdown to occur. No strong shock waves or irregular turbulence around the discharge were observed, in contrast to breakdown types of POD. Significant laser beam refraction measured in the beam cross-section behind the discharge zone was explained by the gas heating in the discharge up to 10 kK, providing high gradients of gas density and refraction index. Intense convective flow was detected on the schlieren images as thermal traces of the laser-induced gas streams flowing from the discharge zone, directed mainly normally to the optical axis. Repeated relaxation of the gas expanding after being rapidly heated by the laser pulse is proposed to explain the effect. The periodic-pulsed discharge located in the elongated beam waist generates an anisotropic heated region with gas streams and vortices, which may form the observed regular convective flow at the late stages of expanding.

Femtosecond pump-probe spectroscopy of I2 in a dense rare gas environment: a mixed quantum/classical study of vibrational decoherence.

The process of decoherence of vibrational states of I2 in a dense helium environment is studied theoretically using the mixed quantum/classical method based on the Bohmian formulation of quantum mechanics [E. Gindensperger, C. Meier, and J. A. Beswick, J. Chem. Phys. 113, 9369 (2000)]. Specifically, the revival of vibrational wave packets is a quantum phenomena which depends sensitively on the coherence between the vibrational states excited by an ultrafast laser pulse. Its detection by a pump-probe setup as a function of rare gas pressure forms a very accurate way of detecting vibrational dephasing. Vibrational revivals of I2 in high pressure rare gas environments have been observed experimentally, and the very good agreement with the simulated spectra confirms that the method can accurately describe decoherence processes of quantum systems in interaction with an environment.

Stable high brightness radio frequency driven micro-discharge lamps at 193 (ArF*) and 157 nm ()

A stable discharge between two pin electrodes separated by several hundred micrometres in a high pressure rare gas (∼900 mbar) halogen (∼1 mbar) mixture is shown to yield continuous wave (CW) ultra violet (UV) and vacuum UV light sources. Lamps operating at 193 (ArF*) and 157 nm () have been demonstrated. Total CW output power in the UV was measured to be 30 for ArF* and 20 mW for . The brightness of the light sources is estimated to be of the order of several W cm−2 sr−1. With direct current excitation, electrode lifetimes are limited to a few minutes due to fluorine salt deposits. However, using a radio frequency (RF) field to drive the discharge, the lifetime of the lamps increased to hundreds of hours. A one-dimensional model of the RF micro-discharge explaining the increase in electrode lifetime is presented. The technology described can be adapted to many other wavelengths and promises even higher powers in future.

Absolute number of scintillation photons emitted by alpha particles in rare gases

In order to determine absolute scintillation yields due to alpha particles in high pressure rare gases, the number of scintillation photons N/sub p/ was measured by using a VUV sensitive photodiode (PD) with a MgF/sub 2/ window and a Cs-Te photocathode with spectral quantum efficiency q/sub e/(/spl lambda/) measured as a function of wavelength /spl lambda/. The number of photoelectrons from the photocathode N/sub pe/ was measured absolutely by using charge-sensitive preamplifier calibrated in numbers of electrons. A collection efficiency F/sub ce/ at the photocathode for scintillation photons can be determined from solid angles subtended by the photocathode at a scintillation point under the condition that there is no photon reflected at surrounding wall. Then, N/sub p/ was determined from N/sub p/=N/sub pe//(Q/sub e/F/sub ce/), where Q/sub e/ is effective quantum efficiency calculated from q/sub e/(/spl lambda/) and a relative intensity I(/spl lambda/) of scintillation in rare gases at wavelength /spl lambda/. Although luminescence spectrums from rare gases emitted by radiation have been measured by many researchers, these spectrums were scarcely corrected by an efficiency of apparatus (e.g. efficiency of monochromator and scintillation detector) for /spl lambda/. In order to exactly determine the luminescence spectrums, these were also measured on our own terms. And, since it was reported that scintillation intensity from rare gases change with a pressure of rare gases, this experiments was carried out in a pressure range from 1.0/spl times/10/sup 5/ Pa to 1.0/spl times/10/sup 6/ Pa. The measurements were carried out in gaseous argon, krypton and xenon. In xenon of 1.0/spl times/10/sup 5/ Pa, N/sub p/ was measured to be 1.6/spl times/10/sup 5/.

