FEMTOSECOND PHOTOEMISSION-STUDY OF ELECTRON RELAXATION IN 2-DIMENSIONAL LAYERED ELECTRON-SYSTEMS

By using the femtosecond two photon photoemission technique, we direct ly measured the lifetimes of photoexcited electrons in two layered mat erials, graphite and SnS2. The inverse lifetimes of photoexcited elect rons in these two-dimensional layered materials exhibit linear energy dependence, i.e., 1/tau proportional to(E-E-F), instead of quadratic, i.e., l/tau proportional to(E-E-F)(2) as predicted by Fermi liquid the ory. This can be explained with the layered electron gas theory, accor ding to which the electron-plasmon interaction is strong, even for ele ctrons at low excitation energies, due to the formation of plasmon ban d in layered electron systems. Furthermore, our observations in SnS2 a lso suggest that in layered semiconductors, photogenerated electron-ho le pairs will recombine directly by emitting plasmons, and this causes the lifetimes of the conduction band electrons to be much shorter tha n those in three-dimensional semiconductors. (C) 1997 American Vacuum Society.