Quasiparticle lifetimes in metallic quantum-well nanostructures

Quasiparticle lifetimes in metals as described by Fermi-liquid theory(1) are essential in surface chemistry(2) and determine the mean free path of hot carriers'. Relaxation of hot electrons is governed by inelastic electron-electron scattering, which occurs on femtosecond timescales owing to the large scattering phase space competing with screening effects(4). Such lifetimes are widely studied by time-resolved two-photon photoemission(5,6), which led to understanding of electronic decay at surfaces(6-8). In contrast, quasiparticle lifetimes of metal bulk(5,9-12) and films(11,13-15) are not well understood because electronic transport(10,16,17) leads to experimental lifetimes shorter than expected theoretically(13,15,18). Here, we lift this discrepancy by investigating Pb quantum-well structures on Si(111), a two-dimensional model system(19-29). For electronic states confined to the film by the Si bandgap we find quantitative agreement with Fermi-liquid theory and ab initio calculations(4,7) for bulk Pb, which we attribute to efficient screening. For states resonant with Si bands, extra decay channels open for electron transfer to Si, resulting in lifetimes shorter than expected for bulk. Thereby we demonstrate that for understanding electronic decay in nanostructures coupling to the environment is essential, and that even for electron confinement to a few (a) over circle ngstroms Fermi-liquid theory for bulk can remain valid.