A structural study of the Al(111)( square root 3* square root 3)R30 degrees -Rb phase at different temperatures

Abstract

Normal-incidence standing x-ray wavefield (NISXW) measurements have been made of the local adsorption site of Rb on Al(111) surfaces, particularly in an ordered ( square root 3* square root 3)R30 degrees phase, as a function of the sample temperature during adsorption or subsequent annealing. The results confirm the a top-site occupation for low-temperature (around 150 K) preparation, but show that room-temperature preparation leads to a structure having Rb atoms in surface substitutional sites. The overall structural situation is therefore essentially the same as that found previously by low-energy electron diffraction LEED for the Al(111)( square root 3* square root 3)R30 degrees -K phases. However, experiments involving annealing of the low-temperature prepared surface to room temperature indicate that only a small part of the surface easily transforms to the higher-temperature form, and indeed there is evidence that even in room-temperature preparations some fraction of the adsorbed atoms may remain in atop sites. The apparent conflict of this result with that from recent photoemission core-level shift and LEED data is discussed.

Theory of Alkali-Metal Adsorption on Close-Packed Metal Surfaces

“…The calculations, which take the full atomic structure into account including relaxation of atoms from their ideal positions as well as surface reconstructions, show that the traditional view of alkali metal adsorption outlined above, is only part of the whole picture, and that we must now adopt an updated view. Phenomena such as the following may occur: 1) The alkali metal atoms may not necessarily assume highly coordinated sites on the surface [21][22][23][24][25][26][27][28][29][30][31][32][33][34]. 2) The alkali metal adatom may kick out surface substrate atoms and adsorb substitutionally [20,21,[30][31][32][33][34][35][36][37][38][39].…”

“…Phenomena such as the following may occur: 1) The alkali metal atoms may not necessarily assume highly coordinated sites on the surface [21][22][23][24][25][26][27][28][29][30][31][32][33][34]. 2) The alkali metal adatom may kick out surface substrate atoms and adsorb substitutionally [20,21,[30][31][32][33][34][35][36][37][38][39]. Substitutional adsorption has been shown in three cases to occur as the result of an irreversible phase transition from an "on-surface" site by warming to room temperature, without change in the periodicity of the surface unit cell or of the coverage [32][33][34][35][36][37][38].…”

“…2) The alkali metal adatom may kick out surface substrate atoms and adsorb substitutionally [20,21,[30][31][32][33][34][35][36][37][38][39]. Substitutional adsorption has been shown in three cases to occur as the result of an irreversible phase transition from an "on-surface" site by warming to room temperature, without change in the periodicity of the surface unit cell or of the coverage [32][33][34][35][36][37][38]. 3) The alkali metal atom may switch site on variation of coverage [29,31,[40][41][42][43], and 4) island formation may occur [13,31,[44][45][46][47][48].…”

Inelastic energy loss in100−keVH+scattering from single atoms: Theory and experiment for K, Rb, and Cs

“…Rather complete quantitative structural information is available for K and Rb on Al͑111͒. [11][12][13][14] At low temperature, these species surprisingly form ͑ͱ3 ϫ ͱ 3͒R30°surface phases in which the adatom occupies sites atop the surface layer Al atoms, but at room temperature the alkali atoms displace some of the surface layer Al atoms to form substitutional surface alloys ͑that retain the ͑ͱ3 ϫ ͱ 3͒R30°periodicity͒. An important feature of these surface alloy phases, however, is that because of the large effective radii of the alkali metal atoms, the outermost alloy layer is very heavily rumpled, with the alkali atoms much higher above the surface than the surrounding Al atoms.…”