  A theoretical study of the surface energy-loss function of freestanding
Pb(111) thin films is presented, starting from the single monolayer case. The
calculations are carried applying the linear response theory, with inclusion of
the electron band structure by means of a first-principles pseudopotential
approach using a supercell scheme. Quantum-size effects on the plasmon modes of
the thinnest films are found in qualitative agreement with previous work based
on the jellium model. For thicker films, results show a dispersionless mode at
all thicknesses, in agreeement with electron energy-loss measurements. For
sizeable values of the momentum, the raising of the surface plasmon with
increasing thickness is retrieved.
