The dynamics of two-dimensional smalt-polaron formation at ultrathin alkane layers on a silver(111) surface have been studied with femtosecond time-and angle-resolved two-photon photoemission spectroscopy. Optical excitation creates interfacial electrons in quasi-free states fbr motion parallel to the interface, These initially delocalized electrons self-trap as small polarons in a localized state within a few hundred femtoseconds. The localized electrons then decay back to the metal within 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 analysis determines the inter-and intramolecular vibrational modes of the overlayer responsible for self-trapping as well as the relaxation energy of the overlayer molecular lattice. These results for a model interface contribute to the fundamental picture of electron behavior in weakly bonded solids and can lead to better understanding of carrier dynamics in many different systems, including organic light-emitting diodes.