Room-temperature kinetics of short-chain alkanethiol film growth on Ag(111) from the vapor phase

We have used time-of-flight (TOF) direct recoiling spectroscopy (DRS) to follow propanethiol adsorption at 300 K from the vapor phase on an Ag(111) surface, for exposures ranging from 10(-1) to 10(5) L. Results show that the adsorption proceeds with changes in the sticking coefficient, consistent with at least three phases. At low exposures, the alkanethiol molecules adsorb with high probability at defect sites, followed by a slower growth mode that essentially covers the whole surface. A third change in the sticking coefficient is associated with the final saturation stage, corresponding to a thicker layer related to molecules in a more upright orientation. The adsorption kinetics for hexanethiol is similar to that of propanethiol but taking place at higher rates, stressing the importance of the hydrocarbon chain length in the growth process. ISS-TOF measurements during thermal desorption show that most of the C, H, and S go away together, suggesting that the molecules adsorb and desorb from flat regions without S-C bond cleavage. Fitting the desorption maximum at 450 K with a first-order desorption curve gives a desorption energy of 1.43 eV. A small final S content that is correlated with the initial Ag(111) surface roughness is observed after desorption.     

Determining the electron-phonon mass enhancement parameter λ on metal surfaces

We discuss various approaches for determining the strength of the electron-phonon coupling on metal surfaces by angle-resolved photoemission. The first is to extract the electron-phonon mass enhancement parameter directly from the re-normalization of the band dispersion very close to the Fermi level. The second is to use the temperature-dependence of photoemission spectra such as energy distribution curves (EDCs) or momentum distribution curves (MDCs). We illustrate the advantages and disadvantages of both methods with the help of simulated spectral functions. Using experimental data, we find =0.40(5) for Bi(111) by analyzing temperature-dependent MDCs. PACS 73.20.-r; 63.20.Kr; 71.18.+y; 79.60.Bm     

Electronic structure of Sr2FeMoO6

We have used electron spectroscopies to investigate the electronic structure of the double perovskite Sr₂FeMoO₆. The valence-band photoemission spectra present a well-defined cut-off at the Fermi level, indicative of the metallic character of the material. The O 1s X-ray absorption spectrum presents three peaks, which are in good correspondence with the main structures in the unoccupied density-of-states of DF-LDA calculations. The electron energy-loss spectra present two structures which are also interpreted in terms of transitions between the bands obtained in the DF-LDA calculations.