Average Sticking Research Articles

Experimental sticking coefficients of CO and N_2 on sub-micrometric cosmic grain analogs

Measuring the sticking coefficient of molecules pertinent to astrochemistry - such as CO - on substrates that mimic interstellar dust grains is crucial for the comprehensive understanding of gas-grain chemical processes. Although astrochemical models assume a sticking coefficient of 1, recent laboratory experiments on H2O and CO2 have revealed significantly lower values when measured on small grain analogs. As the effect of grain size on molecular adsorption has been largely ignored to date, further experiments are needed to determine the accretion rates of species known to freeze out on dust grains. Our aim is to determine the sticking coefficients of CO and N2 on sub-micrometric silicate and carbon grains. By quantifying realistic sticking coefficients on these dust grain analogs, we can improve the accuracy of astrochemists' predictions of molecular abundances as affected by gas-grain interactions. The molecules of interest were added to various substrates at 10 K in an ultra-high vacuum. The amount of adsorbate that stuck to the substrate was quantified using X-ray photoelectron spectroscopy. These quantities were compared to a reference with a sticking coefficient of 1, allowing the deduction of the sticking coefficient for each substrate. The average sticking coefficients of CO and N2 on grain analogs are 0.17 for CO and 0.14 for N2 on olivine powder, and 0.05 for CO and 0.07 on N2 on soot, instead of the presumed 1. This is in line with the low values previously reported for H2O and CO2 These laboratory results indicate that CO and N2 in addition to H2O and CO2 also exhibit a low sticking coefficient on dust grain analogs. It is thus necessary to reconsider the interactions between gaseous species and dust particles as a low-efficiency process. This reduction in accretion and reaction rates has important implications for how we understand astrochemistry.

State-resolved studies of CO2 sticking to CO2 ice.

Internal vibrations may affect the adsorption, scattering, and reactions of molecules impinging onto a surface. The energy of the ν 3 antisymmetric stretch vibration of CO2 slightly exceeds the desorption energy of CO2 bound to CO2 ice. We use supersonic molecular beam techniques and rovibrationally state-resolved excitation to determine whether this vibration affects condensation of gas phase CO2 to its ice. We detect sticking and CO2 ice formation using RAIRS and quantify the sticking probability using the King and Wells method with modulation of the vibrational excitation and Fourier transform based detection. We find that the influence of this vibration on the structure of the formed ice and on the sticking probability is negligible under our conditions. Based on our detection limit, we quantify the weighted average sticking probability at approximately 0.9 and the difference between the state-resolved and weighted average sticking probability as below 0.03%.

Phenomenological modeling of low-bias sulfur hexafluoride plasma etching of silicon

Low-bias etching of silicon (Si) using sulfur hexafluoride (SF6) plasma is a valuable tool in the manufacturing of electronic devices and microelectromechanical systems (MEMS). This kind of etching offers an almost isotropic etching behaviour, since the low voltage bias does not provide enough vertical acceleration and kinetic energy to the ions. Due to this near-isotropic behavior, the aforementioned plasma etching finds application as an alternative to wet etching in, e.g., MEMS and optical applications since it provides a cleaner and more precisely controllable process. However, the degree of isotropy and, consequently, the final surface profile remain difficult to control. In this work, we apply a three-dimensional feature-scale topography simulation to low-bias SF6 etching experiments in Si to aid in process development and to investigate the physical etching mechanisms which govern the final surface geometry. We achieve this by accurately reproducing three distinct experimental data sets and by discussing the meaning of the phenomenological model parameters involved in the topography simulation in detail. We show that our phenomenological top-down flux calculation approach more accurately reproduces the experimental results than conventional strictly isotropic and bottom-up approaches. The reactor loading effect is taken into account as a general reduction of the model etch rates, which is supported by comparing simulated to experimentally determined etch depths in different loading regimes. Our model is also able to accurately reproduce reported trench geometries for different mask openings and etch times using a single parameter set for a given reactor configuration. Hence, we propose that the model parameters, in particular the average effective sticking coefficient, can be taken as a proxy of the reactor configuration. We provide an empirical relationship linking the average sticking coefficient of a reactor recipe to a measurable degree of isotropy of etched geometries. This empirical relationship can be used in practice to (i) estimate the average effective sticking coefficient of independent experiments and to (ii) fine-tune the etched geometry.

