ELECTRONIC-STRUCTURE OF (ROOT-3 X ROOT-3)R30-DEGREES-NA AND (ROOT-3X-ROOT-3)R30-DEGREES-K ON AL(111) - COMPARISON OF NORMAL AND SUBSTITUTIONAL ADSORPTION SITES

Recently, it was shown that Na and K on Al(111) will kick out a surfac e Al atom and occupy the substitutional site. For K also the on-top po sition is possible. In this contribution, we give an analysis of the a dsorbate wave functions, surface density of states, and surface band s tructures. Different adsorption sites are considered, and it is shown that the mechanism of bonding in the substitutional geometry on Al dif fers from that in the ''normal'' on-surface geometry. In the first cas e the adatom-substrate interaction dominates, and the surface electron ic structure is largely determined by the character of the Al surface vacancy, whereas in the second case the adatom-adatom interaction domi nates. The theoretical results are compared with experimental studies of direct and inverse photoemission, suggesting the need of a re-analy sis of the data.