Scattering of hyperthermal argon atoms from clean and D-covered Ru(0001) surfaces

Hyperthermal Ar atoms were scattered from a Ru(0001) surface held at temperatures of 180, 400 and 600 K, and from a Ru(0001)-(1×1)D surface held at 114 and 180 K. The resultant angular intensity and energy distributions are complex. The in-plane angular distributions have narrow (FWHM ≤ 10°) near-specular peaks and additional off-specular features. The energy distributions show an oscillatory behavior as a function of outgoing angle. In comparison, scattered Ar atoms from a Ag(111) surface exhibit a broad angular intensity distribution and an energy distribution that qualitatively tracks the binary collision model. The features observed for Ru, which are most evident when scattering from the clean surface at 180 K and from the Ru(0001)-(1×1)D surface, are consistent with rainbow scattering. The measured TOF profiles cannot be adequately described with a single shifted Maxwell-Boltzmann distribution. They can be fitted by two components that exhibit complex variations as a function of outgoing angle. This suggests at least two significantly different site and∕or trajectory dependent energy loss processes at the surface. The results are interpreted in terms of the stiffness of the surface and highlight the anomalous nature of the apparently simple hcp(0001) ruthenium surface.     

The interaction of hyperthermal argon atoms with CO-covered Ru(0001): scattering and collision-induced desorption

Hyperthermal Ar atoms were scattered under grazing incidence (θ(i) = 60°) from a CO-saturated Ru(0001) surface held at 180 K. Collision-induced desorption involving the ejection of fast CO (～1 eV) occurs. The angularly resolved in-plane CO desorption distribution has a peak along the surface normal. However, the angular distribution varies with the fractional coverage of the surface. As the total CO coverage decreases, the instantaneous desorption maximum shifts to larger outgoing angles. The results are consistent with a CO desorption process that involves lateral interaction with neighboring molecules. Furthermore, the data indicate that the incident Ar cannot readily penetrate the saturated CO overlayer. Time-of-flight measurements of scattered Ar exhibit two components-fast and slow. The slow component is most evident when scattering from the fully covered surface. The ratio and origin of these components vary with the CO coverage.     

Rainbow scattering of CO and N2 from LiF(001)

The angular intensity distributions of CO and N(2) molecules scattered from a LiF(001) surface have been measured as functions of surface temperature, incident translational energy, and incident azimuthal direction affecting surface corrugation at a high resolution. Although both molecules have the same molecular mass and linear structure, only the CO molecule shows a rainbow feature in its scattering pattern, while the N(2) molecule shows a single peak distribution. From the comparisons of the obtained results with the calculated predictions based on the newly developed classical theory of the ellipsoid-washboard model, the differences in scattering distribution are attributed to the effects of molecular anisotropy and center-of-mass position. With an increase in the extent of the molecular anisotropy such as that of N(2) and CO as compared with rare-gas atoms, the summation of several scattering distributions depending on molecular orientation results in smearing the rainbow scattering on the corrugated surface. This smearing effect, however, attenuates when center-of-mass position deviates from the molecular center, as that for CO.     

C2H6 scattering from LiF(0 0 1): Influence of the molecular anisotropy on rainbow scattering in both intensity and speed distributions

We have measured the angle-resolved intensity and speed distributions of C₂H₆ scattered from LiF(0 0 1) along the [1 0 0] azimuthal direction, the largest structural corrugation direction, to investigate the effect of the molecular anisotropy on the gas–surface interaction at the corrugated surface. While clear rainbow feature is observed in the mean energy angular distribution, no rainbow feature is detected in the intensity angular distribution. From the comparisons of the obtained results to the calculated predictions based on the simple classical theory of the ellipsoid-washboard model, the effect of the molecular anisotropy is found to play a crucial role in the rainbow feature. With an increase in the extent of the molecular anisotropy such as that of C₂H₆ as compared with rare gas atoms, the integration of the intensity angular distributions for various molecular orientations results in the smearing of the rainbow feature on the corrugated surface. The rainbow scattering in the mean energy angular distribution, however, is not completely smeared out.     

