Temperature-dependent local geometries in the system Al(100)-c(2×2)-Na: A surface extended x-ray-absorption fine-structure study

Electronic structure of high- and low- temperaturec(2×2)-Na/Al(001) phases from angle-scanned ultraviolet photoemission

“…[7][8][9][10][11] In contradiction to earlier results, 12,13 it has been shown by high-resolution core-level photoelectron spectroscopy that Na adsorption on Al͑001͒ at room temperature ͑RT͒ leads to a disruption of the Al surface and to the formation of a surface alloy. 7 A recent surface-extended x-rayabsorption fine-structure ͑SEXAFS͒ study of the RT c͑2ϫ2͒-Na/Al͑001͒ system at 0.5 ML ͑ML; 1 ML equals the number of surface Al atoms͒ coverage proposed Na adsorption beneath a reconstructed surface Al layer, 8 whereas very recent investigations using density-functional theory ͑DFT͒ conclude that the adsorption energy of the hollow site is the lowest for very low coverages, and that at a coverage of about 0.15 ML a transition from hollow to substitutional site occupation should occur. 9 Substitutional adsorption in this c͑2ϫ2͒ structure implies that every second surface Al atom has been kicked out and replaced by a Na atom.…”

“…7 Whereas the local geometry in the RT c͑2ϫ2͒ phase-and accordingly the nature of the irreversible order-order phase transition-are controversial, there is general agreement in that the LT c͑2ϫ2͒ structure is a chemisorbed phase where the Na atoms occupy the fourfold hollow sites. [7][8][9]11 In order to characterize the surface electronic structure of the two c͑2ϫ2͒-Na/Al͑001͒ phases and the buildup of the controversial RT c͑2ϫ2͒ structure, we have investigated these systems by means of angle-resolved ultraviolet photoelectron spectroscopy ͑ARUPS͒. 14,15 For Na deposition at RT, normal emission spectra of the Al͑001͒ sp surface state have been measured as a function of Na coverage from 0 ML up to the saturation coverage of 1 ML.…”

Theory of Adsorption on Metal Substrates

“…The right side of the figure shows a calculation for Na on Al(001), which takes the atomic structure into account. We note that for coverages Θ Na > ∼ 0.15 the on-surface geometry is a metastable structure which only exists at temperatures below 160 K (Andersen et al 1992;Aminpirooz et al, 1992).…”

“…5.21b). Consequently, the nature of the adsorbate chemical bond, the surface electronic structure, and the origin of the coverage dependence of the work function are in fact qualitatively different to what was assumed before 1992 (Andersen et al, 1992;Aminpirooz et al, 1992;Stampfl et al, 1994b;Berndt et al, 1995). Thus, good agreement between theory and experiment and/or a convincing physical picture are no guarantee that the description and trusted understanding is indeed correct.…”

“…5.22a). The process may be kinetically hindered, but this hindrance may be overcome even at rather low temperatures, e.g., for Na on Al(001) for T ≥ 160 K (Andersen et al 1992;Aminpirooz et al, 1992) and/or by the heat of adsorption. Whether the solubility in the substrate bulk is low or even zero is of no relevance at all for substitutional adsorption.…”