Femtosecond dynamics and transport of optically excited electrons in epitaxial Cu films on Si(111)-7x7

Time-resolved two-photon photoemission spectroscopy is used to study ultrafast electron dynamics of epitaxial Cu films grown in situ on a Si(111)-7x7 substrate with 6 to 44 nm thickness. For excitation with femtosecond laser pulses at hnu=2.35 eV a pronounced increase of the electron relaxation rates is observed with increasing film thickness and even further in comparison to bulk data. This is attributed to an enhanced energy dissipation in thicker films due to transport of excited carriers into the bulk.