Energy accommodation and reactivity of O2 on tungsten

We have determined by direct molecular beam velocity measurements that translational energy accommodation of O₂ molecules scattered from a hot polycrystalline tungsten target is inefficient at high surface temperatures. Translational energy accommodation is inefficient whether the surface is clean or covered with oxygen to a varying extent, even though in the latter case the scattering is diffuse. On a clean tungsten surface the scattering of the O₂ was approximately specular and the reaction probability of O₂ was constant and greater than 90% over the temperature range 1000K to 2800 K. It was shown by simultaneous helium scattering that atomic surface roughness of an oxygen chemilayer, rather than trapping, is a major cause of the observed diffuse scattering of oxygen. At the lowest surface temperature of 1000 K, with an oxygen chemilayer present, the velocity of the most probable number density of the scattered O₂ was lower than in the incoming beam or than that expected for complete equilibration with the surface.     

气体-表面相互作用的分子动力学模拟研究

采用分子动力学模拟方法研究了气体分子Ar在光滑和粗糙Pt表面上的散射规律.提出了一种速度抽样方法,计算了不同温度条件下气体分子对光滑和粗糙表面的切向动量适应系数和吸附概率.结果显示：光滑表面条件下,气体分子的切向动量系数和吸附概率都随着温度的升高而降低;粗糙度对气体分子切向动量与表面的适应具有极大的促进作用,当粗糙度足够大时,切向动量适应系数的大小趋近于1.0,对温度的敏感性也逐渐降低.采用粒子束方法对气体分子在光滑和粗糙表面上的散射规律进行了定量分析.总结了散射过程中气体分子的典型轨迹和动量变化规律,将气体分子在光滑表面的散射分为两种类型：单次碰撞后散射和多次碰撞后散射.单次碰撞后散射的气体分子平均切向动量有所减小,而经过多次碰撞后散射的气体分子则倾向于保持原有的平均切向动量.对于粗糙表面,粗糙度的存在使气体分子与表面间的动量和能量适应更加充分,导致气体分子在较粗糙表面上散射后的平均切向动量大幅减小并接近于0,且气体分子在表面上经历的碰撞次数越多,其散射后的能量损失越严重.     

Scattering of N2 from Ni(111)

A cold (T_rot<10 K) beam of N₂ with an initial translational energy of 0.40 eV strikes an Ni(111) surface at surface temperatures from 300 to 873 K at several incident angles from 15 to 60°. The rotational energy and angular distributions of the scattered molecules are probed using (2+1) resonance-enhanced multiphoton ionization. Molecules scattered in the specular direction have mean rotational energies that are independent of surface temperature, whereas those scattered at angles far from the specular show a dependence on surface temperature, caused likely by multiple collisions with the surface before escape. A rotational rainbow, seen in systems such as CO–Ni(111) and N₂–Ag(111), is not seen in this system. For molecules that scatter close to the specular direction, approximately 10% of the initial translational energy is converted into rotational energy of the scattered N₂. For surface temperatures above room temperature, the angular distributions indicate that molecules that scatter into low-J states also tend to exit in a broad peak (10–20° FWHM) near the specular, and this peak is broadened with increasing incident angle. The molecules that scatter into high-J states have a much broader distribution, indicating that they are trapped rotationally during the initial collision and suffer multiple collisions before leaving the surface.     

On the effect of the parallel velocity on Li− formation in Li+-surface scattering

Li⁻ formation is studied by scattering a Li⁺ beam from a cesium covered tungsten (110) surface. The primary energy ranges from 1000 to 3000 eV. The measurements show that variation of the velocity component of the scattered particles parallel to the surface, can either enhance or decrease the Li⁻ formation probability. The sign of this parallel velocity effect depends on the value of the normal velocity component. The experimental results are qualitatively explained by a semi-classical model.     

