Torr Pressure Range Research Articles

Novel surface wave technique for moderate-pressure plasma measurements

Most in situ plasma diagnostics are only reliable for low-pressure systems, e.g., systems where the collision frequency, νen, is small compared to the plasma frequency, ωpe. In this article, we describe a novel diagnostic technique for simultaneous measurement of the plasma density, ne; electron temperature, Te; and νen in systems in which νen approaches ωpe. We illustrate the operational setup of our probe system along with an analytical framework for ascertaining ne, Te, and νen. Furthermore, we show experimental results from argon capacitively coupled plasma operating in the 0.5–1 Torr pressure range.

Kinetic mechanisms and species densities in N2/(0–5%)H2 afterglows

N atoms absolute concentrations were obtained in the afterglows of N2/(x = 0–5%)H2 mixtures by optical emission spectroscopy (OES) coupled with NO titration in the 2–6 Torr pressure range and at distances to the discharge varying between 20 and 50 cm. Band intensity measurements were then used to obtain absolute concentrations of N2(A) and N2(X, v > 13) metastable molecules in the same conditions. From these data, absolute NH and H-atoms densities were deduced from a set of kinetic mechanisms. Independently, absolute N and H atoms densities were also measured using the TALIF technique. The comparison between TALIF and OES results suggests that the NH species can be generated outside the discharge (i.e. in the afterglow) by the 3-body recombination process N+H+N2→NH+N2, with a rate coefficient of 1.0 10−33 cm6 s−1, the NH(A) state being itself produced by the reaction N2(X,v>13)+NH→NH(A)+N2, with a rate equal to 5 10−11 cm3 s−1. The agreement between the TALIF and OES measurements is further improved considering the role of N(2D) metastable atoms in the H and NH kinetics.

Formation of a Resonance-Stabilized Radical Intermediate by Hydroxyl Radical Addition to Cyclopentadiene.

The reaction of the OH radical with cyclopentadiene (C5H6) was investigated at room temperature using multiplexed photoionization mass spectrometry. OH radicals in their ground electronic state were generated in the gas phase by 248 nm photolysis of H2O2 or 351 nm photolysis of HONO. Analysis of photoion spectra and temporal profiles reveal that at room temperature and over the 4-8 Torr pressure range, the resonance-stabilized 5-hydroxycyclopent-2-en-1-yl (C5H6OH) is the main observed reaction product. Abstraction products (C5H5) were not detected. The C5H6OH potential energy surface calculated at the CCSD(T)/cc-pVTZ//M06-2X/6-311++G** level of theory suggests that the resonance-stabilized radical product is formed through barrierless addition of the OH radical onto cyclopentadiene's π system to form a van der Waals complex. This weakly bound adduct isomerizes through a submerged energy barrier to the resonance-stabilized addition adduct. Master Equation calculations, including two OH-addition entrance pathways, predict that 5-hydroxycyclopent-2-en-1-yl remains the sole addition product up to 500 K. The detection of an OH-containing resonance-stabilized radical at room temperature further highlights their importance in carbon- and oxygen-rich environments such as combustion, planetary atmospheres, and the interstellar medium.

CO and O2 Adsorption and CO Oxidation on Pt Nanoparticles by Indirect Nanoplasmonic Sensing.

We used indirect nanoplasmonic sensing (INPS) coupled with mass spectrometry to study CO and oxygen adsorption as well as CO oxidation, on Pt nanoparticles, in the Torr pressure range. Due to an optimization of the physical parameters of our plasmonic sample, we obtain a highly sensitive probe that can detect gas adsorption of a few hundredths of a monolayer, even with a very low number density of Pt particles. Moreover and for the first time, a similarity is observed between the sign and the evolution of the localized surface plasmon resonance (LSPR) peak shift and the work function measurements for CO and oxygen chemisorption. Controlling the size, shape, and surface density of Pt particles, the turnover frequency (TOF) has also been accurately determined. For similar experimental conditions, the TOF is close to those measured on Pt/oxide powder catalysts and Pt(100) single crystals.

Kinetic fall-off behavior for the Cl + Furan-2,5-dione (C4H2O3, maleic anhydride) reaction.

