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 hydrogen and aluminum atoms on Si(111) is investigated using ab initio density functional cluster techniques. These two systems provide an example of chemisorption bonds whose electric dipoles point in opposite directions. The changes in charge distributions, vibrational frequencies and adsorbate desorption barriers are calculated as a function of the strength and direction of the electric field. The vibrational frequency shifts due to the electric field are shown to be sufficiently large to inhibit lateral energy transfer and thereby lead to adsorbate excitation localization, as is observed in recent STM experiments.