THE EFFECTS OF AN EXTERNAL ELECTRIC-FIELD ON THE ADATOM-SURFACE BOND - H AND AL ADSORBED ON SI(111)

The influence of an external electric field, such as that present in a scanning tunnelling microscope (STM), on the chemisorption bond of hy drogen and aluminum atoms on Si(111) is investigated using ab initio d ensity functional cluster techniques. These two systems provide an exa mple of chemisorption bonds whose electric dipoles point in opposite d irections. The changes in charge distributions, vibrational frequencie s and adsorbate desorption barriers are calculated as a function of th e strength and direction of the electric field. The vibrational freque ncy shifts due to the electric field are shown to be sufficiently larg e to inhibit lateral energy transfer and thereby lead to adsorbate exc itation localization, as is observed in recent STM experiments.