Rate coefficients, k, for the gas-phase Cl + Furan-2,5-dione (C4H2O3, maleic anhydride) reaction were measured over the 15-500 torr (He and N2 bath gas) pressure range at temperatures between 283 and 323 K. Kinetic measurements were performed using pulsed laser photolysis (PLP) to produce Cl atoms and atomic resonance fluorescence (RF) to monitor the Cl atom temporal profile. Complementary relative rate (RR) measurements were performed at 296 K and 620 torr pressure (syn. air) and found to be in good agreement with the absolute measurements. A Troe-type fall-off fit of the temperature and pressure dependence yielded the following rate coefficient parameters: ko(T) = (9.4 ± 0.5) × 10-29 (T/298)-6.3 cm6 molecule-2 s-1, k∞(T) = (3.4 ± 0.5) × 10-11 (T/298)-1.4 cm3 molecule-1 s-1. The formation of a Cl·C4H2O3 adduct intermediate was deduced from the Cl atom temporal profiles and an equilibrium constant, KP(T), for the Cl + C4H2O3 ↔ Cl·C4H2O3 reaction was determined. A third-law analysis yielded ΔH = -15.7 ± 0.4 kcal mol-1 with ΔS = -25.1 cal K-1 mol-1, where ΔS was derived from theoretical calculations at the B3LYP/6-311G(2d,p,d) level. In addition, the rate coefficient for the Cl·C4H2O3 + O2 reaction at 296 K was measured to be (2.83 ± 0.16) × 10-12 cm3 molecule-1 s-1, where the quoted uncertainty is the 2σ fit precision. Stable end-product molar yields of (83 ± 7), (188 ± 10), and (65 ± 10)% were measured for CO, CO2, and HC(O)Cl, respectively, in an air bath gas. An atmospheric degradation mechanism for C4H2O3 is proposed based on the observed product yields and theoretical calculations of ring-opening pathways and activation barrier energies at the CBS-QB3 level of theory.

Understanding Catalyst Surfaces during Catalysis through Near Ambient Pressure X-ray Photoelectron Spectroscopy.

Heterogeneous catalysis occurs on the surface of a catalyst particle in a gas or liquid environment of reactants. The surface of the catalyst particle acts as an active chemical agent directly participating in a chemical reaction performed at a solid-gas or solid-liquid interface. Thus, authentic surface chemistry and the structure of a catalyst particle during catalysis are key descriptors for understanding catalytic performance of this catalyst. However, identification of the authentic surface of a catalyst particle during catalysis is not a simple task. We are far from knowing the fact. Photoelectron spectroscopy is one of the main techniques for characterizing surface of a catalyst since it's a surface sensitive technique. When used to track the surface of a catalyst particle at relatively high temperature in gas phase in the torr pressure range, it is called near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) or AP-XPS for simplicity. In the last several years, AP-XPS has been used to observe surface chemistry of catalysts of single crystals and nanoparticles of metal, metal oxide, and carbide. In this review, instrumentation of the near ambient pressure X-ray photoelectron spectrometers and observation of catalyst surfaces in gases phase under reaction conditions and during catalysis with AP-XPS are discussed with the following objectives: (1) to present how the surface of a catalyst particle can be characterized in gas phase, (2) to interpret how surface chemistries observed during catalysis are correlated with measured catalytic performances, (3) to demonstrate how the uncovered correlations between surface structures and catalytic performances help to understand catalytic mechanisms at a molecular level, and (4) to discuss challenges and prospects of using AP-XPS to explore the authentic surface of a catalyst under a condition near to an industrial catalytic condition. This review focuses on the application of AP-XPS to studies of catalysis and how the insights gained from AP-XPS studies can be used to achieve fundamental understanding of the catalytic mechanism at a molecular level.

