Ultrafast electron dynamics at the Ge(111)2×1 surface

Using angle-resolved laser photoemission we have studied the ultrafast electron relaxation processes on the Ge(111) π-bonded 2×1 surface with subpicosecond time resolution. Photoexcitation of the bulk bands and subsequent bulk-surface coupling results in a transient population of the normally unoccupied ${\mathrm{\pi }}^{\mathrm{*}}$-antibonding surface band. Our studies permit the direct observation of a unique process, that of the recombination of surface electrons with bulk holes mediated by phonons at the surface, which we find to be the primary mechanism involved in the ultrafast decay of the transient surface electron population.