Energy transfer in hyperthermal Xe-graphite surface scattering

The scattering of a hyperthermal Xe from a graphite (0001) surface has been studied using a molecular beam-surface scattering technique and molecular dynamics (MD) simulations. The angular and velocity distributions of scattered Xe atoms were measured at incidence energies from 0.45 to 3.5 eV, three incidence angles of 15°, 35° and 60° and the surface temperatures of 300 K and 550 K. The observed time-of-flight spectra exhibit a sharp velocity distribution with only one velocity component, which is ascribed to the direct inelastic scattering process. The angle-resolved energy ratios of the mean final translational energy over the mean incidence energy E_f/E_i agree well with those predicted based on the assumption of the conservation of the momentum parallel to the surface. The Hard-Cube model, where the mass of the cube is approximately 310 u, has reproduced the angle-resolved flux distributions of scattered Xe atoms. In the Hard-Cube model almost 80% of the normal component of the incidence translational energy is found to be lost in collision. The MD simulations reproduce well the experimental results by using the Brenner potential for intralayer C atoms and a Lennard-Jones potential for interlayer C–C pair interactions.     

Charge states of 25–150 keV H and 4He backscattered from solid surfaces

Measurements have been made of the ion-fractions of H and ⁴He backscattered with energies of 25–160 keV from Cu, Au, and Si surfaces which were etched and washed but not atomically clean. The ion-fractions for H range from 0.37 at 25keV to 0.92 at 160 keV, and for ⁴He from 0.10 at 30keV to 0.58 at 150 keV, depending to a small extent on the target material. Where comparisons can be made the data agree rather closely with results of others for particles traversing thin foils. The data are useful for calibration of an electrostatic analyzer in surface analysis. Plots of ion-fraction against particle velocity show a primary dependence on velocity, as expected, but there is a small difference in slope between the H and He curves. Charge states of particles scattered from surface impurities did not deviate significantly from those of particles scattered from the substrate at the same energy.     

Scattering of NO from a graphite surface at cryogenic temperatures

The scattering of NO molecules from a graphite surface at cryogenic temperatures (T_s<80 K) allows to study the molecular translation-rotation energy transfer without surface phonon contributions in the entrance channel. The angular distributions show that there are no diffusely scattered molecules at surface temperatures below T_s=70 K, whereas the signal of forward scattered molecules remains present down to T_s=20 K, the lowest temperature investigated. The rotational behavior of the scattered molecules can be described by a Boltzmann distribution characterized by a rotational temperature T_rot. It is nearly constant below T_s=80 K and is determined by the molecular kinetic energy in surface normal direction. The data are consistent with the formation of a short-lived collision complex (NO··C_n) between the NO molecule and a few surface atoms. The complex decomposes in a unimolecular fashion. The cryogenic surface temperatures require effective shielding of the crystal from the heat radiation of the surrounding experimental equipment. The data show that the heat radiation influences the crystal temperature, however, it has only negligible influence on the molecule–surface interaction.     

Scattering of linear molecules from solid surfaces

Scattering of linear molecules, which are internally deactivated initially, from solid surfaces is investigated classical mechanically. The solid is modeled with a three-dimensional isotropic Debye solid whose surface is essentially flat but thermally roughened by lattice vibration, and the molecule is described as a rigid body, which interacts with the solid via an exponential potential wall and a stationary attractive potential well. A variation in the interaction energy due to thermal roughening of the surface and due to nonsphericity of the molecule being regarded as perturbations on the system, a second order perturbation theory is developed to construct a Gaussian velocity distribution function of molecules scattered off the exponential wall and still moving inside the potential well. Among the molecules in the distribution, those with sufficient energy to escape from the potential well are regarded as being scattered into the free space. This model shows that the major differences between the scattering of monatomic molecules and that of linear ones result from less normal momentum transfer to the linear molecules than to the monatomic ones.     

Energy spectra and charge fractions for keV hydrogen and helium backscattered from silicon

Energy spectra and charge fractions for hydrogen and helium backscattered from silicon targets are reported. The primary energy of the incident particle varies from 5 to 15 keV. The backscattered energy distributions are measured down to 500 eV and the results are compared to a Monte-Carlo computer simulation. Good agreement is found between the theoretical model and the experimental data. Charge fractions are measured by differentiating between scattered ions and neutrals. For hydrogen, neutralization occurs primarily at the surface for the backscattered particles and no depth effects are found. Helium shows a large peak in the ion yield for surface scattering with a much reduced ion yield for particles scattered from within the solid.     

