Optical Diagnostics in the Gaseous Electronics Conference Reference Cell

A number of laser-induced fluorescence and absorption spectroscopy studies have been conducted using Gaseous Electronics Conference Reference Cells. Laser-induced fluorescence has been used to measure hydrogen atom densities, to measure argon metastable spatial profiles, to determine the sheath electric field, and to infer the electron density and temperature. Absorption spectroscopy, using lamp sources and diode lasers, has been used to measure metastable atom densities in helium and argon discharges and fluorocarbon densities in silicon etching discharges. The experimental techniques and sample results of these investigations are reviewed.     

Optical Observation of Atomic Hydrogen on the Surface of Liquid Helium

We have optically detected hydrogen atoms adsorbed on the surface of liquid helium, a system relevant for the study of Base degeneracy in two dimensions. The atoms are excited by 121.6 nm light and detected both in fluorescence and in absorption. The optical spectrum of the adsorbed hydrogen atoms was not known a priori. It shows a resonance that is much broader than that of a hydrogen atom in vacuo, and it is shifted to lower frequencies. From the fluorescence intensity we determine that we have reached a surface density corresponding to one atom per square De Broglie wavelength. This means that our experiments take place at the edge of quantum degeneracy. In the regime where the adsorption isotherm is known we can use the measured hydrogen densities to infer the temperature of the helium surface. We use this information to determine the thermal conductance between the surface and the bulk of liquid helium. We find quantitative agreement between the measured temperature drops and the prediction of ripplon-phonon coupling theory.     

Electron beam formation in an open discharge

The results of calculations reported for the first time indicate that the electron emission from a cathode, which is necessary to explain the main properties of an open discharge, is stimulated by the cathode bombardment by fast atoms rather than by photons (as was believed for a long time). The calculations are based on the results of measurements of the electric field strength in the discharge region and of the coefficient of electron emission from a cathode bombarded by helium atoms and ions. It is also demonstrated that the efficiency of the gas ionization by fast atom bombardment is significant at a voltage of several keV and becomes comparable with the electron-impact ionization in the anode plasma at a voltage equal to a few tens of keV.     

Determination of metastable atom concentration by use of electrostatic probe technique

A method for determination of the metastable atom concentration in high pressure (>100 Torr) high density (> 10¹²cm²³); helium plasma from current–voltage characteristics of a single electrostatic probe is described. It is shown, that the flux of metastable atoms to the probe is controlled by ion sheath thickness and consequently by probe bias. The method for calculation of metastables concentration from the negative part of the current–voltage probe characteristics is proposed. The metastables concentrations measured in pulsed microwave discharge are in agreement with values calculated from the metastable balance equation.     

Experimental study of atmospheric pressure surface discharge in helium

The surface discharge generated at atmospheric pressure in helium was examined by monitoring the current and voltage at the discharge electrode. The discharge generated in helium behaves differently when compared to that generated in other gases (e.g. air). The single discharge duration and the time between consecutive discharges are longer because there is a different mechanism of discharge evolution. The metastable helium atoms play the most important role for discharge generation. Streamer-like and glow types of discharge were observed. The decay of helium metastables concentration determines the discharge regime. Hence, operation conditions have strong influence on the discharge regime. The introduction of gas flow removes metastable quenchers (gaseous products from dielectric and electrode surfaces) and transition to glow discharge is observed. Also covering the discharge electrode with thin dielectric foil to suppress Auger de-excitation of metastables at metal surface leads to generation of atmospheric pressure glow surface discharge. Properties of this discharge are comparable with properties of glow discharge at low pressure (e.g. the electron concentration).     

Experimental study of atmospheric pressure surface discharge in helium

The surface discharge generated at atmospheric pressure in helium was examined by monitoring the current and voltage atthe discharge electrode. The discharge generated in helium behaves differently when compared to that generated in other gases (e.g. air). The single discharge duration and the time between consecutive discharges are longer because there is a different mechanism of discharge evolution. The metastable helium atoms play the most important role for discharge generation. Streamer-like and glow types of discharge were observed. The decay of helium metastables concentration determines the discharge regime. Hence, operation conditions have strong influence on the discharge regime. The introduction of gas flow removes metastable quenchers /gaseous products from dielectric and electrode surfaces) and transition to glow discharge is observed. Also covering the discharge electrode with thin dielectric foil to suppress Auger de-excitation of metastables at metal surface leads to generation of atmospheric pressure glow surface discharge. Properties of this discharge are comparable with properties of glow discharge at low pressure (e.g. the electron concentration).     