Accurate deuterium spectroscopy for fundamental studies

We present an accurate measurement of the weak quadrupole S(2) 2-0 line in self-perturbed D2 and theoretical ab initio calculations of both collisional line-shape effects and energy of this rovibrational transition. The spectra were collected at the 247–984 Torr pressure range with a frequency-stabilized cavity ring-down spectrometer linked to an optical frequency comb (OFC) referenced to a primary time standard. Our line-shape modeling employed quantum calculations of molecular scattering (the pressure broadening and shift and their speed dependencies were calculated, while the complex frequency of optical velocity-changing collisions was fitted to experimental spectra). The velocity-changing collisions are handled with the hard-sphere collisional kernel. The experimental and theoretical pressure broadening and shift are consistent within 5% and 27%, respectively (the discrepancy for shift is 8% when referred not to the speed averaged value, which is close to zero, but to the range of variability of the speed-dependent shift). We use our high pressure measurement to determine the energy, ν0, of the S(2) 2-0 transition. The ab initio line-shape calculations allowed us to mitigate the expected collisional systematics reaching the 410 kHz accuracy of ν0. We report theoretical determination of ν0 taking into account relativistic and QED corrections up to α5. Our estimation of the accuracy of the theoretical ν0 is 1.3 MHz. We observe 3.4σ discrepancy between experimental and theoretical ν0.

Enhanced control of the ionization rate in radio-frequency plasmas with structured electrodes via tailored voltage waveforms

Radio-frequency capacitively coupled plasmas that incorporate structured electrodes enable increases in the electron density within spatially localized regions through the hollow cathode effect (HCE). This enables enhanced control over the spatial profile of the plasma density, which is useful for several applications including materials processing, lighting and spacecraft propulsion. However, asymmetries in the powered and grounded electrode areas inherent to the hollow cathode geometry lead to the formation of a time averaged dc self-bias voltage at the powered electrode. This bias alters the energy and flux of secondary electrons leaving the surface of the cathode and consequentially can moderate the increased localized ionization afforded by the hollow cathode discharge. In this work, two-dimensional fluid-kinetic simulations are used to demonstrate control of the dc self-bias voltage in a dual-frequency driven (13.56, 27.12 MHz), hollow cathode enhanced, capacitively coupled argon plasma over the 66.6–200 Pa (0.5–1.5 Torr) pressure range. By varying the phase offset of the 27.12 MHz voltage waveform, the dc self-bias voltage varies by 10%–15% over an applied peak-to-peak voltage range of 600–1000 V, with lower voltages showing higher modulation. Resulting ionization rates due to secondary electrons within the hollow cathode cavity vary by a factor of 3 at constant voltage amplitude, demonstrating the ability to control plasma properties relevant for maintaining and enhancing the HCE.

Modeling of low pressure plasma sources for microelectronics fabrication

Chemically reactive plasmas operating in the 1 mTorr–10 Torr pressure range are widely used for thin film processing in the semiconductor industry. Plasma modeling has come to play an important role in the design of these plasma processing systems. A number of 3-dimensional (3D) fluid and hybrid plasma modeling examples are used to illustrate the role of computational investigations in design of plasma processing hardware for applications such as ion implantation, deposition, and etching. A model for a rectangular inductively coupled plasma (ICP) source is described, which is employed as an ion source for ion implantation. It is shown that gas pressure strongly influences ion flux uniformity, which is determined by the balance between the location of plasma production and diffusion. The effect of chamber dimensions on plasma uniformity in a rectangular capacitively coupled plasma (CCP) is examined using an electromagnetic plasma model. Due to high pressure and small gap in this system, plasma uniformity is found to be primarily determined by the electric field profile in the sheath/pre-sheath region. A 3D model is utilized to investigate the confinement properties of a mesh in a cylindrical CCP. Results highlight the role of hole topology and size on the formation of localized hot-spots. A 3D electromagnetic plasma model for a cylindrical ICP is used to study inductive versus capacitive power coupling and how placement of ground return wires influences it. Finally, a 3D hybrid plasma model for an electron beam generated magnetized plasma is used to understand the role of reactor geometry on plasma uniformity in the presence of E × B drift.