Imaging the dynamics of reactions between Cl atoms and the cyclic ethers oxirane and oxetane

Direct current slice velocity map ion images of the HCl(nu' = 0, J') products from the photoinitiated reactions of ground state Cl atoms with ethane, oxirane (c-C2H4O), and oxetane (c-C3H6O), at respective mean collision energies of 5.5, 6.5, and 7.3 kcal mol-1(-1), were analyzed using a Legendre moment fitting procedure. The experimental method and the fitting technique were tested by comparing the derived center-of-mass (CM) frame angular scattering distribution for the HCl(v' = 0, J' = 1) products from the reaction of Cl + C2H6 with those determined by Suits and co-workers from a crossed molecular beam experiment. For the Cl + c-C2H4O reaction, a broad, forward, and backward peaking CM frame angular distribution of HCl(nu' = 0, J' = 2) products was determined, with an average fraction of the available energy released as product translational energy of f t, equal to 0.52 +/- 0.18. The HCl consumes only 1% of the available energy, and conservation arguments dictate that the radical coproduct is significantly internally excited, corresponding to an average fraction of the available energy of f int(c-C2H3O), equal to 0.47 +/- 0.18. For the reaction of oxetane with Cl atoms, abstraction of H atoms is possible from carbon atoms from positions either alpha or beta to the O atom. The contributions to the reaction from these two H-atom abstraction channels were estimated to be 63 and 37%, consistent with an unbiased propensity for removal of alpha- and beta-H atoms that are present in 2:1 abundance. The angular scatter of products in the CM frame is also broad and forward-backward peaking and is reminiscent of the products of the Cl + CH3OH and CH3OCH3 reactions. The derived mean fraction of the available energy channelled into product translation is f t = 0.54 +/- 0.12 for each of the two abstraction pathways. With only a small amount of energy in the rotation of the HCl(nu' = 0), the remainder is accounted for by excitation of the radical coproduct internal modes, with f int(c-C3H5O) = 0.42 +/- 0.12 for both alpha- and beta-H abstraction. The broad product scattering in the CM frame observed for both reactions of Cl atoms with the cyclic ethers is consistent with reactive collisions over a wide range of impact parameters, as might be expected for barrierless reactions with loose transition states.     

Small-angle x-ray scattering by crystalline polymer fibers 2. Investigation of the linear paracrystalline model using various materials

The linear position-sensitive detector is well-suited to measure quantitatively the distribution parallel to the fibre axis of the intensity of small-angle x-ray scattering (SAXS) by polymer fibres, except that in the case of four-point patterns their width is greater than that of the detector window. A method is described which overcomes this problem, and which has high angular resolution. Using this method, the variation of scattered intensity with angles from 0.3° to 2.5° has been measured for fibres of poly(ethylene terephthalate), nylon, and low density poly(ethylene) (LDPE), and compared with that predicted by the linear paracrystalline model. In all cases except LDPE, when the distribution of phase lengths was given by the Reinhold function, there was no significant disagreement between the measured and predicted scattering except for a very small range of angles on the low angle side of the peak intensity. With LDPE small but significant discrepancies were found at other angles as well, and these were worse if the symmetrical Gaussian distribution function was used. The method enabled quantitative parameters describing the morphology to be obtained. It is concluded that the morphology of the linear paracrystalline stack is consistent with the SAXS intensity distribution, and that the Reinhold function is a reasonable approximation to the distribution of phase lengths. A small modification so that this decays more rapidly at long lengths might be necessary to explain the scattering for all materials over the entire angular range and other small changes might be needed with LDPE, although the asymmetrical nature of the distribution must be retained.On leave from Department of Physics, University of Technology Malaysia, 81300 Sekudai, Malaysia.     

Reactive ion surface scattering as an Eley-Rideal process: a molecular dynamics study into the abstraction reaction mechanism by low energy Cs+ from Pt(111).

We have employed a classical molecular dynamics simulation for the direct pick-up reaction of adsorbates by very low energy (1-60 eV) ions scattered at a surface. The system investigated is the reactive ion scattering (RIS) of Cs+ with an adsorbate on a Pt(111) surface. The ion-dipole attraction between the projectile and the physisorbed adsorbate drives the abstraction reaction, in which the ion projectile at first collides with the surface to release a substantial amount of its kinetic energy, and subsequently pulls the adsorbate along in the outgoing trajectory. Desorption induced by the ion-dipole attraction is a precursor to the formation of the Cs(+)-adsorbate product. This Eley-Rideal-type mechanism must accommodate the inertia of the adsorbate. Consequently, a successful abstraction works well only for low mass adsorbates and slow outgoing Cs+ ions. The efficient energy transfer to the Pt(111) surface makes Cs+ a better candidate for RIS than lighter projectile ions. Optimal conditions for the efficient RIS abstraction mechanism are found for physisorbed adsorbates with a mass below 32 amu, and for 10 eV Cs+ ions at a 45 degrees incidence.     

