Surface scattering

Metallic solids have mobile electrons that can be excited at low energies. These are the electrons of the partly full conduction band and the excited electron leaves a hole at the lower energy. Electronic excitation of the surface can therefore take place at lower collision energies than between molecules in the gas phase. Similarly, charge transfer between an incident projectile and the surface is more common than in collisions between molecules in the gas phase. This alters the bonding in the molecule and facilitates bond stretching and dissociation. Non-adiabatic processes are more common in surface-induced chemistry. 

Just as for gas-phase molecular collisions, gas-surface encounters can be elastic, inelastic, or reactive in nature. A wide range of scattering behavior is observed depending upon the gas molecules, the composition, structure, and temperature 

Molecular projectiles offer the possibility of additional direct inelastic channels, namely, the excitation or de-excitation of the molecular internal modes, much as for gas-phase molecular inelastic scattering. Unlike a gas-phase collision of a molecule with a structureless projectile, here the energy balance of the internal modes of the molecule need not be met entirely by the translation. The participation of the surface degrees of freedom is possible and the low-energy modes of the surface, not only phonons but also electron-hole pairs, are particularly important in bridging the gap (remember the exponential gap principle) and thereby making such inelastic collisions quite efficient. 

The analogous situation in the gas phase is ion-molecule collisions. Section 6.2.3.1, where there is a deep well in both the entrance and exit valleys. 

In Section 6.1.4.1 we have seen how transition state theory can account for the pre-exponential factor being smaller than k Tjh. There it was due to the loss of entropy in forming the transition state due to the steric requirements. Here the steric requirements work in the opposite direction. It is 

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MSS Molecule surface scattering [159-161] Translational and rotational energy distribution of a scattered molecular beam Quantum mechanics of scattering processes... 

Harris J 1987 Notes on the theory of atom-surface scattering Phys.scr. 36 156... 

The dynamics of ion surface scattering at energies exceeding several hundred electronvolts can be described by a series of binary collision approximations (BCAs) in which only the interaction of one energetic particle with a solid atom is considered at a time [25]. This model is reasonable because the interaction time for the collision is short compared witii the period of phonon frequencies in solids, and the interaction distance is shorter tlian the interatomic distances in solids. The BCA simplifies the many-body interactions between a projectile and solid atoms to a series of two-body collisions of the projectile and individual solid atoms. This can be described with results from the well known two-body central force problem [26]. 

The summation of pair-wise potentials is a good approximation for molecular dynamics calculations for simple classical many-body problems [27], It has been widely used to simulate hyperthennal energy (>1 eV) atom-surface scattering ... 

Mowrey R C and Kouri D J 1987 Application of the close coupling wave packet method to long lived resonance states in molecule-surface scattering J. Chem. Phys. 86 6140... 

P. O. Goodman and H. Y. Wachman, Dynamics of Gas Surface Scattering, Academic Press, Inc., New York, 1976. 

For the simple case of surface scattering (or scattering from a very thin layer), the ratio of to is given by... 

The RWP method also has features in common with several other accurate, iterative approaches to quantum dynamics, most notably Mandelshtam and Taylor s damped Chebyshev expansion of the time-independent Green s operator [4], Kouri and co-workers time-independent wave packet method [5], and Chen and Guo s Chebyshev propagator [6]. Kroes and Neuhauser also implemented damped Chebyshev iterations in the time-independent wave packet context for a challenging surface scattering calculation [7]. The main strength of the RWP method is that it is derived explicitly within the framework of time-dependent quantum mechanics and allows one to make connections or interpretations that might not be as evident with the other approaches. For example, as will be shown in Section IIB, it is possible to relate the basic iteration step to an actual physical time step. 

Bird RB, Stewart WE, Lightfoot EN. Transport Phenomena. New York Wiley, 1960. Goodman F, Wachman H. Dynamics of Gas-Surface Scattering. New York Academic Press, 1976. 

Just as in gas phase kinetics, reactive molecular beam-surface scattering is providing important molecular level insight into reaction dynamics. There is no surface reaction for which such studies have proven more illuminating than the carbon monoxide oxidation reaction. For example Len, Wharton and co-workers (23) found that the product CO exits a 700K Pt surface with speeds characteristic of temperatures near 3000K. This indicates that the CO formed by the reactive encounter of adsorbed species is hurled off the surface along a quite repulsive potential. 

Chao, C. Y. Guo, L. J., Reduction of surface scattering loss in polymer microrings using thermal reflow technique, IEEE Photon. Technol. Lett. 2004, 16, 1498 1500... 

The LEIS technique owes its excellent surface sensitivity to the high neutralization probability of the rare gases. The fraction of He+ ions that survives a single collision without being neutralized is only between HT4 and 10 2. This implies automatically that the probability that a He+ ion will penetrate the surface, scatter off deeper atoms and return as an ion is practically zero. However, a finite probability exists that the backscattered neutral He atom will be ionized upon leaving the sample, and this is the reason that an LEIS spectrum still contains some information on the state of a sample below its surface. 

Equation Approach for Atom/Solid-Surface Scattering General Formulation for Classical Scattering off Harmonic Solids. 

MO LCAO methods, 34 136 Molecular-beam surface scattering, 26 26, 27 Molecular Cage, 34 226 Molecular design in cyclodextrin, 32 427 Molecular dynamics diffusion in zeolites, 42 2, 4-6 argon, 42 20... 

THEORY OF RESONANT CHARGE TRANSFER IN ATOM-SURFACE SCATTERING... 

Accurate numerical techniques are available, which allow quantitative investigation of the atom-surface scattering process to be made, and which provide a standard for assessing the validity of the various approximations Despite the different approaches adopted by the numerical methods reviewed here, they are, nonetheless, interrelated and do lead to substantially the same results. 

In this chapter we have shown that the TDAN model gives a good description of the resonant charge-transfer process in atom-surface scattering. While it is unfortunate that exact solutions for the TDAN wavefunction cannot be obtained, the one-electron method can be used to find approximate solutions which allow qualitative predictions to be made. On the whole, these predictions are in reasonable accord with experimental Hndings. 

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