Anisotropic molecule-surface potential

Figure 16. Measure of translational order of N2 patches on graphite with respect to the ideal adsorption sites on the (VS x J3)R3Q° lattice from molecular dynamics simulations. Anisotropic molecule-surface potential (3.3) with 7 = 0.4 and 7r = -0.9 (triangles), 7 = 0.4 and 7 = —0.54 (cirlces), and isotropic special case 7.4 = 0 and Jr = 0 (squares). Unfilled (filled) symbols refer to 140 (210) molecules. (Adapted from Fig. 1 of Ref. 165.)...

Figure 17. Molecular dynamics trajectories above the melting transition at a coverage of 0.5 monolayers. The center of mass of the N2 molecules is projected on the graphite basal plane (a) for the isotropic molecule-surface potential (3.3) with 7 = 7 = 0 at 44 K and (b) for the anisotropic molecule-surface potential (3.3) with = 0.4 and y — —0.54 at 55 K. (c) Top view of the final configuration of (Z>). (Adapted from Figs. 3 and 9 of Ref. 165.)...

The presence of the metal or insulator does not only add the molecule-substrate interaction as a formative influence, but can also alter the effective intermolecular interactions. For example, whereas the crystallisation of bulk tetraeene is governed by the attractive interaction between molecules in a particular relative orientation, the surface-confined molecules (on Ag( 111)) repel each other. The modification of the effective intermolecular interaction may originate both from substrate-mediation and from the intrinsically anisotropic molecular interaction potentials. As the possibly entropy-driven ordering of tetraeene on Ag(lll) shows, the modified interactions may introduce new ordering mechanisms at the interface. 

Various methods of statistical mechanics are applied to the calculation of surface orientation of asymmetric molecules, by introducing an angular dependence to the intermolecular potential function. The Boltzmann distribution can also be used to estimate the orientational distribution of molecules. The pair potential V(r) may be written as V(r, 6) if it depends on the mutual orientation of two anisotropic molecules, and then we can write for the angular distribution of two molecules at a fixed distance, r, apart... 

Only very recently it became feasible to carry out classical trajectory calculations of transport properties of nitrogen (Heck Dickinson 1994 Heck et al. 1994). In these calculations the nitrogen molecule has been treated as a rigid rotor, and the intermolec-ular pair potential was based on the anisotropic ab initio potential energy surface of van der Avoird et al. (1986). The details of these computations as well as the uncertainties of their results are described in the given ptq)ers and will not be repeated here. 

Ar-N2 (10). and Ar-HCl (16). In this section, we present the results for the Ar-H2 and Ar-HD systems. The Ar-H2 system Is one of the most extensively studied vdW molecules, and several reliable anisotropic potential energy surfaces derived from experimental data are available. Although rotational predissociation (RP) line widths for Ar-H2 systems have not been measured, McKellar (25) has... 

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