Molecule-surface scattering

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

For example, energy transfer in molecule-surface collisions is best studied in nom-eactive systems, such as the scattering and trapping of rare-gas atoms or simple molecules at metal surfaces. We follow a similar approach below, discussing the dynamics of the different elementary processes separately. The surface must also be simplified compared to technologically relevant systems. To develop a detailed understanding, we must know exactly what the surface looks like and of what it is composed. This requires the use of surface science tools (section B 1.19-26) to prepare very well-characterized, atomically clean and ordered substrates on which reactions can be studied under ultrahigh vacuum conditions. The most accurate and specific experiments also employ molecular beam teclmiques, discussed in section B2.3. 

The site specificity of reaction can also be a state-dependent site specificity, that is, molecules incident in different quantum states react more readily at different sites. This has recently been demonstrated by Kroes and co-workers for the Fl2/Cu(100) system [66]. Additionally, we can find reactivity dominated by certain sites, while inelastic collisions leading to changes in the rotational or vibrational states of the scattering molecules occur primarily at other sites. This spatial separation of the active site according to the change of state occurring (dissociation, vibrational excitation etc) is a very surface specific phenomenon. 

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... 

Hyperthermal surface ionisation (HSI) is an ultrasensitive tuneable selective ion source [222,223] which is based on the very effective ionisation of various hyperthermal molecules upon their scattering from a surface with a high work function, such as rhenium oxide. Molecule-surface electron transfer constitutes the major and most important HSI mechanism for GC-MS. 

Observed angular distributions were quasi-specular and scattered rotational distributions were strongly dependent upon the incidence energy, both observations indicating the direct nature of the interaction. The most important observation of the work was the approximately Arrhenius surface temperature dependence of the vibrational excitation probability, exhibiting an effective activation energy close to the vibrational excitation energy of the scattered molecule (see Fig. 2). The authors also showed that the... 

The energy of an incident molecule will not normally be the same as that of the molecule when it is scattered from the surface, i.e., ZsP Ef There will be an accommodation to the surface and an exchange of energy with the surface. Complete accommodation or equilibration with the surface would imply that the scattered molecules have the same temperature as the surface. The energy accommodation coefficient, ac, is defined for each surface involved in the problem by the expression... 

While for a given surface the incident and scattered molecules have different energies (unless ac = 0), the molecule scattered from surface 1 is the molecule that is incident for surface 2. That is, since there are no gas-gas collisions when the molecule travels between the two surfaces, there is no mechanism to change the energy that it has when it leaves surface 1. Likewise, the energy of the molecules scattered from surface 2 is the same as the energy of those molecules incident upon surface 1. Thus, the temperatures are related by... 

Direct NO scattering from Ag(lll) is perhaps the most extensively experimentally studied molecule-surface scattering system. This principally reflects the ease... 

State-resolved inelastic scattering for a wide range of incident conditions ( ), d,) are measured for this system by combining molecular beam techniques with (2 + 1) ion TOF REMPI detection of the scattered molecules [58]. Energy transfer parallel to the surface is measured from the Doppler broadening of the REMPI spectra. Trapping... 

The 10 11 M solution used for emission had an average of just 10 analyte molecules in the volume probed by the 514-nm excitation laser. [From Pj. G. Goulet, N.P.W. Pleczonka, and R. F. Aroca, "Overtones and Combinations in Single-Molecule Surface-Enhanced Resonance Raman Scattering Spectra," Anal. Chem. 2003, 75, 1918.]... 

Mowrey, R.C., Sun, Y., and Kouri, D.J. (1989). A numerically exact full, wave packet approach to molecule-surface scattering, J. Chem. Phys. 91, 6519-6524. 

Figure 15 A swarm of classical trajectories incident on a model PES [49]. The site labelled bridge is initially attractive, but ultimately there is a barrier to dissociation at this site. At die atop site dissociation is activationless (downhill), but molecules can fail to take this path because they are initially steered to die bridge site. Some molecules can trap because die momentum normal to die surface is converted into parallel motion and rotations. After making several bounces, the trapped molecules dissociate or return to die gas-phase. For H2/Pd(l 1 1) die trapping channel contributes a large fraction of the scattered molecules [50].

Figure 17 Angular distributions for direct scattering of preferentially oriented NO from Pt(l 11), presented in a polar plot. E = 0.18 eV, Ts = 573 K, i = 50°. The hnes through the angular distributions are drawn to guide the eye. The arrows indicate the angle of incidence and the specular angle. In case of N-end collisions less molecules are directly scattered. Molecules with are directly scattered after an N-end collision come off closer to the surface normal than molecules with an O-end collision. From Kuipers et al. [94].

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