Molecular orientation surface energy

Polymer films were produced by surface catalysis on clean Ni(100) and Ni(lll) single crystals in a standard UHV vacuum system H2.131. The surfaces were atomically clean as determined from low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Monomer was adsorbed on the nickel surfaces circa 150 K and reaction was induced by raising the temperature. Surface species were characterized by temperature programmed reaction (TPR), reflection infrared spectroscopy, and AES. Molecular orientations were inferred from the surface dipole selection rule of reflection infrared spectroscopy. The selection rule indicates that only molecular vibrations with a dynamic dipole normal to the surface will be infrared active [14.], thus for aromatic molecules the absence of a C=C stretch or a ring vibration mode indicates the ring must be parallel the surface. 

This treatment of melting in confined geometries is obviously oversimplified and the molecular nature of the phases and interactions between the adsorbent walls and the adsorbate should be taken into account by considering not only the surface energies but also the exact nature of the solid phase (structure, crystalline orientation, crystal defects, and so on). 

The translational and internal energy dependences of the dissociation probability can yield a great deal of information regarding the PES, but the final state is not fully specified (only given as dissociated or not dissociated) and this leads to some loss of information. Much more detail can be obtained by examining the scattered fraction instead. Diffraction intensities tell us about the surface site dependence of the PES, while comparison of the internal state populations before and after scattering tells us about the changes of vibrational and rotational state, and hence about the curvature of elbow PESs and the molecular orientation dependence of the PES. 

A variation of XANES or NEXAFS has been used to determine the structure of molecules chemisorbed on surfaces. In this approach photoemitted electrons excite molecular orbitals in the chemisorbed molecules. By varying the polarization of the incident photons, molecular orientation can be determined from selection rules for excitation. The bond lengths can be determined from a quasi-empirical correlation between bond-length and the shift in the molecular orbital excitation energy. This technique has been used to study the chemisorption of several hydrocarbon molecules on different metal surfaces./17/... 

The hexapole technique has been extensively exploited for the study of oriented open shell molecules such as OH (see Ref [46] and references therein) and NO (see Ref [47] and references therein), the latter also for scattering on surfaces [48]. This is a very important topic, because the basic tool for enhancement of chemical reactivity is catalysis at surfaces. In Ref [49], for examples, the oxidation of Si (001) induced by incident energy of O2 molecules is studied by synchrotron radiation photoemission spectroscopy and mass spectrometry, a process of a kind which may show propensities regarding molecular orientation of O2 as it impinges on the surface, possibly controlled by techniques of the kind described in previous sections. 

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