  Scanning tunneling spectroscopy measurements of superconducting thin
lanthanum films grown on a normal metal tungsten substrate reveal an
extraordinarily large broadening of the coherence peaks. The observed
broadening corresponds to very short electron-like quasiparticle lifetimes in
the tunneling process. A thorough analysis considering the different relaxation
processes reveals that the dominant mechanism is an efficient quasiparticle
relaxation at the interface between the superconducting film and the underlying
substrate. This process is of general relevance to scanning tunneling
spectroscopy studies on thin superconducting films and enables measurements of
film thicknesses via a spectroscopic method.