Exact solutions for the flow of Casson fluid over a stretching surface with transpiration and heat transfer effects

The effects of transpiration on forced convection boundary layer non-Newtonian fluid flow and heat transfer toward a linearly stretching surface are reported.The flow is caused solely by the stretching of the sheet in its own plane with a velocity varying linearly with the distance from a fixed point.The constitutive relationship for the Casson fluid is used.The governing partial differential equations corresponding to the momentum and energy equations are converted into non-linear ordinary differential equations by using similarity transformations.Exact solutions of the resulting ordinary differential equations are obtained.The effect of increasing Casson parameter,i.e.,with decreasing yield stress(the fluid behaves as a Newtonian fluid as the Casson parameter becomes large),is to suppress the velocity field.However,the temperature is enhanced as the Casson parameter increases.It is observed that the effect of transpiration is to decrease the fluid velocity as well as the temperature.The skin-friction coefficient is found to increase as the transpiration parameter increases.     

Quasi-triple collisions in low energy ion scattering; A possibility to determine surface debye temperatures

Calculations and measurements of energy spectra of 10 keV Kr⁺ ions, scattered from Cu(100) face at different temperatures, are reported. One of the observed phenomena is the existence of a new peak. From the temperature behaviour of this peak we obtain the surface Debye temperature.     

Correlated electron effects in low energy Sr(+) ion scattering

Resonant charge transfer during low energy ion scattering reveals correlated-electron behavior at high temperature. The valence electron of a singly charged alkaline-earth ion is a magnetic impurity that interacts with the continuum of many-body excitations in the metal, leading to Kondo and mixed valence resonances near the Fermi energy. The occupation of these resonances is acutely sensitive to the surface temperature, which results in a marked temperature dependence of the ion neutralization. We report such a dependence for low energy Sr(+) scattered from polycrystalline gold.     

Direct-inelastic and trapping-desorption scattering of N2 and CH4 from Pt(111)

Angle and velocity distributions for supersonic chopped beams of N₂ and CH₄ scattered from clean close-packed Pt(111) surfaces are reported. For specular direct-inelastic scattering N₂ and CH₄ velocity distributions can be characterized by empirical relationships used for Ar scattering. For instance, for specular scattering the following relation is found for Ar, N₂ and CH₄: 〈KE_f〉 = A(KE_i) + B(2kT_s), where 〈KE_f〉 is the average final kinetic energy, KE_i is the incident kinetic energy and T_s is the surface temperature. The beam and surface temperature independent coefficients A and B are, respectively: Ar 0.87, 0.17; N₂ 0.79, 0.19 and CH₄ 0.84, 0.25. Unlike Ar, N₂ desorbs from Pt with a Maxwell-Boltzmann velocity distribution near the surface temperature. Qualitatively the trapping probabilities for these molecules on Pt(111) are ordered: Xe > N₂ > CH₄> Ar.     

Scattering and recoiling mapping of the Kr–Pt(1 1 1) system by SARIS

The technique of angle resolved mapping of scattering and recoiling imaging spectra (SARIS) combined with computer simulations is demonstrated to be a valuable tool for characterization of atomic collision events on surfaces. The energy distributions of scattered Kr and fast recoiled Pt atoms from a Pt(1 1 1) surface were measured as a function of exit angle. The use of a large area microchannel plate detector and time-of-flight techniques decreases the collection time and increases the number of detected trajectories above that of other designs. Classical ion trajectory simulations using the three-dimensional scattering and recoiling imaging code are used to simulate the kinematics of the scattering and recoiling particles. It is shown that SARIS mapping allows one to probe the kinematics of both scattered and recoiled particles, the probability for their occurrence in specific trajectories, their detection probabilities, and their threshold detection velocity. The measured and simulated energy distributions agree quantitatively if the detection efficiency is taken into account. The observed value of the threshold detection velocity for Pt atoms, ν^th=3.78(5)×10⁴ m/s, is in good agreement with previous studies.     

