Surface atomic vibration

The rotational mobility of adsorbed molecules is caused by its rotational degree of freedom (resulting from the fact that the molecule is tightly bound to the substrate through the only atom) and by the coupling of molecular vibrations with surface atomic vibrations. The rotational motion intensity is strongly temperature-dependent and affects spectroscopic characteristics. As a result, the rotational mobility of surface hydroxyl groups was reliably detected.200 203... 

Primary surface information bonding geometry, surface atom vibrations. 

Such a potential energy function gives rise to the famihar parabolic curve (Figure 22) where the curvature of the function is related to the force constant. The success of this simple harmonic model in treating surface atom vibrations lies in the relatively small displacement of surface atoms during a period of vibration. For some crystal properties, such as thermal expansion at elevated temperature, anharmoitic contributions to the potential must be included for an accurate description. 

The effect of surface atom vibrations is seen clearly in low energy electron diffraction (EEED). Experimentally, an exponential decrease in the intensity of scattered beams and an increase in background intensity are observed with increasing temperature. This arises as a result of the increased vibrational amplitude of the surface atoms that occurs at... 

High-energy scattering may be used also to investigate surface-atom vibrations and should yield adsorbate-site geometry with high accuracy. 

Fig. 7.1 Modelling the intensity amplification of surface atom vibrations by a onedimensional linear chain of five carbon atoms. The chain has four vibrations, the LAMs, LAMl has the highest intensity and LAM4 the lowest. When the very strong C—H stretching mode is at low frequencies this relative amplification is lost. See text for details.

SURFACE ATOM VIBRATIONS 4.2.1 The Harmonic Oscillator Model... 

Excited surface-atom vibrations induced by incident electrons have many applications in surface science. 

Adsorbates should have a marked influence on surface-atom vibrations, since they change the bonding environment with respect to that on the clean surface. The adsorption of oxygen on tungsten increases the surface Debye temperature with respect to the bulk value due to the stronger W—O bond as compared to the W—W bond. Studies of surface-atom vibrations in the presence of adsorbates provide information on the nature of the surface bond. 

For the same reason of higher amplitude of near-surface atomic vibrations, crystallization cannot start from the liquid-phase surface. Because of this, unlike the melting ptrooess, the crystallization process is homogeneous and needs overcooling of the system by dozens or even hundreds degrees relative to the equilibrium temperature of the phase transition. 

Oshima, C. (1986) New type of surface atomic vibrational modes of the (100) surface of the NaCl-type lattice. Phys. Rev. B, 34, 2949. 

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