Fusion and quasifission studies for the Ca40+W186,Os192 reactions

Background: All elements above atomic number 113 have been synthesized using hot fusion reactions with calcium beams on statically deformed actinide target nuclei. Quasifission and fusion-fission are the two major mechanisms responsible for the very low production cross sections of superheavy elements.Purpose: To achieve a quantitative measurement of capture and quasifission characteristics as a function of beam energy in reactions forming heavy compound systems using calcium beams as projectiles.Methods: Fission fragment mass-angle distributions were measured for the two reactions $^{40}\mathrm{Ca}+^{186}\mathrm{W}$ and $^{40}\mathrm{C}+^{192}\mathrm{Os}$, populating $^{226}\mathrm{Pu}$ and $^{232}\mathrm{Cm}$ compound nuclei, respectively, using the Heavy Ion Accelerator Facility and CUBE spectrometer at the Australian National University. Mass ratio distributions, angular distributions, and total fission cross sections were obtained from the experimental data. Simulations to match the features of the experimental mass-angle distributions were performed using a classical phenomenological approach.Results: Both $^{40}\mathrm{Ca}+^{186}\mathrm{W}$ and $^{40}\mathrm{C}+^{192}\mathrm{Os}$ reactions show strong mass-angle correlations at all energies measured. A maximum fusion probability of $60\ensuremath{-}70%$ is estimated for the two reactions in the energy range of the present study. Coupled-channels calculations assuming standard Woods-Saxon potential parameters overpredict the capture cross sections. Large nuclear potential diffuseness parameters $\ensuremath{\sim}1.5$ fm are required to fit the total capture cross sections. The presence of a weak mass-asymmetric quasifission component attributed to the higher angular momentum events can be reproduced with a shorter average sticking time but longer mass-equilibration time constant.Conclusions: The deduced above-barrier capture cross sections suggest that the dissipative processes are already occurring outside the capture barrier. The mass-angle correlations indicate that a compact shape is not achieved for deformation aligned collisions with lower capture barriers. The average sticking time of fast quasifission events is ${10}^{\ensuremath{-}20}$ s.

Exploring quasifission characteristics forS34+Th232formingSg266

Background: Fission fragments from heavy ion collisions with actinide nuclei show mass-asymmetric and mass-symmetric components. The relative probabilities of these two components vary rapidly with beam energy with respect to the capture barrier, indicating a strong dependence on the alignment of the deformed nucleus with the partner in the collisions.Purpose: To study the characteristics of the mass-asymmetric quasifission component by reproducing the experimental mass-angle distributions to investigate mass evolution and sticking times.Methods: Fission fragment mass-angle distributions were measured for the $^{34}\text{S}+^{232}\text{Th}$ reaction. Simulations to match the measurements were made by using a classical phenomenological approach. Mass ratio distributions and angular distributions of the mass-asymmetric quasifission component were simultaneously fit to constrain the free parameters used in the simulation.Results: The mass-asymmetric quasifission component---predominantly originating from tip (axial) collisions with the prolate deformed $^{232}\text{Th}$---is found to be peaked near $A=200$ at all energies and center-of-mass angles. A Monte Carlo model using the standard mass equilibration time constant of $5.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}$ s predicts more symmetric mass splits. Three different hypotheses assuming (i) a mass halt at $A=200$, (ii) a slower mass equilibration time, or (iii) a Fermi-type mass drift function reproduced the main experimental features.Conclusions: In tip collisions for the $^{34}\text{S}+^{232}\text{Th}$ reaction, mass-asymmetric fission with $A\ensuremath{\sim}200$ is the dominant outcome. The average sticking time is found to be $\ensuremath{\sim}7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}$ s, independent of the scenario used for mass evolution.

The advertised price of cigarette packs in retail outlets across Australia before and after the implementation of plain packaging: a repeated measures observational study

ObjectiveThis study monitored the advertised price of the most prominently promoted and the cheapest single packs of cigarettes in Australian retail outlets before and after the implementation of plain packaging.MethodsA...