A high-power low-pressure iodine lamp pumped by glow discharge

We present the working characteristics of a high-power UV-VUV electric-discharge lamp filled with a working mixture of helium and iodine vapor (He-I₂) at a low pressure (0.1–1.5 kPa) and pumped by a dc glow discharge at a power of 15–200 W. The power of the total output UV radiation and the main emission peak at λ = 206.2 nm were studied as dependent on the electric power supplied to the glow discharge and on the partial pressure of helium in the He-I₂ mixture. The emission characteristics of the glow discharge plasma were studied in the spectral range from 200 to 350 nm. In this range, the lamp is operating predominantly on a resonance emission line of excited iodine atoms (λ = 206.2 nm, FWHM = 0.10–0.12 nm) and on a system of electronic-vibrational bands of excited iodine molecules with the main peak at λ = 342 nm. The contribution of the resonance emission due to excited iodine atoms to the total UV emission from the glow discharge plasma does not exceed 50%. The optimum partial pressure of helium is within 400–800 Pa. The total UV radiation power of the lamp operating in the optimum regime reaches 25 W at an efficiency of η ≤15%.     

Properties of excited xenon atoms in a plasma display panel

The luminance efficiency of a plasma display panel is directly related to the vacuum ultraviolet (VUV) light that is emitted from excited xenon (Xe) atoms and molecules. It is therefore necessary to investigate the properties of excited xenon atoms. This study presents experimental data associated with the behavior of excited xenon atoms in a PDP discharge cell and compares the data with the theoretical results obtained using an analytical model. The properties of excited xenon atoms in the discharge cells of a plasma display panel are investigated by measuring the excited atom density through the use of laser absorption spectroscopy. The density of the excited xenon atoms increases from zero, reaches its peak, and decreases with time in the discharge cells. The profile of the excited xenon atoms is also studied in terms of the xenon mole fraction. The typical density of the excited xenon atoms in the metastable state is on the order of 10¹³ atoms per cubic cm.     

Influence of small amounts of organic solvents in aqueous samples on argon inductively coupled plasma spectrometry

The presence of organic solvents in aqueous samples in concentrations as low as 0.1% by volume had a large effect on emission and fluorescence signals. Both atom and ion emission signals were depressed when organic solvents were present. In contrast, Sr II fluorescence intensities were enhanced by up to a factor of 3.5 when the sample contained 2% (v/v) organic solvent. More ions were produced when the sample contained organic solvent. However, the fractions of atoms and ions that were excited and emitted light decreased. Ar and H emission intensities can be used as diagnostic signals to warn the operator when plasma conditions have changed due to the presence of organic solvents.     

UV emission from capacitive discharge in inert gas-iodine vapor mixture

We have studied the optical characteristics of capacitive discharge in binary mixtures of helium, neon, argon, and krypton with iodine vapor in a spectral range of 180–300 nm. It is established that the main power of UV radiation from the discharge plasma is concentrated in the emission lines at 183.0 and 206.2 nm. The intensity of emission due the spectral lines of iodine atom was optimized with respect to the inert gas type and partial pressure. The optimum results were obtained using He-I₂ mixture with partial pressures of helium within 0.8–2.0 kPa and iodine vapor below 50–60 Pa. Being excited with a trains of nanosecond current pulses at a repetition rate of 10–100 Hz, the capacitive discharge emitted pulses with duration not exceeding 400–500 ns.     

Unconventional geometric phase gates using two four-level rf-SQUIDs with cyclic population transfer in a cavity

We propose a scheme for realizing two-qubit quantum phase gates via an unconventional geometric phase shift using two four-level superconducting artificial atoms with cyclic population transfer (broken symmetry) in a cavity. In this scheme, the logical gates' operation only involves the metastable states of each artificial atom. Moreover, under certain conditions, the atoms are disentangled with the cavity mode. Thus, the gate is insensitive to both the atomic spontaneous emission and the cavity decay.     

