FEMTOSECOND DYNAMICS OF ELECTRON LOCALIZATION AT INTERFACES

The dynamics of two-dimensional smalt-polaron formation at ultrathin a lkane layers on a silver(111) surface have been studied with femtoseco nd time-and angle-resolved two-photon photoemission spectroscopy. Opti cal excitation creates interfacial electrons in quasi-free states fbr motion parallel to the interface, These initially delocalized electron s self-trap as small polarons in a localized state within a few hundre d femtoseconds. The localized electrons then decay back to the metal w ithin picoseconds by tunneling through the adlayer potential barrier, The energy dependence of the self-trapping rate has been measured and modeled with a theory analogous to electron transfer theory. This anal ysis determines the inter-and intramolecular vibrational modes of the overlayer responsible for self-trapping as well as the relaxation ener gy of the overlayer molecular lattice. These results for a model inter face contribute to the fundamental picture of electron behavior in wea kly bonded solids and can lead to better understanding of carrier dyna mics in many different systems, including organic light-emitting diode s.