Movement Variability of Professional Pool Billiards Players on Selected Tasks

Parameter values characterizing the motion of the cue during impact in pool billiards have been determined for selected shots. 20 elite players performed 18 predefined tasks comprising follow, draw, stop shots and breaks. 3D-kinematics were obtained using a motion analysis system comprising 8 cameras operating at 250Hz and a high speed camera capturing with 5000Hz. Longitudinal accelerations of the cue stick were recorded with 5kHz using an accelerometer, mounted on the butt cap of the cue. Coefficients of variation for the parameter values obtained range from 3.9% (height of impact point of maximum follow shot) to 58.3% (elevation angle of 10-ball break). The average cue stick motion is basically non-accelerated (-0.060 +/- 0.508 ms-2) at ball impact for all of the tasks except for the breaks (3.918 +/- 0.164 ms-2). Despite the high number of DOF in the input configuration the tested pool billiard players achieved very similar outcomes.

Effects of Electron Impact on Ionization of Short-Lived Isotopes in a Hot Cavity

A Monte Carlo method based numerical model of hot cavity surface ion source that takes into account radioactive decay and the delay due to the sticking of particles to ionizer surface is considered. The in uence of electron impact ionization by thermionic electrons accelerated by the extraction eld is investigated. It is shown that the contribution from that process rises with decreasing half-life period and cannot be neglected, especially for substances of small surface ionization coe cient. The changes of relative yields from electron impact and surface processes with the length of the average sticking time are also studied.

Physics and chemistry of hot-wire chemical vapor deposition from silane: Measuring and modeling the silicon epitaxy deposition rate

We measure and successfully model the deposition rate (R) of epitaxial Si by hot-wire chemical vapor deposition (HWCVD) onto (100) silicon over a wide range of growth conditions. A deposition rate model based on the fundamentals of gas-filament and gas-substrate interactions is presented; the results are consistent with the observed dependences of R on gas pressure, flow, and filament area. Gas-phase measurements of silane depletion allow calculation of the average radical sticking coefficient from the film deposition rate. Our findings indicate that the epitaxial deposition rate can be increased sufficiently to enable an economical epitaxial film-silicon photovoltaic tech-nology on low-cost foreign substrates. The model can be simply adapted to apply to the HWCVD of amorphous, nanocrystalline, and polycrystalline Si.

Method to Determine the Sticking Coefficient of O2 on Deposited Al During Reactive Magnetron Sputtering, Using Mass Spectrometry

AbstractModels describing the reactive sputtering process need experimental input for some important parameters as the knock‐on yield, the reaction rate constant, the sticking coefficient on the target and the sticking coefficient on the deposited material. Here, a method is described to calculate the sticking coefficient of O2 on the deposited material, during reactive magnetron sputtering. Mass spectrometry is used to measure local, effective O2 partial pressure during sputtering, at the position of the sample. Combined with electron probe microanalysis of the oxygen content in deposited layers, we calculate the sticking coefficient. At our conditions, a local, effective pressure of (5.26 ± 0.44) × 10−6 mbar was found and an average sticking coefficient of 0.107 ± 0.032 for O2 on deposited aluminium during reactive magnetron sputtering.

Reactive Uptake of Ozone on Solid Potassium Iodide

Heterogeneous processes impact the rate of both trace gas uptake and chemical conversion within the atmosphere. Sea-salt particles are among the most abundant aerosols globally and provide reactive surface sites for heterogeneous reactions. Ozone, a powerful trace gas oxidant found in both the troposphere and stratosphere, is believed to oxidize alkali halides in the form of sea-salt aerosols. This chemistry is considered to be a significant candidate for the formation of reactive gas-phase molecular halogens. Recent studies suggest that although iodide is a minor component of sea-salt spray its concentration at the interface of aqueous solutions and at the interface of dried solid crystalline sea-salt particles is greatly enhanced relative to the bulk concentration. X-ray photoelectron spectroscopy studies described here show, under conditions in which a low defect density KI surface is completely devoid of adsorbed water molecules, that surface oxidation by ozone results in a thin layer of KIO3 that is self-passivating. An average reactive sticking probability is determined to be γavg = 1.4 (±0.7) × 10-4. Results of the low defect density system are compared with the results of a defect-rich polycrystalline KI surface which does not form a passivating film under the conditions of our experiments. In the presence of water, under atmospheric conditions, the surface oxidation of KI to KIO3 is likely to have important consequences on the release of gas-phase molecular iodine into the atmosphere via the Dushman reaction.