In-situ optical analysis of the gas phase during the formation of carbon nanotubes

A reactor has been developed at ONERA to investigate the gas phase during carbon nanotube formation by laser-induced fluorescence (LIF), Laser-induced incandescence (LII), coherent anti-Stokes Raman Scattering (CARS), and emission spectroscopy. Continuous vaporization is achieved with a continuous wave CO2 laser. Optimized conditions are used for single-walled nanotube growth, that is, a graphite target doped with 2 atom % Ni and 2 atom % Co, helium as buffer gas at a flow rate of 50 ml/s, and a pressure of 300 hPa. Temperature profiles are measured by CARS on H2, and soot images are obtained by LII in the hot carbonaceous flow. LIF and spontaneous emission of the C2 radical and Ni and Co atoms are presented. Spectral investigations are conducted at 3100 and 3200 K to have an optimized pair of excitation/detection wavelengths. Spatial investigations of the relative concentrations in the hot carbonaceous flow are performed up to 3500 K. The concentrations are measured as a function of target temperature. Two regimes of vaporization are observed. Vaporization is slow up to 3350 K and becomes much faster above this temperature. The fast regime in the 3350-3500 K range corresponds to the observed spatial extent of the metal vapors region. At 3500 K, the C2 profiles obtained with and without catalysts are very different as a result of carbon coalescence as well as carbon dissolution into the metal nanoparticles when these are present in the gas phase. The shape of the C2 profile can be related to nanotube formation and growth at a target temperature of 3500 K.     

Microplasma source based on a dielectric barrier discharge for the determination of mercury by atomic emission spectrometry

A low-power, atmospheric-pressure microplasma source based on a dielectric barrier discharge (DBD) has been developed for use in atomic emission spectrometry. The small plasma (0.6 mm x 1 mm x 10 mm) is generated within a glass cell by using electrodes that do not contact the plasma. Powered by an inexpensive ozone generator, the discharge ignites spontaneously, can be easily sustained in Ar or He at gas flow rates ranging from 5 to 200 mL min(-1), and requires less than 1 W of power. The effect of operating parameters such as plasma gas identity, plasma gas flow rate, and residual water vapor on the DBD source performance has been investigated. The plasma can be operated without removal of residual water vapor, permitting it to be directly coupled with cold vapor generation sample introduction. The spectral background of the source is quite clean in the range from 200 to 260 nm with low continuum and structured components. The DBD source has been applied to the determination of Hg by continuous-flow, cold vapor generation and offers detection limits from 14 (He-DBD) to 43 pg mL(-1) (Ar-DBD) without removal of the residual moisture. The use of flow injection with the He-DBD permits measurement of Hg with a 7.2 pg limit of detection, and with repetitive injections having an RSD of <2% for a 10 ng mL(-1) standard.     

Active roles of helium in the growth of hydrogenated microcrystalline silicon germanium thin films

A combination of hydrogen and helium dilutions was introduced when the microcrystalline silicon germanium (μc-SiGe:H) thin films were prepared by very high frequency plasma enhanced chemical vapor deposition on a low-temperature substrate. An optimum helium flow rate was found to achieve the structural uniformity in the growth direction, while Ge content was found to nearly keep constant with varying flow rates of helium. An abundance of atomic H was detected in plasma due to the attendance of helium and no obvious photosensitivity deterioration was observed on the thin film with a high crystalline volume fraction. The active roles of helium were identified by analyzing the mechanism in the plasma, where both metastable He_m^? and He⁺ can accelerate the diffusion of Ge related radicals and passivation of the dangling bonds on the growth surface, respectively. These phenomena have been revealed by experimental results. Therefore, a combination of hydrogen and helium dilutions can improve the structure of the μc-SiGe:H thin films with little degradation of photo-electronic properties.     

