ULTRAFAST ELECTRON DYNAMICS AT THE GE(111) SURFACE INVESTIGATED WITH SUBPICOSECOND LASER PHOTOEMISSION

The technique of subpicosecond angle resolved laser photoemission spectroscopy is described. Investigations of the dynamics of electrons excited into normally unoccupied states at the cleaved Ge(111)2 x 1 surface are detailed. In particular, electrons which are excited into bulk states near the surface are observed to scatter into the normally unoccupied antibonding pi* band whose minimum is at JBAR in the surface Brillouin zone. Cooling of the sample to near liquid nitrogen temperatures has permitted us to directly delineate the role played by optical phonons in the surface electron dynamics. In addition hole dynamics are reported and renormalization of the surface band gap is observed.