Insertion of the bacterial type III translocon: not your average needle stick

Bacterial type III secretion systems are thought to translocate virulence proteins directly from the bacterial cytoplasm into host cells through a continuous molecular channel. Little is known about how the apparatus itself interacts with membranes and whether insertion of this structure into the host membrane has consequences for the bacteria apart from its beneficial role in delivering virulence proteins. New evidence suggests that membrane insertion of the bacterial type III apparatus might turn on a calcium-dependent signaling pathway resulting in phagolysosomal fusion.

The role of the silyl radical in plasma deposition of microcrystalline silicon

Expanding thermal plasma chemical-vapor deposition has been used to deposit microcrystalline silicon films. We studied the behavior of the refractive index, crystalline fraction, and growth rate as a function of the silane (SiH4) flow close to the transition from amorphous to microcrystalline silicon. It was found that the refractive index, a measure for film density, increases when the average sticking probability of the depositing radicals decreases. Furthermore, we studied the influence of the position at which SiH4 is injected in the expanding plasma on the film density. It was found that the film density becomes higher when the SiH4 is injected closer to the substrate. Both findings strongly suggest that the film density benefits from a high contribution of the SiH3 radical to the growth of microcrystalline silicon.

Removal of bacteria in laboratory filters: models and experiments

Five filtration models were used to predict the effect of particle size on removal in several types of laboratory filters (nylon, cellulose and glass fiber). All models predicted similar trends in removal with particle size, although the capillary-pore model predicted substantially lower removals than the other four fibrous filtration models. These models indicated that bacterium-size (∼ 1 μm) particles are removed in filters primarily through the filtration mechanism of particle interception. Based on advertised pore sizes, removal of bacterium-size particles was a function of filter material in the order cellulose > nylon > glass fiber. The percent of bacteria removed in a nylon filter (5 μ pore diameter) was similar to removal in GF/C filters (1.2 μm pore diameter) as a result of a higher sticking coefficient of bacteria with nylon than with glass. The average sticking coefficients of three bacteria, Pseudomonas sp. JS6, Pseudomonas fluorescens P17 and Bacillus pumilus, in glass fiber (GF/C) filters, were 0.01, 0.24 and 0.20, respectively. These results indicate that filtration models can be used to describe particle removal during laboratory filtration of water and wastewater samples, and that bacteria have relatively high sticking coefficients for filter materials.

Growth kinetics of amorphous and microcrystalline silicon studied by using radical beam

Neutral fragments of SiH 4 are directly extracted from the plasma, and the growth of a-Si:H and μc-Si:H from such precursors has been demonstrated for the first time. It was found that the average sticking probability of SiH x radicals is near unity and the nucleation of microcrystallites on the growing surface proceeds more easily than the case of plasma deposition.

Estimating hazards in traffic situations

Hazards are attributes of road traffic. Safe driving requires perception of hazards and their combination into an overall evaluation of hazardousness of traffic situations. This process has been called hazard cognition. Results of previous studies could not show how drivers come to those cognitions. Because of the number of factors which contribute to these cognitions hazardousness can be seen as a multidimensional attribute of traffic situations. Drivers must recognize the dimensions of this attribute. To find out if drivers can do this, two experiments were conducted. In both experiments methods of multidimensional scaling were applied to pictorial representations of traffic situations. In Experiment I subjects comparing 16 situations with respect to hazardousness estimated more or less the magnitude of hazardousness. Results from indirect similarity ratings of 38 situations in Experiment II demonstrated a strong influence of driving experience. Less experienced drivers (75,000 km on average) stick more closely to details of situations and judge types of hazardousness, whereas more experienced drivers (150,000 km on average) judge hazardousness in a more holistic manner, obviously integrating different aspects of traffic situations. However, a common characteristic of all situations estimated as to be hazardous was a fairly high load of information on the driver. This certainly does not mean an equivalence of information load and hazardousness but results of these studies demonstrated the loading character of hazard control.

