Incident atom

Conceptually similar studies have since been carried out for the reaction of Ft atoms with Cl/Aii(l 11). More recently, quantum-state distributions have been obtamed for both the Ft + Cl/Aii(l 11)[, and M and Ft(D) + D (Ft)/Cii(l 11) systems. The results of these studies are in good qualitative agreement widi calculations. Even for the Ft(D) + D (Ft)/Cii(l 11) system [89], where we know that the incident atom caimot be significantly accommodated prior to reaction, reaction may not be direct. Detailed calculations yield much smaller cross sections for direct reaction than the overall experimental cross section, indicating that reaction may occur only after trapping of the incident atom [90]. 

Here we have utilized Eq. (147) and assumed that the electronic ground state of the transition state has been raised by AE (to refer partition functions to the transition state s own ground state) and qto-vih is referred with respect to the bottom of the potential, as in Fig. 3.10. Expression (156) shows that the adsorption rate per area is the collision number for that area times a factor So(T), the so-called sticking coefficient, which must always be smaller than one. The sticking coefficient describes how many of the incident atoms were successful in reaching the adsorbed state... 

Much of the theory of scaling analysis was developed for molecular beam epitaxy (MBE), and there are some challenges in transferring the treatment to electrodeposition. In MBE, the incident atoms originate at a source at high temperature, arrive at the growth front from a vapor phase that is not in internal equilibrium, attach... 

Here D is the depth of the atomic potential as sensed by the incident atom and Mg and Ms are the masses of the scattering atoms and surface atoms, respectively, and 6 is the surface Debye temperature. For the diffracting X-rays and electrons that have much... 

Although the division of surface reaction mechanisms into LH or ER dates to the early days of catalysis, ER/HA surface reactions have only been demonstrated recently and only for strongly reactive atomic gas phase species, e.g., H, O. There are many differences between the ER/HA mechanism and the LH mechanism that can be used to separate them experimentally. For example, ER/HA reactions of reactive incident atoms are very exothermic relative to the equivalent LH reaction, typically by several eV. Much of this released energy should end up in the gas-phase product molecule. ER/HA are direct non-activated reactions whose final state properties depend on the initial conditions of the incoming atom and not Ts. This is of course the exact opposite of LH properties. 

Let us now consider the atomic kinetic equation (4.70). There are also two contributions to the collision integral I (Pt), the first one leaves the incident atom unchanged. The relevant processes are... 

Atomic beams are suitable for diffraction experiments. The repulsive interaction with the surface is so strong that low-energy atoms are already reflected by the topmost atomic layer. Due to its pronounced sensitivity to the topmost surface layer, the method of atomic beam diffraction is especially useful for the study of adsorbates and superlattices. Typical energies of the incident atoms are under 0.1 eV. At such low energies no radiation damage occurs. In... 

Neutral atoms, usually He, in electronically excited states collide with a surface at thermal energies. A surface electron may tunnel into an unoccupied electron level of the incoming gas atom, causing the incident atom to ionize and eject an electron, which is then detected. This technique measures the density of states near the Fermi level of the substrate and is highly surface-sensitive. 

Therefore, if the incident atomic velocity vz is normal to the surface, the best -position to place the surface on which the atoms are to be deposited is at a distance ... 

Fig. 11. Total cross section for the interaction of relativistic positronium atoms with carbon as a function of the kinetic energy expressed in the rest masses of the incident atom (T = 7—1). The solid curve is the theoretical dependence, - the measured value. The arrow marks the region (7 < 1.2) investigated in experiments on the interaction of hydrogen atoms with carbon...

Since DFT calculations are in principle only applicable for the electronic ground state, they cannot be used in order to describe electronic excitations. Still it is possible to treat electronic exciations from first principles by either using quantum chemistry methods [114] or time-dependent density-functional theory (TDDFT) [115,116], First attempts have been done in order to calculate the chemicurrent created by an atom incident on a metal surface based on time-dependent density functional theory [117, 118]. In this approach, three independent steps are preformed. First, a conventional Kohn-Sham DFT calculation is performed in order to evaluate the ground state potential energy surface. Then, the resulting Kohn-Sham states are used in the framework of time-dependent DFT in order to obtain a position dependent friction coefficient. Finally, this friction coefficient is used in a forced oscillator model in which the probability density of electron-hole pair excitations caused by the classical motion of the incident atom is estimated. 

Figure 2 Computed potential energy curves for two H atoms over Ni(l 00), plotted as a function of Zi, the height of the incident atom above the surface. The adsorbed H atom is held fixed in the hollow site at the equilibrium position. Results are shown for the incident H atom moving normal to the surface directly over the neighboring bridge (open circles), atop (filled diamonds) and hollow (filled circles) sites, as well as another hollow site an infinite separation from the target H (open diamonds).

Our flat-surface model is well-described elsewhere [17, 81, 82], and only the essential features are noted here. Consider the reaction between an incident atom of mass n, located atrj, and a target atom of mass mt, located at rt and initially adsorbed onto a flat static surface. Our initial wave function is ... 

For the fixed-puckered lattice case, we find very similar behavior, with roughly 1 eY less energy in the products. For the case where the target H is initially physisorbed, the dynamics differ. While a is large at low energy (about 10 A2), it drops rapidly to zero as Zq increases above about 0.1 eV. At these energies, the propensity is for the incident atom to simply knock the target, and itself, into the gas phase. What H2 does form, however, is very vibrationally excited, with (n) on the order of 10. 

A review of the semidassical method is given by Cottrell and McCoubrey [9] and by Rapp and Kassal [13]. In this method, the translational motion is treated classically, while the molecule BC is assumed to have quantized vibrational levels. By converting the force V (x) on the oscillator due to the incident atom to V (t) by utilization of the classical trajectory x(t), one may apply time dependent perturbation theory. The wave function for the perturbed system is written as a sum of the stationary-state wave functions Y (y)exp( —icojf), with coefficients ck given by... 

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