Structural Changes of Cu(110) and Cu(110)-(2 × 1)-O Surfaces under Carbon Monoxide in the Torr Pressure Range Studied with Scanning Tunneling Microscopy and Infrared Reflection Absorption Spectroscopy

The atomic structure of the clean Cu(110) and the oxygen covered Cu(110) surfaces in the presence of carbon monoxide (CO) gas in the Torr pressure range at 298 K is studied using scanning tunneling microscopy (STM) and infrared reflection adsorption spectroscopy (IRRAS). We found that the initially clean surface reconstructs to form short rows of Cu atoms along the [1–10] direction separated by missing rows. The adsorbed CO molecules show two different C–O stretch vibration modes originating from molecules bound to Cu atoms with different coordination numbers, in the middle and at the end of the atomic rows. On the oxygen covered p(2 × 1) surface, adsorbed CO is observed only after removal of surface O atoms by reaction with CO. In the presence of 1:5 and 1:1 mixtures of O2 and CO at 298 K, the p(2 × 1)-O reconstructed surface transforms into Cu2O, instead of reducing to metallic Cu.

Determining the microwave coupling and operational efficiencies of a microwave plasma assisted chemical vapor deposition reactor under high pressure diamond synthesis operating conditions.

The microwave coupling efficiency of the 2.45 GHz, microwave plasma assisted diamond synthesis process is investigated by experimentally measuring the performance of a specific single mode excited, internally tuned microwave plasma reactor. Plasma reactor coupling efficiencies (η) > 90% are achieved over the entire 100-260 Torr pressure range and 1.5-2.4 kW input power diamond synthesis regime. When operating at a specific experimental operating condition, small additional internal tuning adjustments can be made to achieve η > 98%. When the plasma reactor has low empty cavity losses, i.e., the empty cavity quality factor is >1500, then overall microwave discharge coupling efficiencies (η(coup)) of >94% can be achieved. A large, safe, and efficient experimental operating regime is identified. Both substrate hot spots and the formation of microwave plasmoids are eliminated when operating within this regime. This investigation suggests that both the reactor design and the reactor process operation must be considered when attempting to lower diamond synthesis electrical energy costs while still enabling a very versatile and flexible operation performance.

Reaction of CO with Preadsorbed Oxygen on Low-Index Copper Surfaces: An Ambient Pressure X-ray Photoelectron Spectroscopy and Scanning Tunneling Microscopy Study

The reaction of CO with chemisorbed oxygen on three low-index faces of copper was studied using ambient pressure X-ray photoelectron spectroscopy (XPS) and high-pressure scanning tunneling microscopy. At room temperature, the chemisorbed oxide can be removed by reaction with gas-phase CO in the 0.01–0.20 Torr pressure range. The reaction rates were determined by measuring the XPS peak intensities of O and CO as a function of time, pressure, and temperature. On Cu(111) the rate was found to be one order of magnitude faster than that on Cu(100) and two orders of magnitude faster than that on Cu(110). The apparent activation energies for CO oxidation were measured as 0.24 eV for O/Cu(111), 0.29 eV for O/Cu(100), and 0.51 eV for O/Cu(110) in the temperature range between 298 and 473 K. These energies are correlated to the oxygen binding energies on each surface.

Enhanced simulation of an RF ion funnel including gas turbulence.

Electrodynamic ion funnels are used to enhance the transmission of ions in electrospray-based ion injection systems in 0.1 to 30 Torr pressure range. Jet disrupters are commonly employed to prevent droplets and high pressure jets from entering subsequent vacuum regions. This study presents the simulation and testing of an ion funnel containing a jet disrupter using computational fluid dynamics (CFD) and SIMION ion trajectory simulations. Traditional modeling approaches have utilized approximations for the bulk fluid flow fields without including the time-varying nature of the turbulent flow present in the system, thus yielding idealized results. In this study, the fluid flow fields are calculated using CFD. In an effort to include time dependence, a random velocity vector, whose magnitude is proportional to the square root of the turbulence kinetic energy, was calculated at each time step and added to the velocity of the background gas. These simulations predicted that the transmitted ion current is effectively modulated by the variation of the jet disrupter voltage. The addition of the random velocity vector produced results that closely matched the experiments. The simulations yielded the dependence of the transmission on the jet disrupter voltage, and the voltage necessary for maximum ion throughput was accurately predicted. In addition, the magnitude of the predicted transmission closely matched that of the experimental results. This modeling approach could be extended to similar ion transport and filtering systems in which the effects of turbulent fluid flow cannot be ignored.

Kinetic study of the OH radical reaction with phenylacetylene.