Dynamics of hyperthermal collisions of O(3P) with CO

The dynamics of O(3P) + CO collisions at a hyperthermal collision energy near 80 kcal mol-1 have been studied with a crossed molecular beams experiment and with quasi-classical trajectory calculations on computed potential energy surfaces. In the experiment, a rotatable mass spectrometer detector was used to monitor inelastically and reactively scattered products as a function of velocity and scattering angle. From these data, center-of-mass (c.m.) translational energy and angular distributions were derived for the inelastic and reactive channels. Isotopically labeled C18O was used to distinguish the reactive channel (16O + C18O 16OC + 18O) from the inelastic channel (16O + C18O 16O + C18O). The reactive 16OC molecules scattered predominantly in the forward direction, i.e., in the same direction as the velocity vector of the reagent O atoms in the c.m. frame. The c.m. translational energy distribution of the reactively scattered 16OC and 18O was very broad, indicating that 16OC is formed with a wide range of internal energies, with an average internal excitation of approximately 40% of the available energy. The c.m. translational energy distribution of the inelastically scattered C18O and 16O products indicated that an average of 15% of the collision energy went into internal excitation of C18O, although a small fraction of the collisions transferred nearly all the collision energy into internal excitation of C18O. The theoretical calculations, which extend previously published results on this system, predict c.m. translational energy and angular distributions that are in near quantitative agreement with the experimentally derived distributions. The theoretical calculations, thus validated by the experimental results, have been used to derive internal state distributions of scattered CO products and to probe in detail the interactions that lead to the observed dynamical behavior.     

A molecular-beam study of the collision dynamics of methane and ethane upon a graphitic monolayer on Pt(111)

Utilizing a supersonic molecular-beam scattering technique, the angular intensity distributions of alkane molecules (CH4 and C2H6) have been measured, which are scattered from a chemically inert and highly oriented monolayer graphite (MG) on Pt(111). A MG which covers the Pt(111) surface with a full monolayer is found to induce a large energy loss of alkanes during collision with the surface by phonon creation due to the large mass ratio of an alkane molecule with respect to MG. Based on the classical cube model, only applicable to the molecules without internal mode excitation, the effective masses of MG of 76 (six atoms of carbon) and Pt(111) of 585 (three atoms of platinum) are determined from rare-gas atom scattering data. Despite the difference in the degree of freedom between CH4 and rare-gas atoms, CH4 scattering is found to be well described by the simple hard-cube model as a result of the high symmetry of the CH4 structure. With the recently developed ellipsoid-washboard model, an extension of the hard-cube model to include some internal mode excitation of impinging molecules in addition to the surface corrugation, it is found that unlike CH4 the cartwheel rotation mode of C2H6 is significantly excited during collision, while the helicopter mode excitation is negligible on a flat MG surface.     

Angular intensity distribution of a molecular oxygen beam scattered from a graphite surface

The scattering of the oxygen molecule from a graphite surface has been studied using a molecular beam scattering technique. The angular intensity distributions of scattered oxygen molecules were measured at incident energies from 291 to 614 meV with surface temperatures from 150 to 500 K. Every observed distribution has a single peak at a larger final angle than the specular angle of 45° which indicates that the normal component of the translation energy of the oxygen molecule is lost by the collision with the graphite surface. The amount of the energy loss by the collision has been roughly estimated as about 30-41% based on the assumption of the tangential momentum conservation during the collision. The distributions have also been analyzed with two theoretical models, the hard cubes model and the smooth surface model. These results indicate that the scattering is dominated by a single collision event of the particle with a flat surface having a large effective mass. The derived effective mass of the graphite surface for the incoming oxygen is 9-12 times heavier than that of a single carbon atom, suggesting a large cooperative motion of the carbon atoms in the topmost graphene layer.     

