H-atom Density Research Articles

Gas temperature measurements from ps-TALIF in highly collisional plasmas

In this work, we present a straightforward approach to retrieve gas temperatures directly from the effective lifetime τH of the ps-laser excited species measured with picosecond Two Photon Absorption Laser Induced Fluorescence (ps-TALIF). This approach allows for simultaneous measurements of absolute atom densities and gas temperatures performed in moderate-pressure reactive plasmas using an advanced ps-TALIF diagnostic and has been demonstrated for highly collisional hydrogen microplasmas. Furthermore, this methodology was applied to obtain a 2D distribution of gas temperature and H-atom densities in the downstream region of the microplasma discharge.

Ammonia production in a dual crossed atom beam experiment

Production of ammonia by surface reactions of H and N atoms on surfaces not wetted by partially ionized plasma may represent an important technological issue in fusion reactors where puffing nitrogen is employed to cool plasma in the divertor region. The H and N atoms are likely to interact on such surfaces forming NH3 molecules. The interaction efficiency was studied in a laboratory setup consisting of two separate sources of either N or H atoms. Both sources enabled experiments with atoms at room temperature in the range of H-atom density of the order of 1021 m−3 and N-atom density of the order of 1020 m−3. The production of ammonia was measured with a calibrated residual gas analyser. The production depended on the fluxes of both atoms onto the surface of selected materials. As a general rule, the higher H-atom flux at a constant N-atom flux caused an increase in ammonia production. The highest efficiency of up to 50% was found for nickel. It was up to 30% for tungsten, whereas for P92 alloy, it was up to about 20%. The accuracy of these results is within about ±20% of the measured values. Methods for suppressing ammonia formation in fusion reactors will have to be invented in order to enable appropriate long-term operation.

Barrier-discharge source of cold hydrogen atoms in supersonic beams: Stark effect in the 1s–2s transition

An intense source of cold hydrogen atoms in a supersonic beam is presented. H and D atoms are produced from pure samples of H2 and D2 using a barrier discharge near the orifice of a cryogenic pulsed valve. The atoms are entrained in the supersonic expansion of the precursor molecules and cool down to translational temperatures as low as mK. The H-atom density at a distance of 1.2 m from the valve orifice is estimated to be around 108 cm−3. The mean velocity of the supersonic beam containing the H (D) atoms can be adjusted between 2600 and 1200 m s−1 (between 1850 and 820 m s−1). The barrier discharge also produces and ions with n ranging from 2 to 8, which can be deflected off the beam by electric fields. The use of this H-atom source in spectroscopic measurements is demonstrated by an investigation of the Stark effect in the 1s–2s two-photon transition of H and D. A near-Fourier-transform-limited pulsed laser generating pulses of UV radiation of adjustable pulse length between 10 and 200 ns near 243 nm is used to record spectra with full resolution of the hyperfine structure in the s states, providing a textbook example of the Stark effect in the simplest set of near-degenerate states of one-electron atoms.

High H-atom Density in R(He,Ar)/x%(N2–5%H2) Early Afterglows

Absolute densities of N atoms and N2(A) molecules and estimated densities of H atoms and N2(X,v > 13) and NH molecules are determined in R(He,Ar)/x%(N2–5%H2) flowing microwave discharges by a band intensity ratio method after calibration of the N atom density by NO titration. By varying the plasma parameters: pressure of 8 and 4 Torr, flow rates of 0.5, 1.0 and 2.0 slpm, HF power up to 150 W, a high N + N recombination fraction (> 80%) has been observed with He dilution in the early afterglow (t = 5 × 10–3 s). A maximum value of the H-atom density has been found in the He/2%(N2–5%H2) gas mixture at 8 Torr, 0.5 slpm and 150 W, with a [H]/[N] ratio of 0.3. With Ar dilution, it is found that the Ar/5%(N2–5%H2) gas mixture at 4 Torr, 0.5 slpm, 100 W brought a high H2 dissociation (80%), with a [H]/[N] ratio of about 0.6. From the variation of the H-atom density along the quartz tube, the wall destruction probability of the H-atoms was calculated to be $${\gamma }_{H}^{He}$$ = $${\gamma }_{H}^{Ar}$$ = (1–4) × 10–3 in helium and argon mixtures. Such results are of interest for the enhancement of surface nitriding by the combined effects of N and H atoms inclusion inside a TiO2 surface.