The reaction of the OH radical with phenylacetylene is studied over the 298-423 K temperature range and 1-7.5 Torr pressure range in a quasi-static reaction cell. The OH radical is generated by 266 nm photolysis of hydrogen peroxide (H2O2) or 355 nm photolysis of nitrous acid (HONO), and its concentration monitored using laser-induced fluorescence. The measured reaction rates are found to strongly depend on laser fluence at 266 nm. The 266 nm absorption cross-section of phenylacetylene is measured to be 1.29 (±0.71) × 10(-17) cm(2), prohibiting any accurate kinetic measurements at this wavelength. The rates are independent of laser fluence at 355 nm with an average value of 8.75 (±0.73) × 10(-11) cm(3) s(-1). The reaction exhibits no pressure or temperature dependence over the studied experimental conditions and is much faster than the estimated values presently used in combustion models. These results are consistent with the formation of a short lifetime intermediate that stabilizes by collisional quenching with the buffer gas. The structures of the most likely formed products are discussed based on both the computed energies for the OH-addition intermediates and previous theoretical investigations on similar chemical systems.

Influence of Step Geometry on the Reconstruction of Stepped Platinum Surfaces under Coadsorption of Ethylene and CO.

We demonstrate the critical role of the specific atomic arrangement at step sites in the restructuring processes of low-coordinated surface atoms at high adsorbate coverage. By using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), we have investigated the reconstruction of Pt(332) (with (111)-oriented triangular steps) and Pt(557) surfaces (with (100)-oriented square steps) in the mixture of CO and C2H4 in the Torr pressure range at room temperature. CO creates Pt clusters at the step edges on both surfaces, although the clusters have different shapes and densities. A subsequent exposure to a similar partial pressure of C2H4 partially reverts the clusters on Pt(332). In contrast, the cluster structure is barely changed on Pt(557). These different reconstruction phenomena are attributed to the fact that the 3-fold (111)-step sites on Pt(332) allows for adsorption of ethylidyne-a strong adsorbate formed from ethylene-that does not form on the 4-fold (100)-step sites on Pt(557).

Kinetics Study of Reactions of α-Pinene and β-Pinene with Hydroxyl Radical at 1–8 Torr and 240–340 K Using the Relative Rate/Discharge Flow/Mass Spectrometry Method

The kinetics of reactions of α-pinene and β-pinene with hydroxyl radicals (OH) has been investigated at 1-8 Torr and 240-340 K using the relative rate/discharge flow/mass spectrometry (RR/DF/MS) technique. Our kinetic results indicate that at 298 K the rate constant of the reactions of α-pinene and β-pinene with hydroxyl radicals has little pressure dependence over the 1-8 Torr pressure range, suggesting that the high pressure limit of these reactions has been reached at 1 Torr. The rate constant of these reactions was found to negatively depend on the temperature at 240-340 K, which is consistent with previous investigations using different techniques. The Arrhenius equation for α-pinene and β-pinene with hydroxyl radical was determined to be k(α-pinene) = (1.21 ± 0.20) × 10(-11)exp[(441 ± 46)/T] cm(3) molecule(-1) s(-1) and k(β-pinene) = (1.65 ± 0.10) × 10(-11)exp[(470 ± 17)/T] cm(3) molecule(-1) s(-1), respectively. Using the rate constant determined at 277 K in this work and the average global hydroxyl radical concentration, the atmospheric lifetime of α-pinene and β-pinene was estimated to be 5.8 and 3.8 h, respectively.

Deposition of refractory coatings on carbon fibers from volatile Hf-containing precursors

The possibility to modify the carbon fiber surface by refractory hafnium compounds via chemical vapor deposition of volatile metal-organic complexes is studied. Thermodynamic modeling is applied to calculate the CVD diagram of the Hf-C-O-F-H system within a 25–1200°C temperature range, 10−1–10 Torr pressure range, and 0–2000 hydrogen/precursor vapor ratio. It is demonstrated that the deposition should be carried out at the maximum hydrogen flow and the minimum total pressure in the system. SEM and AFM techniques are used to examine the morphology and topography of the coatings on the fibers. Continuous coatings that are uniform in length and diameter of monofilaments and are tightly bound to them are obtained.