Angular distribution of photoelectrons in small molecules: a molecular quantum defect calculation

The molecular quantum defect orbital (MQDO) method, previously used in the determination of molecular photoionization cross sections, is applied here to calculate the angular distribution of photoelectrons arising from the molecular photoionization. Calculations are performed for the ionization from outer valence orbitals of HF, H(2)O, NH(3), N(2)O, and H(2)CO molecules. The results are compared with previous measurements and with theoretical curves found in the literature. Profiles of the angular distribution parameter as a function of photoelectron energy covering a range from the photoionization threshold to 120 eV are presented for the above molecules. The energy dependence of the angular distributions predicted by the MQDO calculations agrees fairly well with predictions from more sophisticated theories and with observed results.     

Two-dimensional angle-resolved photoelectron spectroscopy using display analyzer—Atomic orbital analysis and characterization of valence band

The information obtained by two-dimensional angle-resolved photoelectron spectroscopy in UPS (ultraviolet photoelectron spectroscopy) region is described. A display-type spherical mirror analyzer can measure wide-angle angular distribution of photoelectrons of one particular kinetic energy (binding energy) without changing the angles of incident light and the sample. The shape of the cross section of valence band, especially of the Fermi surface can be observed directly on the screen. Three-dimensional energy band and Fermi surface are obtained by scanning the binding energy of two-dimensional band mapping. In the case of linearly polarized light excitation the symmetry relation in the photoelectron excitation process can also be displayed as “angular distribution from atomic orbital ADAO”, which is used to distinguish the atomic orbitals constituting the energy band. An example is shown for the atomic orbital analysis of Cu Fermi surface at each k point. It was successfully revealed that the Cu 4p orbitals are aligned with their axes pointing outwards. Another important angular distribution is the “photoemission structure factor PSF”, which originates from the interference among photoelectron waves from individual atoms. PSF determines the intensity inequivalency between Brillouin zones and reveals the bonding character of the energy band.     

High translational energy release in H2 (D2) associative desorption from H (D) chemisorbed on C(0001)

Highly energetic translational energy distributions are reported for hydrogen and deuterium molecules desorbing associatively from the atomic chemisorption states on highly oriented pyrolytic graphite (HOPG). Laser assisted associative desorption is used to measure the time of flight of molecules desorbing from a hydrogen (deuterium) saturated HOPG surface produced by atomic exposure from a thermal atom source at around 2100 K. The translational energy distributions normal to the surface are very broad, from approximately 0.5 to approximately 3 eV, with a peak at approximately 1.3 eV. The highest translational energy measured is close to the theoretically predicted barrier height. The angular distribution of the desorbing molecules is sharply peaked along the surface normal and is consistent with thermal broadening contributing to energy release parallel to the surface. All results are in qualitative agreement with recent density functional theory calculations suggesting a lowest energy para-type dimer recombination path.     

Dissociative scattering of hyperthermal energy CF3+ ions from modified surfaces

Dissociative scattering of CF3+ ions in collision with a self-assembled monolayer surface of fluorinated alkyl thiol on a gold 111 crystal has been studied at low ion kinetic energies (from 29 to 159 eV) using a custom built tandem mass spectrometer with a rotatable second stage energy analyzer and mass spectrometer detectors. Energy and intensity distributions of the scattered fragment ions were measured as a function of the fragment ion mass and scattering angle. Inelastically scattered CF3+ ions were not observed even at the lowest energy studied here. All fragment ions, CF2+, CF+, F+, and C+, were observed at all energies studied with the relative intensity of the highest energy pathway, C+, increasing and that of the lowest energy pathway, CF2+, decreasing with collision energy. Also, the dissociation efficiency of CF3+ decreased significantly as the collision energy was increased to 159 eV. Energy distributions of all fragment ions from the alkyl thiol surface showed two distinct components, one corresponding to the loss of nearly all of the kinetic energy and scattered over a broad angular range while the other corresponding to smaller kinetic energy losses and scattered closer to the surface parallel. The latter process is due to delayed dissociation of collisionally excited ions after they have passed the collision region as excited parent ions. A similar study performed at 74 eV using a LiF coated surface on a titanium substrate resulted only in one process for all fragment ions; corresponding to the delayed dissociation process. The intensity maxima for these fragmentation processes were shifted farther away from the surface parallel compared to the thiol surface. A new mechanism is proposed for the delayed dissociation process as proceeding via projectile ions' neutralization to long-lived highly excited Rydberg state(s), reionization by the potential field between the collision region and entrance to the energy analyzer, and subsequent dissociation several microseconds after collisional excitation. A kinematic analysis of experimental data plotted as velocity Newton diagrams demonstrates that the delayed dissociation process results from the collisions of the ion with the bulk surface; i.e., the self-assembled monolayer surface acts as a bulk surface. A similar analysis for the highly inelastic collision processes shows that these are due to stronger collisions with a fraction of the thiol molecular chain, varying in length (mass) with the ion energy.     