Absolute density of precursor SiH3 radicals and H atoms in H2-diluted SiH4 gas plasma for deposition of microcrystalline silicon films

Microcrystalline hydrogenated silicon films were produced at a high deposition rate of about 2 nm/s by using a capacitively coupled plasma under a practical pressure of around 1 kPa. The SiH4 source gas was almost fully dissociated when highly diluted with H2 gas, and the dominant species in the gas phase were found to be SiH3 radicals, which are film-growth precursors, and H atoms. The absolute density of these species was measured as the partial pressure of SiH4 gas was varied. With the increasing SiH4 gas flow rate, the SiH3 radical density, which was on the order of 1012 cm−3, increased linearly, while the H-atom density remained constant at about 1012 cm−3. The film growth mechanism was described in terms of precursors, based on the measured flux of SiH3 radicals and H atoms, and the relative fraction of higher-order radicals.

Spectroscopic analysis of H2/CH4 microwave plasma and fast growth rate of diamond single crystal

One of the best ways to increase the diamond growth rate is to couple high microwave power to the plasma. Indeed, increasing the power density leads to increase gas temperature the atomic hydrogen density in the plasma bulk, and to produce more hydrogen and methyl at the diamond surface. Experimental and numerical approaches were used to study the microwave plasma under high power densities conditions. Gas temperature was measured by optical emission spectroscopy and H-atom density using actinometry. CH3-radical density was obtained using a 1D model that describes temperatures and plasma composition from the substrate to the top of the reactor. The results show that gas temperature in the plasma bulk, atomic hydrogen, and methyl densities at the diamond surface highly increase with the power density. As a consequence, measurements have shown that diamond growth rate also increases. At very high power density, we measured a growth rate of 40 μm/h with an H-atom density of 5 × 1017 cm−3 which corresponds to a H2 dissociation rate higher than 50%. Finally, we have shown that the growth rate can be framed between a lower and an upper limit as a function depending only on the maximum of H-atom density measured or calculated in the plasma bulk. The results also demonstrated that increasing fresh CH4 by an appropriate injection into the boundary layer is a potential way to increase the diamond growth rates.

Wire diameter dependence in the catalytic decomposition of H2

Jansen et al. have demonstrated that the dissociaiton rate of H2 molecules on hot wire surfaces, normalized per unit surface area, depends on the wire diameter based on the electrical power consumption measurements [J. Appl. Phys. 66, 5749 (1989)]. Mathematical modeling calculations have also been presented to support their experimental results. In the present paper, it is shown that such a wire diameter dependence cannot be observed and that the H-atom density normalized by the wire surface area depends little on the wire diameter. Modeling calculations also show that the wire diameter dependence of the dissociation rate cannot be expected under typical decomposition conditions.

Optical emission spectroscopy and Langmuir probe diagnostics of CH3F/O2 inductively coupled plasmas

A CH3F:O2 (50%:50%) inductively coupled discharge, sustained in a compact plasma reactor, was investigated as a function of power (20–400 W) and pressure (9–30 mTorr), using optical emission spectroscopy and Langmuir probe measurements. The electron density increased linearly with power but only weakly with pressure. The effective electron temperature was nearly independent of power and pressure. The gas temperature, obtained from the rotational structure of N2 (C → B) optical emission, increased from 400 to 900 K as a function of inductive mode power between 75 and 400 W at 10 mTorr. For constant feed gas flow, the absolute H, F, and O atom densities, estimated by optical emission rare gas actinometry, increased linearly with power. The absolute number density ratios H/Ar, F/Ar, and O/Ar, increased, decreased, and remained constant, respectively, with pressure. The H-atom density was estimated to be 5.4 × 1013 cm−3 at 400 W and 10 mTorr (gas temperature = 900 K), implying a high degree of dissociation of the CH3F feedstock gas. The F and O atom number densities were much lower (8.3 × 1012 cm−3 and 5.9 × 1012 cm−3, respectively) for the same conditions, suggesting that most of the fluorine and oxygen is contained in reaction products HF, CO, CO2, H2O, and OH. The relative number densities of HF, CO, and CO2 were observed to first rapidly increase with power, and then reach a plateau or decay slightly at higher power. Reaction mechanisms were proposed to explain the observed behavior of the number density of F and HF vs. power and pressure.