Branching Ratios in the Hydroxyl Reaction with Propene

The branching ratios for the reactions of attachment of hydroxyl radical to propene and hydrogen-atom abstraction were measured at 298 K over the buffer gas pressure range 60-400 Torr (N(2)) using a subatmospheric pressure turbulent flow reactor coupled with a chemical ionization quadrupole mass spectrometer. Isotopically enriched water H(2)(18)O was used to produce (18)O-labeled hydroxyl radicals in reaction with fluorine atoms. The β-hydroxypropyl radicals formed in the attachment reactions 1a and 1b , OH + C(3)H(6) → CH(2)(OH)C(•)HCH(3) (eq 1a ) and OH + C(3)H(6) → C(•)H(2)CH(OH)CH(3) (eq 1b ), were converted to formaldehyde and acetaldehyde in a sequence of secondary reactions in O(2)- and NO-containing environment. The (18)O-labeling propagates to the final products, allowing determination of the branching ratio for the attachment channels of reaction 1. The measured branching ratio for attachment is β(1b) = k(1b)/(k(1a) + k(1b)) = 0.51 ± 0.03, independent of pressure over the 60-400 Torr pressure range. An upper limit on the hydrogen-abstraction channel, OH + C(3)H(6) → H(2)O + C(3)H(5) (eq 1c ), was determined by measuring the water yield in reactions of OH and OD radicals (produced via H(D) + NO(2) → OH(OD) + NO reactions) with C(3)H(6) as k(1c)/(k(1a) + k(1b) + k(1c)) < 0.05 (at 298 K, 200 Torr N(2)).

Evolution of Structure and Chemistry of Bimetallic Nanoparticle Catalysts under Reaction Conditions

Three series of bimetallic nanoparticle catalysts (Rh(x)Pd(1-x), Rh(x)Pt(1-x), and Pd(x)Pt(1-x), x = 0.2, 0.5, 0.8) were synthesized using one-step colloidal chemistry. X-ray photoelectron spectroscopy (XPS) depth profiles using different X-ray energies and scanning transmission electron microscopy showed that the as-synthesized Rh(x)Pd(1-x) and Pd(x)Pt(1-x) nanoparticles have a core-shell structure whereas the Rh(x)Pt(1-x) alloys are more homogeneous in structure. The evolution of their structures and chemistry under oxidizing and reducing conditions was studied with ambient-pressure XPS (AP-XPS) in the Torr pressure range. The Rh(x)Pd(1-x) and Rh(x)Pt(1-x) nanoparticles undergo reversible changes of surface composition and chemical state when the reactant gases change from oxidizing (NO or O(2) at 300 degrees C) to reducing (H(2) or CO at 300 degrees C) or catalytic (mixture of NO and CO at 300 degrees C). In contrast, no significant change in the distribution of the Pd and Pt atoms in the Pd(x)Pt(1-x) nanoparticles was observed. The difference in restructuring behavior under these reaction conditions in the three series of bimetallic nanoparticle catalysts is correlated with the surface free energy of the metals and the heat of formation of the metallic oxides. The observation of structural evolution of bimetallic nanoparticles under different reaction conditions suggests the importance of in situ studies of surface structures of nanoparticle catalysts.

Effects of biomedical sterilization processes on performance characteristics of MEMS pressure sensors

The effects of steam and gamma sterilization on the performance of bulk-micromachined pressure sensors were investigated using a variable pressure setup. Commercially available piezoresistive MEMS (microelectromechanical systems) pressure sensor die were characterized prior and subsequent to sterilization over a 0-500 Torr pressure range. The effects of sterilization were examined as changes in sensor output voltage (DeltaV) at various applied pressures. For steam sterilization, DeltaV decreased with applied pressure ranging from +0.27 mV at 100 Torr to -0.14 mV at 500 Torr. In contrast, the corresponding values for gamma-sterilized sensors were lower, decreasing from +0.01 mV 100 Torr to -0.06 mV at 500 Torr. The increased variation in DeltaV for the steam-sterilized sensors was attributed to the formation of an oxide film, which was confirmed using energy dispersive X-ray (EDX) spectroscopy. Statistical analysis revealed that the effect of both sterilization procedures on sensor performance was insignificant.