Quantum-size effects in the loss function of Pb(111) thin films: An ab initio study

A theoretical study of collective electronic excitations in free-standing Pb(111) thin films consisting of 1--5 monolayers (MLs) and a 21-ML film is presented. The calculations are carried out applying the linear response theory, with full inclusion of the electron band structure by means of a first-principles pseudopotential approach in a supercell scheme. In the case of the thickest film, we find that, due to strong bulklike interband transitions, at the Pb(111) surface there are two surface modes. For thin films, a mechanism of transformation of these modes to the symmetric and antisymmetric classic hybrid plasmons is investigated. Pronounced quantum-size effects on plasmon modes of the thinnest films are found. Strong influence of the band structure on dispersion and lifetime of such modes is demonstrated. The present results are in good agreement with available experimental data for a Pb surface and thin films.