Nouvelle méthode de préparation pour les complexes endoédriques atome de gaz rare–fullerène C 60

New route for preparation of endohedral complexes of rare gas atom–C60. Preparation of endohedral complexes of a rare gas atom–fullerene is described, in which a sample of C60 powder, surrounded by a high pressure rare gas, is sublimated and excited by a strong nanosecond pulse laser beam. Experiments were carried out with the following rare gases: pure 4He, a mixture of 2 % of 3He in 4He, and pure Ar. Macroscopic quantities of these complexes were obtained in estimated concentations up to 20 %. On décrit une méthode de préparation pour des complexes endoédriques atome de gaz rare–fullerène, faisant appel à l'ablation et à l'excitation par le faisceau de lumière à λ=532 nm, fourni par un laser impulsionnel Nd–YAG, d'un échantillon de poudre de C60 placé au sein d'un gaz rare comprimé jusqu'à 0,3 GPa au maximum. On a utilisé les gaz rares suivants : 4He pur, mélange 3He à 2 % dans 4He et Ar pur. On a obtenu des quantités macroscopiques de ces complexes, avec des concentrations estimées jusqu’à 20 %.

Single and multiple collision effects observed in the femtosecond spectroscopy of I2-rare gas collision complexes: a statistical description

We use a simple statistical model to simulate recent femtosecond experiments on the caging dynamics of I2 in high pressure rare gas environments [Wan et al., J. Chem. Phys. 106, 4353 (1997)]. The collision-induced bound state motion in I2 and its appearance in the pump/probe signal is perfectly reproduced in our calculation. In the case where He is used as the I2 collision partner we find multiple collisions to be important. The numerical results can be explained analytically under simplified assumptions.

Modeling of high-pressure rare gas plasmas excited by an energetic flash X-ray source

In this paper, a comprehensive description of the kinetics of argon plasmas has been formulated for atmospheric pressures. It has been used to attribute to molecular species the ultraviolet-vacuum ultraviolet (UV-VUV) continua, observed between 150 and 300 nm in pure argon. The participation of both ionic dimers and trimers has been found to be necessary to explain the whole fluorescence emitted by the rare gases (Rg). A simple kinetic model of high-pressure (1-30 bar) argon plasmas created by flash X-ray excitation is reported. The photoexcitation of Rg at high pressure by X-ray photons with energy of about 10 keV is shown to produce plasmas with significant density gradients of ionic and excited species. Spatial analysis of the energy position along the X-ray penetration path is performed in order to account for these concentration gradients. The calculation of electron density in different Rg (Ar, Kr, Xe) excited by X-ray photons of various energies (0.2-10 keV) is also reported. From these results, it appears that hash X-ray excitation of Rg can be a very interesting means to produce high-electron-density plasmas designed to selectively emit intense fluorescence in the UV-VUV spectral domain.

Time-resolved spectroscopy of high pressure rare gases excited by an energetic flash X-ray source

Fluorescence from high pressure (0.1–30 bar) rare gas plasmas has been excited using an intense flash X-ray source specially developed and optimized for this experiment. Spectral analysis of the so-called “third continuum” of rare gases is presented. Time-resolved spectroscopy, absorption measurements and pressure dependence studies have provided an extended database on this longer wavelength continuum of rare gases. These data allow us to understand some of the previously contradictory results reported in the literature. They strongly support a multi-component spectral aspect of the previously called “third continuum”. In fact, the observed fluorescence result from the superposition of several continua whose relative intensities are strongly dependent upon pressure. This suggests that different species are at its origin. The name of “third continuum” does not seem anymore appropriate when speaking of the whole fluorescence emitted in the longer wavelength continuum by high pressure rare gas plasmas. Generally speaking, the flash X-ray device was proven to be a very convenient and powerful tool for the study of high pressure plasmas resulting in UV-VUV fluorescence.

Laser Spectroscopy of Large Polyatomic Molecules in Supersonic Jets

Spectroscopic study of large polyatomic molecules had long been pre­ vented by spectral complexity. The complexity arises from the fact that a large molecule has a large number of vibrational degrees of freedom. Another complication comes from the thermal distribution of molecules over many ground-state vibrational levels, which causes many hot bands to appear in the spectrum. Molecular collisions in gas and condensed phases also contribute to broadening of spectral bands. All these com­ plicating factors become increasingly significant as a molecule becomes larger. Eventually, the spectrum becomes a continuous feature, even in the gas phase, owing to heavy spectral band congestion. A typical example of this spectral congestion is seen in the electronic absorption spectrum of biphenyl in the gas phase. The S 1 <So absorption spectrum of benzene is well known to exhibit prominent vibrational structure. However, in bi­ phenyl, which is composed of two benzene rings, the corresponding spec­ trum is completely structureless (1). We often encounter such a broad and structureless absorption spectrum when a molecule is larger than benzene. Clearly, detailed information on the energy levels cannot be obtained from such a broad spectrum. The recent development of the supersonic jet technique (2-5) has greatly improved the above situation. Supersonic expansion of sample molecules, seeded in high-pressure rare gas, into vacuum through a small nozzle

