The Ni single crystal was cleaned by repeated cycles of In the light of structure data and with respect to limits of VI we discuss our best-fit vertical distance of sulfur Vertical distances of sulfur and different force fields. V weĬompare the theoretical and experimental results on theĭispersion curves of sulfur and nickel modes for different IV we brieflyĭescribe our theoretical model calculation. We give some experimental details with regard to sample The data is not as good if the sulfur atom is allowed to sit Sulfur atoms are placed at least 1.45 A above the surface.Įven with a more complex force field, the agreement with However, this satisfactory match with the data is obtained only when the Modes and resonances, can be matched with a simple The dispersion curves for the parallel and perpendicular sulfur modes, as well as the substrate surface Was expected, we indeed find that no reduction in theīonding between the first and second nickel layers is involved. In this paper we report on the measurement of the adsorbate and substrate dispersion curves and the theoreticalĪnalysis accompanying the data. Thus we do notĮxpect a c (2X2) sulfur overlayer to greatly influence the %'eīelieve that, in the case of oxygen overlayers, this softening in frequency is related to the change in the couplingīetween the first- and second-layer nickel atoms in the Is observed for the case of similar sulfur overlayers. Of overlayer atoms, no such drastic softening of this mode Plained on the basis of the mere difference in the number Which cannot be exp(2X2) to the c(2X2) structure' ' It is intriguing that, whileįor oxygen overlayers there is a considerable softening in The available vibrational data for sulfur at the I" point of The remarkable difference in the behavior of the sulfurĪnd oxygen overlayers on this surface, as is evident from Sulfur overlayer on the Ni(100) surface also stems from Our choice of the study of the dispersion of the c(2 X 2) Studies, sulfur "sits" in the fourfold hollow sites at (approximately) -1.4 A above the nickel surface plane. (LEED) and other structural methods such as extendedĪnd photoelectron diffraction. As with theĬ(2X2) overlayer, the sulfur system has been investigated extensively with low-energy electron diffraction An example of the same overlayer with weaker bonding is the c (2 X 2) overlayer of sulfur. It should therefore be interesting to pursue this issueįurther, with other adsorbates on the same surface forming either weaker bonds or even stronger bonds with niCel. layer nickel atoms is reduced as a result of the Ni - bonds formed on the surface. Thus one has several indications that the bondingīetween the first- and second. When the surface is covered with a c(2X2) overlayer of This finding is in agreement with ionscattering data, suggesting that on the clean surface theĭistance between the first and second nickel layers is contracted by -Ġ.06 A, where as it is relaxed by +0.09 A The coupling between the first- and second-layer nickelĬlean surface this coupling is enhanced to 1.2 times theīulk value. The Rayleigh mode, it was found necessary to assume that Order to also achieve a reasonable fit to the dispersion of Theoretical analysis showed that the dispersion curves could be matched to a simple latticedynamical model with nearest-neighbor Was the Ni(100) surface covered with a c(2X2) overlayer Of adsorbate and substrate surface modes was measured The first system for which the dispersion Of the modes of ordered overlayers of chemisorbed atoms Latter is particularly suitable for studies on the dispersion Techniques are in use, namely inelastic scattering of He The dispersion of surface phonons on clean surfacesĪnd those with adsorbate layers has become amenable toĮxperimental investigations only recently. The optimum fit is obtained when the sulfur atom is placed at 1.45 A Experimental data are compared to a lattice dynamical model using an analytical0 With a c (2& 2) sulfur overlayer has been measured along the I. The dispersion of the adsorbate- and substrate-associated modes of the Ni(100) surface covered Surface phonon dispersion of c (2& 2) S on Ni(100)
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