Infrared optical actinometry to determine N- and H-atom density in a N2–H2 microwave discharge

Infrared (IR) emission spectroscopy was performed on N2 + H2 microwave discharges at pressures ranging between 300 and 3000 mTorr. The relative atomic density of N and H was measured by optical actinometry in the IR region at various total gas pressures. The effect of relative hydrogen partial pressure (between 10 and 90% in the discharge) on N and H relative density was also investigated. Although rarely studied, optical actinometry in the IR region has nevertheless provided numerous advantages over traditional techniques performed in the UV–visible (UV–VIS) spectral region. Results show that despite the decrease in the radiative state of the N and H atoms as a function of pressure, their ground state density increased. With increased relative hydrogen concentration under constant pressure, both the ground and the radiative state density of the H atoms increased similarly to that recorded by actinometry, whereas those of the N atoms decreased as expected. In comparing the results of the H-atom density measured in the well-documented UV–visible region and the IR region, optical actinometry confirms the accuracy of the IR method.

In-situ production of PH3 from red phosphorus and atomic hydrogen

The production of PH3 from red phosphorus and the atomic hydrogen formed in the catalytic decomposition of H2 on heated W surfaces was confirmed. The absolute density of the PH3 could be as high as 1013cm−3; the density was proportional to H-atom density in the absence of red phosphorus, although three H atoms must be involved in the production of one PH3 molecule, but showed minor dependence on the amount of red phosphorus. These results suggest that the rate-determining step for the production of PH3 is that which produces H atoms.

ChemInform Abstract: Production and Detection of H Atoms and Vibrationally Excited H2 Molecules in CVD Processes

AbstractReview: 184 refs.

Hydrogen atom density in a solar plasma reactor

A density of neutral hydrogen atoms was systematically measured in the MESOX solar plasma reactor at different MW powers and flow rates. The H-atom density was measured by a gold fibre optics catalytic probe. The H-atom density was in general increasing with increasing MW power. At a pressure of 40 Pa and a power of 500 W it was about 3.5 × 10 21 m −3 and at a power of 1000 W it was about 4.1 × 10 21 m −3. A degree of dissociation of hydrogen molecules was between 3% and 20% depending on pressure and power. A maximum degree of dissociation was obtained at a pressure of 40 Pa and 1000 W, while the lowest one at 130 Pa and 500 W.

A multi-wavelength study of parent volatile abundances in Comet C/2006 M4 (SWAN)

Volatile organic emissions were detected post-perihelion in the long-period Comet C/2006 M4 (SWAN) in October and November 2006. Our study combines target-of-opportunity infrared observations using the Cryogenic Echelle Spectrometer (CSHELL) at the NASA-IRTF 3-m telescope, and millimeter wavelength observations using the Arizona Radio Observatory (ARO) 12-m telescope. Five parent volatiles were measured with CSHELL (H 2O, CO, CH 3OH, CH 4, and C 2H 6), and two additional species (HCN and CS) were measured with the ARO 12-m. These revealed highly depleted CO and somewhat enriched CH 3OH compared with abundances observed in the dominant group of long-period (Oort cloud) comets in our sample and similar to those observed recently in Comet 8P/Tuttle. This may indicate highly efficient H-atom addition to CO at very low temperature (∼10–20 K) on the surfaces of interstellar (pre-cometary) grains. Comet C/2006 M4 had nearly “normal” C 2H 6 and CH 4, suggesting a processing history similar to that experienced by the dominant group. When compared with estimated water production at the time of the millimeter observations, HCN was slightly depleted compared with the normal abundance in comets based on IR observations but was consistent with the majority of values from the millimeter. The ratio CS/HCN in C/2006 M4 was within the range measured in ten comets at millimeter wavelengths. The higher apparent H-atom conversion efficiency compared with most comets may indicate that the icy grains incorporated into C/2006 M4 were exposed to higher H-atom densities, or alternatively to similar densities but for a longer period of time.