Detector for metastable helium ions

We describe a sensitive detector for metastable ( ) helium ions. The detector operates in a magnetic field of 4–16 kG and has an overall efficiency of about 1%. Metastable He⁺ ions are quenched in an electric field causing them to decay to the ground state ( ) with emission of 304 Å radiation. The radiation is detected in a cylindrical photodiode whose active surfaces are made of copper foils coated with MgF₂. Multiple photocathodes are used in an arrangement which maximizes the overall efficiency.     

Decoherence in grazing scattering of fast H and He atoms from a LiF(001) surface

The influence of decoherence on the diffraction during grazing scattering of fast hydrogen and helium atoms from a LiF(001) single crystal surface with projectile energies of some keV, is investigated by two-dimensional angular distributions for scattered projectiles in coincidence with their energy loss and emitted electrons from the target surface. For keV hydrogen atoms, we identify the excitations of electrons and excitons of the target surface as the dominant mechanisms for decoherence, whereas for keV helium atoms these contributions are negligibly small. The suppression of electronic excitations owing to the band gap of insulators plays an essential role for preserving quantum coherence and thus for the application of fast atom diffraction as a surface analytical tool.     

The energetics of helium and hydrogen atoms in β-SiC: an ab initio approach

Silicon carbide is a prime candidate for plasma-facing materials in future fusion reactors. The formation energies of various interstitial configurations of helium and hydrogen atoms in β-SiC were estimated based on density functional theory. Special consideration was given to the helium and hydrogen interstitials as the bubble seeds in β-SiC. From an energetic point of view, only one helium atom could position itself into the tetrahedral sites. However, up to three hydrogen atoms could easily position themselves into the tetrahedral sites by forming a stable H₂ molecule or a 3H-trimer that was newly identified in this study. Based on the different behaviors of helium and hydrogen, an explanation is proposed for the experimental observations of bubble formation in irradiated β-SiC.     

Helium detection in gas mixtures by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been evaluated as a tool for monitoring trace levels of helium in gas mixtures consisting mostly of hydrogen. Calibration data for helium in hydrogen was investigated at different helium concentration levels. At high concentrations of helium (>7.25%), the LIBS signal is quenched due to Penning ionization. The hydrogen alpha line (656.28 nm) was observed to broaden as the concentration of helium impurities in the hydrogen gas mixture increased. The helium line at 587.56 nm was selected as the analyte line for helium impurity detection. The effects of laser energy, the delay time between the laser pulse and data acquisition, and the gas pressure on the LIBS signal of helium were investigated to determine the optimum conditions for helium detection. The LIBS signal from the helium line at 587.56 nm shows good linear correlation with helium concentration for He concentrations below 1%. Thus, LIBS can be reliably used to detect the low levels of helium. The limit of detection for helium was found to be 78 ppm.     

Observation of Single Compton-Electron Tracks in Superfluid Helium-4 and Trace Detection of Metastable Helium Molecules by Laser-Induced-Fluorescence Imaging

We use laser-induced fluorescence enhanced by a cycling transition to image metastable helium molecules in liquid helium-4. We demonstrate that the method achieves sufficient sensitivity to image the trail of helium molecules that forms along the track of a single high-energy electron created by Compton scattering of a 511 keV gamma ray in liquid helium. The experiments show that a liquid helium detector with optical readout can function as an electron-tracking Compton camera when combined with conventional gamma-ray detectors. The demonstrated sensitivity for imaging helium molecules could also find application in the spectroscopy and imaging of neutrons, detection of electronic and nuclear recoil events, and investigation of superfluid or normal-fluid flow in liquid helium using the molecules as passive tracer particles.     

Generation of intrinsic excitations in superfluid 4He by using gas jet approach

No Heading A radio frequency discharged helium gas jet was used to generate intrinsic excitations in bulk superfluid ⁴He. The present experimental results and our previous estimates show that metastable triplet state He atoms and excimers cannot enter directly the liquid but rather concentrate on the liquid surface. High concentration of metastable species promotes reactions, which then lead to formation of He ions. Upon electron - ion recombination, population of highly excited atomic and diatomic excimer electronic states occurs. Effect of molecular hydrogen on quenching of the helium emissions is demonstrated. Excitations are efficiently transferred from He to H₂.PACS numbers: 33.20 Kf, 33.70 Jg.