No abstract

The normal state temperature dependence of a metallic K3C60 phase pure film has been investigated by means of core levels and valence band photoemission, carbon K edge x-ray absorption, and electron energy loss spectroscopies and compared with that of a Mott–Hubbard insulating K4C60 film. The anomalous temperature behavior of K3C60, in the range 30 K–600 K, can be consistently interpreted considering the presence of orientational disorder of the C60 molecules together with the inhomogeneity of the conduction electron distribution at the molecular level. In particular, the changes observed in the photoemission spectra near the Fermi level, are consistent with the behavior predicted for strongly correlated metallic systems in presence of disorder. Also in the case of K4C60 we observe anomalies in the temperature dependence of core levels photoemission data. These anomalies can be explained with the freezing of the molecular motions at low temperature and point to large thermal fluctuations of the alkali atoms around their equilibrium position at high temperature.

We investigate the nonlinear photoemission from a silver monocrystal with femtosecond laser pulses in a wide range of photon energies (0.8 eVϽhϽ6.3 eV). Electrons with high kinetic energy are observed in all cases of nonlinear photoemission and at intensities much smaller than 1 GW/cm 2 . In the infrared, the electron distribution resembles a decreasing exponential, the tail extending up to tens of eV. At hϭ3.1 eV such exponential distribution coexists with the Fermi-Dirac distribution due to two-photon emission. An explanation in terms of nanosize protuberancies, heated by localized surface plasmons cannot be ruled out, but alternative pictures, however speculative, are worth being considered.

The high-energy resolution O K-edge absorption near-edge x-ray absorption fine structure spectrum has been measured for in situ prepared potassium superoxide. The experimental data have been analyzed in detail by multiple scattering calculations using self-consistent field potentials. In particular, the so-called resonance at the rising edge, which presents a double-peak structure, has been totally resolved and reproduced by the calculations. This analysis indicates that the grown material is arranged in a KO 2 structure with an O-O distance between 1.31 and 1.34 Å. Moreover, the calculation demonstrates both a complete ionic character of the bound between the O 2Ϫ anion and K atoms and a strong interaction between the anion and solid-state matrices.