Recoiling

At the other extreme are materials which flow like liquids but show a tendency to recoil when the stress is removed. This has been called flow-elasticity or elastico-viscosity . Such liquids (e.g. molten Nylon) can be spun to form threads. 

Hand M R and Harris J 1990 Recoil effects in surface dissociation J. Chem. Phys. 92 7610... 

TOF-SARS (time-of-flight scattering and recoiling spectrometry) no 13 1.2... 

There are two basic physical phenomena which govern atomic collisions in the keV range. First, repulsive interatomic interactions, described by the laws of classical mechanics, control the scattering and recoiling trajectories. Second, electronic transition probabilities, described by the laws of quantum mechanics, control the ion-surface charge exchange process. 

The energy of scattered or recoiled ions can be measured directly by means of an electrostatic energy analyser. If the TOF method is used, the relation between scattering energy E and TOF is expressed as... 

Similar to QSS, direct recoil (DR) of surface atoms produces energetic atoms that have a relatively narrow velocity distribution. DR particles are those species which are recoiled from the surface layers as a result of a direct collision of the primary ion. They escape from the surface with little energy loss through collisions with... 

Figure Bl.23.2. (a) Shadow cone of a stationary Pt atom in a 4 keV Ne ion beam, appearing with the overlapping of ion trajectories as a fiinction of the impact parameter. The initial position of the target atom that recoils in the collision is indicated by a solid circle, (b) Plot of the nonnalized ion flux distribution density across the shadow cone in (a). The flux density changes from 0 inside the shadow cone, to much greater than l in the focusing region, converging to 1 away from the shadow cone edge, (c) Blocking cones...

Figure Bl.23.3. Schematic illustrations of backscattering with shadowing and direct recoiling with shadowing and blocking.

B1.23.2.3 SCATTERING AND RECOILING ANISOTROPY CAUSED BY SHADOWING AND BLOCKING CONES... 

A) TIME OF FLIGHT SCATTERING AND RECOILING SPECTROMETRY (TOF-SARS)—SHADOW CONE BASED EXPERIMENT... 

In TOF-SARS [9], a low-keV, monoenergetic, mass-selected, pulsed noble gas ion beam is focused onto a sample surface. The velocity distributions of scattered and recoiled particles are measured by standard TOF methods. A chaimel electron multiplier is used to detect fast (>800 eV) neutrals and ions. This type of detector has a small acceptance solid angle. A fixed angle is used between the pulsed ion beam and detector directions with respect to the sample as shown in figure Bl.23.4. The sample has to be rotated to measure ion scattering... 

Figure Bl.23.4. Schematic diagram of TOE scattermg and recoiling spectrometry (TOF-SARS) illustrating the plane of scattering fonned by the ion beam, sample and detector. TOE spectra (a) are collected with fixed...

A = /W//Wp, P is impact parameter and Tq is the distance of closest approach (apsis) of the collision pair. The transformations from the CM coordinates (scattering angle y) to the laboratory coordinates with the scattering angle 0 for the primary particle and (]) for the recoiled surface atoms Is given by... 

For accurate ion trajectory calculation in the solid, it is necessary to evaluate the exact positions of the intersections of the asymptotes (A A2) of the incoming trajectory and that of the outgoing trajectories of both the scattered and recoiled particles in a collision. The evaluation of these values requires time integrals and the following transfonnation equations ... 

TOF-SARS and SARIS are capable of detecting all elements by either scattering, recoiling or both teclmiques. TOF peak identification is straightforward by converting equation (Bl.23.lt and equation (B 1.23.81 to the flight times of the scattered and recoiled particles as... 

Examples of typical TOF spectra obtained from 4 keV Ar impinging on a Si 100] surface with chemisorbed FI2O and FI2 are shown in figure B 1.23.6 [35]. Peaks due to Ar scattering from Si and recoiled FI, O and Si are observed. The intensities necessary for stnictural analysis are obtamed by integrating the areas of fixed time windows under these peaks. 

Figure Bl.23.10. Schematic diagram of a scattering and recoiling imaging spectrometer (SARIS). A large-area (95 X 75 nnn ), time-resolving, position-sensitive microchannel plate (MCP) detector captures a large...

Figure Bl.23.12. Above selected time-resolved SARIS images of 4 keV Ar recoiling Pt atoms from Pt 111] along (I 12). Below view of Pt 111 ] surface along (112) showing a focused second-layer Pt recoil trajectory (atoms 1-4 fonn a focusing atomic lens ).

B1.23.9 ROLE OF SCATTERING AND RECOILING AMONG SURFACE SCIENCE TECHNIQUES... 

Scattering and recoiling contribute to our knowledge of surface science tln-ough (i) elemental analysis, (ii) structural analysis and (iii) analysis of electron exchange probabilities. We will consider the merits of each of these tluee areas. 

The major role of TOF-SARS and SARIS is as surface structure analysis teclmiques which are capable of probing the positions of all elements with an accuracy of <0.1 A. They are sensitive to short-range order, i.e. individual interatomic spacings that are <10 A. They provide a direct measure of the interatomic distances in the first and subsurface layers and a measure of surface periodicity in real space. One of its most important applications is the direct determination of hydrogen adsorption sites by recoiling spectrometry [12, 4T ]. Most other surface structure teclmiques do not detect hydrogen, with the possible exception of He atom scattering and vibrational spectroscopy. 

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