Behaviour of neutral hydrogen atom density in the presence of a sample holder in a plasma reactor

The behaviour of density of neutral hydrogen atoms in the presence of a sample holder in a MESOX reactor was studied. The MESOX reactor is used for studying interaction of hydrogen atoms with solid state surfaces at extreme conditions. Concentrated solar radiation with power of approximately 6 kW is collimated to a spot with the surface in the order of magnitude 1 cm 2 thus allowing for independent sample heating above 2000 K. Hydrogen plasma is generated in a MW (microwave) discharge. The H-atom density is measured with a FOCP (Fibre Optics Catalytic Probe). The H-atom density in the empty reactor depends slightly on the pressure and MW power and is about 5 × 10 21 m −3 at a power of 1000 W. The sample holder made from quartz glass was mounted in the centre of the reactor and the H-atom density was measured versus time. The H-atom density in the loaded reactor was decreasing continuously. After a time period in the order of 2 min, the H-atom density was reduced by approximately a factor of 3 in regards to the original value. The results were explained by taking into account a temperature dependence of the recombination coefficient for heterogeneous recombination of H atoms on the quartz surface.

Detection of radical species produced by catalytic decomposition of H 2, O 2 and their mixtures on heated Ir surfaces

Efficient production of H atoms, O atoms and OH radicals was confirmed by laser spectroscopic measurements in the catalytic decomposition of H 2, O 2 and their mixtures on a heated Ir filament. No change in electric resistivity was observed when the filament was kept at 2350 K in the presence of 1.0 Pa of pure O 2, showing that oxidation is minor. Arrhenius-type temperature dependences were observed for the densities of H (O) atoms in pure H 2 (O 2) systems. In the H 2/O 2 mixed system, the H-atom density was almost independent of the O 2 partial pressure, although the O-atom and OH-radical densities increased with the O 2 pressure. These O 2 pressure dependences are completely different from those observed for W. Ir is less poisoned by O 2 compared to W. In addition, direct production of H 2O molecules on Ir surfaces must take place besides the production of radical species.

Coating techniques of metal chambers for remote catalytic chemical vapor deposition applications

To obtain information for remote catalytic chemical vapor deposition, the authors constructed a two-chamber system in which the decomposition and deposition areas were separated. H2 molecules were decomposed on a heated tungsten filament in the first area and the H atoms produced were carried to the second area, where the absolute H-atom densities were measured by a vacuum-ultraviolet laser absorption technique. The gas pressure in the decomposition area was maintained ten times higher than that in the deposition area by mounting a diffusion barrier to prevent the possible back-diffusion of the material gases. The inner walls of the decomposition area as well as the diffusion barrier were coated with SiO2 or polytetrafluoroethene to minimize the recombination of H atoms. The H-atom density could be increased by one order of magnitude by such coatings, and it is shown that H-atom densities in the order of 1012 cm−3 can be achieved in the deposition area in the present system.