Parametric study of x-ray preionized high-pressure rare gas halide lasers

Improved laser output is demonstrated in KrF and XeCl lasers with x-ray preionization. The influence of gas composition, preionization geometry, x-ray dose,E/p value and temporal delay between x-ray pulse and laser discharge is discussed. In XeCl lasing has been achieved with an x-ray dose of 0.2 mR whereas approx. 3 mR are required for KrF. For both systems, the highest output energy and the lowest possibleE/p ratio were observed with Ne buffered gas mixtures. Dissociative attachment to halogen molecules on a ns time scale supported by a relatively slow laser voltage rise is recognized as the main electron loss channel preventing a long lifetime of the preionization electrons. High preionization intensity is thus desirable at the moment of voltage breakdown.

Solvent perturbations of extravalence excitations of atomic Xe by rare gases at high pressures

In this paper we present the results of an experimental study of the lowest doublet excitations (1S0→3P1 and 1S0→1P1) of atomic xenon in dense supercritical fluid neon and helium and the 1S0→1P1 xenon excitation in dense supercritical and subcritical fluid argon. The spectral shift, the linewidth, the first moment, and the second moment of the absorption bands exhibit a strong density and a weak temperature dependence. The semiclassical statistical theory of line shapes was applied for the calculations of the first moment, the second moment, and the entire line shape. By fitting to the experimental moments over a broad density range, the excited state guest–host interaction potential parameters were derived. An analysis of the density dependence of the splitting and of the intensity ratio of the Xe doublet perturbed by high pressure rare gases was provided.

Efficient electron excitation mechanism in high-pressure rare gas lasers

Efficient electron excitation mechanism in high-pressure rare gas lasers

Ultraviolet Emission Spectra of High-Pressure Rare Gases

The emissions arising from a short-arc discharge in high-pressure (10–20 atm) rare gases extend well into the vacuum-ultraviolet (vuv) region. Short-wavelength-emission cutoffs of 1600, 1310, and 1150 Å were found for xenon, krypton, and argon, respectively. High-pressure-neon emission extends to even shorter wavelengths. The construction of a high-intensity, high-pressure lamp utilizing a magnesium fluoride window is described. This lamp should be useful as a vuv source. In an unfocused mode, the flux from this lamp (measured at the exit slit of a monochromator with a 0.6-Å bandpass) was 3×109 photons/s using argon gas and 1000 W of power. Approximate ratios of flux emitted for neon, argon, and krypton are 1:5:10, respectively.

Electrical Conduction and Breakdown in High-Pressure (0.25–300 mm) Rare Gases

The current-voltage characteristics of high-pressure (0.25–300 mm) rare-gas diodes have been measured. Data have been taken on diodes containing xenon, argon, and helium as the ambient gas, and tungsten, tantalum, rhenium, thoriated tungsten, and an oxide cathode as filamentary cathodes. Some unexpected phenomena have been observed and some of these general results can be summarized as: (1) The current vs voltage characteristics of argon- and xenon-filled diodes, at gas pressure above 1 mm, do not obey any space-charge law if the cathode temperature is above 2400°K. (2) The above violation of space-charge relations in argon- and xenon-filled diodes is found with tungsten, tantalum, or rhenium filaments, which can be operated above 2400°K. When thoriated tungsten or the oxide cathode, which operate below 2400°K, are used as filaments, the current-voltage characteristics of the diode follow a space-charge relation (current-voltage characteristics independent of temperature). (3) The current vs voltage characteristics of helium-filled diodes, however, do obey the space-charge relations at all filament temperatures available with the present cathode materials. (4) Some very unusual early breakdown phenomena (breakdown at potentials below ionization potential) were observed in these high-pressure rare-gas diodes employing a hot cathode. These data can be explained qualitatively by postulating the existence of thermally generated xenon or argon gas ions at temperatures in the range of 2400°K. An attempt will be made to justify this assumption by a semiquantitative theoretical treatment based on Saha's thermal ionization theory rather than the surface ionization theory of Langmuir and Kingdon.

Electrical Conduction and Breakdown in High-Pressure Rare Gases

Electrical Conduction and Breakdown in High-Pressure Rare Gases