Energy spectra of 6–32 keV neutral and ionized Ar and He scattered from Au targets; ionized fractions as functions of energy

The neutralization of ions is an important aspect of low energy ion scattering for surface analysis. Electrostatic energy analyzers (ESA) have been used almost exclusively in such work, and information on charge neutralization efficiencies is needed for quantitative interpretation of ESA data. In the past, the occurrence in low energy ion spectra of surface peaks and low backgrounds due to scattering from inside the solid has been attributed to preferential neutralization of ions which penetrate beyond the surface. In the work to be described, a time-of-flight technique was used to measure energy spectra of both neutral and ionized Ar and He scattered at 90° from a polycrystalline gold target. Incident energies of 6–32 keV were used. The energy spectra of neutral Ar scattered from polycrystalline gold exhibit sharp surface peaks, and double scattering shoulders, over this entire energy range. For He there is a gradual downward slope toward lower energy rather than a sharp surface peak. The behavior in both cases is attributed to large scattering cross-sections which cause a loss of beam particles during penetration. A calculation using a $\text{1}\text{r}{}^2$ potential illustrates this effect as a function of energy for helium. In the present experiments we find that the ion fraction of scattered argon does indeed depend on depth of penetration. This is in contrast to the behavior of He and H at higher energies, e.g. 100 keV, in which cases the charge state depends on emergent velocity but not on depth of penetration. The characteristic shapes of ion scattering spectra in this energy range appear to result from both neutralization and beam attenuation inside the target.     

IR reflection-absorption spectroscopy of CO adsorbed on palladium

Kinetic theory is used to compute the flux and relative translational kinetic energy incident upon a surface oscillating in a rarefied gas. The flux incident upon the oscillatory surface is deficient in low-velocity molecules from the gas during the reentry half of a vibration cycle, in which the surface moves into the gas, because all molecules which are to strike the oscillatory surface from the gas must cross the plane of maximum surface extension during that cycle. The deficiency is largely compensated for by recapture, during the reentry half of a cycle, of low-velocity molecules emitted during the recession half of the cycle. The result of these two opposing effects is that the average energy of gas-surface collisions, and therefore the temperature rise of an oscillatory surface is greater than that of a constant speed plate of the same rms velocity in the same gas. For argon at 300 K and 10^?3 torr incident upon a surface with an rms velocity of 3.3 × 10³ cm sec^?1 the apparent average temperature of incident molecules is 302.91 ° for an oscillatory surface and 302.63 ° for a constant speed plate. Measurements of the temperature rise of an oscillatory surface offer a way to measure thermal accomodation coefficients.     

Velocity scaling of ion neutralization in low energy ion scattering

The ion fraction P+ is measured for He+ ions scattered by 129 degrees from a Cu surface. Both the primary energy and the angles of incidence and of exit are varied. From our results we conclude the following: along the incoming and outgoing trajectories, neutralization is due to Auger processes and depends on the normal velocity component v( perpendicular ) only. At higher energies, additional charge exchange is due to collision induced neutralization and reionization, both depending on the total ion energy only. Also in this regime P+ depends on v( perpendicular ), but via a two-valued function of the scattering geometry at fixed energy.     

Relaxation of the edge atoms of a stepped Cu(410) surface

The edge atom position of the stepped Cu(410) surface has been investigated with low energy ion scattering (LEIS) using 10–20 keV H⁺. It is possible to determine the atomic position by utilizing atomic shadowing effects. The scattered particles are analysed with a time-of-flight spectrometer. This means that we can detect scattered hydrogen leaving the copper surface in the neutral state as well as in the charged state, so the influence of neutralization of the scattered particles along different outgoing trajectories is eliminated. We find an inward relaxation of the edge atoms; assuming that this relaxation takes place in the (100) plane it amounts to about 8% of the interlayer spacing.     

Direct determination of gas velocity and gas temperature in an atmospheric-pressure argon-hydrogen plasma jet

Direct gas temperature and gas velocity measurements made in the exit plane of a subsonic argon-hydrogen thermal plasma jet from high-resolution lineshape analysis of laser light scattered by the plasma are reported. The lineshapes are in general a superposition of the ion feature of the Thomson-scattered light and the lineshape of Rayleigh scattered light. In the center of the jet Thomson scattering dominates while at larger radii Rayleigh scattering dominates. Because of the complexity of the lineshapes of light scattered by multicomponent plasmas, only those that are predominantly due to Thomson scattering can in practice be analyzed for gas temperature. Gas velocity can be determined from the Doppler shift of the lineshapes relative to the frequency of the incident laser if the velocity is greater than about 50 m s⁻¹. The maximum gas temperature measured was 14,500 K±5%. The maximum gas velocity measured was 1100 m s⁻¹±3%. Temperature values and the radial velocity profile are compared with those previously obtained from a subsonic all-argon plasma jet operated under similar conditions.