  The interfacial band structures of multilayer systems play a crucial role for
the ultrafast charge and spin carrier dynamics at interfaces. Here, we study
the energy- and momentum-dependent quasiparticle lifetimes of excited states of
a lead monolayer film on Ag(111) prior and after the adsorption of a monolayer
of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA). Using time-resolved
two-photon momentum microscopy, we show that the electron dynamics of the bare
Pb/Ag(111) bilayer system is dominated by isotropic intraband scattering
processes within the quantum well state as well as interband scattering
processes from the QWS into the Pb sideband. After the adsorption of PTCDA on
the Pb monolayer, the interband scattering is suppressed and the electron
dynamics is solely determined by intraband or inelastic scattering processes.
Our findings hence uncover a new possibility to selectively tune and control
scattering processes of quantum well systems by the adsorption of organic
molecules.
