  We use angle resolved photoemission spectroscopy (ARPES) to investigate the
coupling of electron quantum well states (QWS) in epitaxial thin Pb and Ag
films. More specifically, we investigate the Ag/Si, Pb/Si, and Pb/Ag/Si
systems. We found that the parallel momentum plays a very profound role
determining how two adjacent quantum wells are coupled electronically across
the interface. We revealed that in the Pb/Ag bimetallic system, there exist two
distinctly different regimes in the energy versus momentum (E vs. k) space. In
one regime the electronic states in Ag and Pb are strongly coupled resulting in
a new set of quantum well states for the bi-metallic system. In the other
regime the electronic states in individual metallic layers are retained in
their respective regions, as if they are totally decoupled. This result is
corroborated by calculations using density functional theory. We further
unravel the underlying mechanism associated with the electron refraction and
total internal reflection across the interface.