Studies of adsorbed molecules by X-ray photoelectron diffraction (XPD)

The angular intensity distribution of photoelectrons (or Augerelectrons) with high kinetic energy (> 400 eV), emitted from atoms of adsorbed molecules on single crystal surfaces, may exhibit pronounced peaks that are due to the high cross section for forward scattering by nearest neighbor atoms. This well-known effect can be utilized to study the orientation of bond axes of such molecules relative to the surface normal if they have the same orientation. The molecules should preferably be located in identical bonding sites but long range order not required. Atom-specific photoelectrons are excited by photons from a laboratory X-ray source or a synchrotron and analyzed for their spatial distribution by rotating the sample. For an ordered adsorbed layer of high coverage it is possible to observe also inter-molecular forward scattering peaks that are related to the registry of the layer. Molecules such as CO, N₂, C₂H₄, CH₃O and C₂H₄O have been studied in this fashion. Several of these results will be reviewed and discussed.     

Elastic electron scattering from state-selected molecules

Electronically elastic, electron scattering cross sections are calculated for molecules in particular rotational states in order to establish the sensitivity of the scattering pattern to the quantum state. For the examples of diatomic molecules and symmetric tops considered here, the scattering pattern provides a unique fingerprint of the quantum state if one measures the scattered intensity for different orientations of the scattering vectors. The structure in the scattered intensity reflects the anisotropy of the square of the rotational wavefunction. Even for low angular momentum states which have diffuse rotational wavefunctions, very large differences in intensity are the result at certain scattering angles for states whose quantum numbers differ even by only one unit.     

Reactive scattering of H and D atoms. II. isotope effect on the angular distribution of H,D + NO2

The chemical reactions of hydrogen and deuterium atoms with nitrogendioxide (NO₂) have been studied with crossed molecular beams. Angular and velocity distributions have been measured. The angular distribution peaks at small angles and shows a pronounced isotope effect; only 2̃4% of the available energy goes into translation. Al-though four very deep potential wells (2.1 eV) exist in the potential hypersurface no statistical complex is formed. The reaction can probably be described as intermediate between direct and complex scattering.     

Dynamics of scattering and dissociative adsorption on a surface alloy: H2/W(100)-c(2 × 2)Cu

H(2) scattering and dissociative adsorption on the W(100)-c(2 × 2)Cu surface alloy is studied based on DFT calculations. A strongly site dependent reactivity is observed in line with results obtained for the density of states projected onto the W and Cu atoms of the topmost layer. H(2) dissociation on a defect free terrace of W(100)-c(2 × 2)Cu is found to be a non-activated process like on W(100), despite the reduction of the number of energetically accessible dissociation pathways at low impact energies due to the presence of Cu atoms. A prominence of dynamic trapping and a reduction of the efficacy of trapping to promote dissociation is also verified, leading to a decrease of the initial sticking probability as a function of the molecular impact energy, in qualitative agreement with experimental findings. The heterogeneous reactivity is also evidenced by two different kinds of reflection events at low energies. Its combination gives rise to a broad specular peak superimposed on a cosine-like angular distribution of scattered molecules which is in good agreement with available experimental data.     

X-ray Scattering in Liquid Pentane

Abstract

X-ray diffraction investigation of liquid n-pentane CH₃—(CH₂)₃—CH₃ was performed at 293 K. An angular distribution of X-ray scattered intensity was measured by applying MoK _α (γ = 0.71069 Å) radiation. The electron density radial distribution function was numerically found using Fourier analysis. The mean distances between the neighbouring atoms were found. A simple model of short-range arrangement of the molecules was proposed.