Production and detection of reducing and oxidizing radicals in the catalytic decomposition of H2∕O2 mixtures on heated tungsten surfaces

H atoms, O atoms, and OH radicals were identified in the catalytic decomposition of H2∕O2 mixtures on heated polycrystalline tungsten surfaces. In order to suppress the oxidization of the tungsten catalyzer surfaces, the H2∕O2 pressure ratio was kept more than 83, while the catalyzer temperature was kept below 2000K. The absolute density of H atoms was determined by a vacuum-ultraviolet laser absorption technique, while one-photon and two-photon laser-induced fluorescence techniques were employed to extend the dynamic range. Since the O-atom density was much smaller, only a vacuum-ultraviolet laser-induced fluorescence technique could be used for the detection. The absolute density could be estimated by comparing the induced fluorescence intensity with that for H atoms. OH radicals could be identified by a laser-induced fluorescence technique in the ultraviolet region. The absolute density was determined by comparing the induced fluorescence intensity with that of Rayleigh scattering caused by Ar. The H-atom density decreased with the increase in the O2 partial pressure stepwisely. The O-atom density increased with the O2 partial pressure monotonously, but the increase was rather slow at low O2 pressures. The OH-radical density showed saturation against the O2 partial pressure. These results can be explained by the change in the coverage conditions of the catalyzer surfaces.

Optical absorption and emission spectroscopy studies of ammonia-containing plasmas

The chemistry of NH3/Ar/He plasmas was investigated, using a combination of ultraviolet (UV) optical absorption spectroscopy (OAS) and optical emission spectroscopy (OES). Absolute NH3 number densities in 1 Torr plasmas were measured by OAS as a function of inductively coupled plasma power and substrate heater temperature (Th). OES and actinometry were used to determine semi-quantitative H-atom density. A ‘self-actinometry’ method was introduced to measure the absolute number density of N2 that formed following the dissociation of NH3 and secondary reactions. In this approach, small amounts of N2 are added to the NH3-containing plasma, leading to an increase in the N2(C 3 Πu → B 3 Πg) emission intensity. This provides an accurate calibration factor for converting relative N2 emission intensities into absolute number densities. The number densities of NH3 were found to decrease with increasing power and Th, reaching >90% dissociation at 400 W and 900 K. N2 densities increased with power and Th. The majority of dissociated NH3 was converted to N2 (i.e. the total nitrogen content was conserved in the sum of these two species). The major hydrogen-containing species appeared to be H2; however, a substantial amount of H-atoms (comparable to H2) was present at the highest powers.

Influence of gas temperature on the loss mechanisms of H-atoms in a pulsed microwave discharge identified by time-resolved LIF measurements

The present work deals with investigating the loss kinetics of atomic hydrogen in an H2/CH4 pulsed microwave discharge used for diamond chemical vapour deposition. The temporal evolution of the hydrogen atom density, particularly in the afterglow phase, was investigated in order to determine the fundamental loss processes occurring in such discharge. Time resolved two-photon laser induced fluorescence (LIF) measurements of the relative concentration of H-atoms in the ground state and observed by Hα transition have been established. The gas temperature was estimated from the Doppler broadening of the LIF Gaussian profile obtained by scanning the laser frequency. The wavelength calibration and the estimation of the effective bandwidth of the LIF profile were accomplished by measuring the spectral response of atomic hydrogen and deuterium. The decay of H-atom density in the afterglow is characterized by a fast decrease in the early afterglow up to 2 ms, followed by a slow evolution. The effect of the gas cooling on the diffusion coefficient and then on the atom density decay in the afterglow was studied. It is found that the fundamental loss processes of atomic hydrogen are governed by the diffusion phenomenon and surface recombinations, especially on the diamond substrate.

Hydrogen dissociation on high-temperature tungsten

H 2 dissociation on polycrystalline tungsten is measured from 1700 to 3000 K using the filament temperature ( T) change and a normalized H-atom density at the chamber surface. The dissociation probability per H 2 filament collision ( P diss) saturates at 0.40 at high T and has a 2.25 ± 0.05 eV apparent activation energy when P diss ≪ 1. This activation energy is consistent with prior data and models, but the H 2 pressure dependence is not. P diss is independent of the H 2 pressure for this entire T range and the 1–85 mTorr pressure range studied, contradicting the primary model that has been used to explain H 2 dissociation on tungsten and other metals. We show that some apparently contradictory prior measurements are actually consistent with our observations and with each other, once this pressure dependence of P diss is recognized.