ELS study on chemisorbed state of CO on Cr(110)

The chemisorbed state of CO on a Cr(110) surface has been investigated at 300 K by electron energy loss spectroscopy (ELS) with the in-situ combined supplementary techniques. The ELS spectrum of the Cr(110) surface after CO adsorption is characterized by the peaks at 2, 4.4, 6–7, 9, 11, 14.5, 19 and 23 eV, and is found to be practically the same as that of the oxygen covered surface. The C-KLL Auger spectra obtained in the range 0.1–900 L CO agree with those of metal carbides. These results are considered to indicate that CO is dissociatively chemisorbed on the Cr(110) surface throughout the whole exposure region examined. The average sticking probability of CO on Cr(110) is 0.7 at below 0.5 L, and the maximum work function increase at 1 L is ~0.1 eV. The adsorbed state of O atoms produced from dissociative adsorption of CO is also discussed.

Clean TiC(001) surface and oxygen chemisorption studied by work function measurement, angle-resolved X-RAY photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and ion scattering spectroscopy

The clean surface of TiC(001) prepared by flash-heating at ~ 1500°C shows a 1 × 1 ordered atomic structure. No gradient of chemical composition exists over the shallow surface region of ~15 Å. The work function of the clean surface is 3.8 ± 0.1 eV at room temperature. The features of oxygen chemisorption are similar to those of noble metals such as Ag and Cu; the average sticking probability is about two order of magnitude less than those of Ti and W metals, and the work-function change is also small. The oxygen atoms are preferentially chemisorbed on the carbon sites at the topmost layer, which is responsible for the small change in work function.

The adsorption of CO on planar and oxygen-etched silicon surfaces

The adsorption of CO on polished (planar) and oxygen etched Si(111) surfaces has been studied by Scanning Electron Microscopy (SEM), Auger Electron Spectroscopy (AES), Electron Stimulated Desorption (ESD), and flash desorption mass spectrometry techniques. Average sticking probabilities, saturation coverages, Arrhenius parameters for desorption, and the effect of reaction etch pits on each of these have been determined. In contrast to what has been reported by other laboratories, we find low sticking probabilities of CO on planar Si surfaces and high saturation coverages. The presence of pits dramatically increases the sticking probabilities and the characteristic desorption temperatures. The low pre-exponential factor in the Arrhenius equation for CO desorbing from the planar and etched surfaces, as well as the anomalous lack of either AES or ESD signals with the adsorption of CO, are discussed in terms of CO penetration below the surface. Preliminary measurements on Si(100) reveal similar features in the desorption and Auger results, suggesting the interaction dynamics are qualitively the same.

Adsorption of HCl and HBr on Si(111): AES, ELS, and EID studies

The combined techniques in situ of Auger electron spectroscopy, electron energy loss spectroscopy, electron impact desorption, and work-function change measurement have been applied to the study of the adsorption of HCl and HBr on thermally cleaned Si(111) surfaces. Major results are summarized as follows: (1) HCl shows a fast adsorption to the saturation coverage of θ s ⋍ 0.3 (estimated using the continuum approximation) by the exposure of about 1 L at room temperature. (2) The average sticking probabilities for HCl and HBr are ∼0.7. (3) Two adsorbed states of HCl or HBr at room temperature are discriminated. For HCl, the first state is characterized by the emission of ∼1.2 eV ions and the electronic transition at 8.4 eV, which is subsequently converted to the second state characterized by the emission of ∼3.2 eV ions and the electronic transitions at 7.0 and 8.4 eV. Heating the sample at ∼800 K causes the desorption of hydrogen and the appearance of the Cl-related peaks at 6.0, 7.0, and 9.0 eV in the loss spectra. For HBr, the first and the second states are characterizied by the emission of ∼1.2 and ∼3.2 eV ions, respectively. The electronic transition is observed at 7.8 eV in both states. (4) It is proposed that HCl and HBr are adsorbed as molecules initially, which are subsequently dissociated into atoms spontaneously at room temperature.