RESUMEN
Closed-shell atoms scattered from a metal surface exchange energy and momentum with surface phonons mostly via the interposed surface valence electrons, i.e., via the creation of virtual electron-hole pairs. The latter can then decay into surface phonons via electron-phonon interaction, as well as into acoustic surface plasmons (ASPs). While the first channel is the basis of the current inelastic atom scattering (IAS) surface-phonon spectroscopy, no attempt to observe ASPs with IAS has been made so far. In this study we provide evidence of ASP in Ni(111) with both Ne atom scattering and He atom scattering. While the former measurements confirm and extend so far unexplained data, the latter illustrate the coupling of ASP with phonons inside the surface-projected phonon continuum, leading to a substantial reduction of the ASP velocity and possibly to avoided crossing with the optical surface phonon branches. The analysis is substantiated by a self-consistent calculation of the surface response function to atom collisions and of the first-principle surface-phonon dynamics of Ni(111). It is shown that in Ni(111) ASP originate from the majority-spin Shockley surface state and are therefore collective oscillation of surface electrons with the same spin, i.e. it represents a new kind of collective quasiparticle: a Spin Acoustic Surface Plasmon (SASP).
RESUMEN
The electronic structures of tris(8-hydroxyquinolinato)-erbium(III) (ErQ(3)) and tris(8-hydroxyquinolinato)-aluminum(III) (AlQ(3)) have been studied by means of core level and valence band photoemission spectroscopy with the theoretical support of hybrid Heyd-Scuseria-Ernzerhof density functional theory, to investigate the role played by the central metal atom. A lower binding energy (0.2 eV and 0.3 eV, respectively) of the O 1s and N 1s core levels has been observed for ErQ(3) with respect to AlQ(3). Differences in the valence band spectra, mainly related to the first two peaks next to the highest occupied molecular orbital (HOMO), have been ascribed to an energetic shift (to 0.4 eV lower energies for ErQ(3)) of the σ molecular orbital between the oxygen atoms and the central metal atom. A lower (by 0.5 eV) ionization energy has been measured for the ErQ(3). The interpretation of these results is based on a reduced interaction between the central metal atom and the ligands in ErQ(3), with increased electronic charge around the ligands, due to the higher ionic radius and the lower electronegativity of Er with respect to Al.
