Scanning tunneling spectroscopy of p(2 × 2) Cs and Na ordered overlayers on Cu(111) reveals similar line widths of quasi two-dimensional quantum well states despite largely different binding energies. Detailed calculations show that 50 % of the line widths are due to electron-phonon scattering while inelastic electron-electron scattering is negligible. A frequently ignored mechanism for ordered structures, i.e., enhanced elastic scattering due to Brillouin zone back folding, contributes the remaining width.PACS numbers: 73.20. At, 68.37.Ef, 73.21.Fg, 71.20.Gj In many electron systems ubiquitous interactions between quasiparticles such as electron-electron, electronphonon or electron-magnon scattering are present and cause inelastic scattering. A fingerprint of this dynamics, which occurs on a femtosecond timescale, is the width of spectroscopic lines. Often, elastic scattering is significant too, except for well defined surface states at special points in the surface Brillouin zone (SBZ). Thus, surface resonances exhibit a finite width owing to a degree of mixing with the bulk continuum. Defect scattering can also increase the line width of electron states at artificial atomic arrays or at electron confining island [1,2,3,4]. Adsorbates on a surface lead to additional scattering channels that may change the dynamics of electrons at surfaces and at the same time can be an important source of elastic scattering for electrons [5]. An intriguing situation occurs when an extended ordered superstructure changes the periodicity of a surface, as in the case of submonolayer coverages of alkali atoms on (111) noble metal surfaces. The SBZs of the substrate and of the overlayer have different sizes. The resulting back folding of the bands can lead to large effects in the dynamics of electrons at the interface, like the appearance of an elastic width of quantum well states (QWS) when energy gaps are closed in certain regions of the SBZ.Inverse photoemission and two-photon photoemission are widely used techniques to probe the dynamics of hot electrons at surfaces [6,7]. Recently, the different mechanisms that contribute to the elastic and inelastic scattering of hot electrons at Cu(001) surfaces with a low coverage of Cu adatoms have been identified [8] using these techniques. The conclusions are deduced from information taken over the whole surface area without probing the local electronic structure. Scanning tunneling spectroscopy (STS) resolves the local electronic structure of surface areas free from defects. It also can provide detailed information about both occupied and unoccupied states. The lifetime broadening of surface states can be extracted from STS measurements by doing a line shape analysis [9,10,11,12] or from electron standing wave patterns [13,14,15]. The progress of these experimental approaches clearly requires refined theoretical methods if one aims to explain experimental data at a quantitative level.Here, in a combined experimental and theoretical study, we analyze the processes that contribute to t...

Scanning tunneling spectroscopy (STS) is used to study the dynamics of hot electrons trapped on a Cu(100) surface in field emission resonances (FER) above the vacuum level. Differential conductance maps show isotropic electron interference wave patterns around defects whenever their energy lies within a surface projected band gap. Their Fourier analysis reveals a broad wave vector distribution, interpreted as due to the lateral acceleration of hot electrons in the inhomogeneous tipinduced potential. A line-shape analysis of the characteristic constant-current conductance spectra permits to establish the relation between apparent width of peaks and intrinsic line-width of FERs, as well as the identification of the different broadening mechanisms.

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Molecular beams have been used to search for evidence for a weakly bound molecular precursor in the interaction of O2 with Al(111). The experiments are consistent with a precursor whose binding energy is smaller than 0.1 eV. The total reflectivity as a function of incidence angle shows a pronounced dip at 25° for Etrans between 90 and 300 meV. This feature corroborates an earlier observation by Österlund et al. in sticking measurements. Modeling using a reduced dimensionality potential energy surface shows a similar behavior which is caused by steering into a shallow molecular adsorption well located at the same site in the unit cell as the maximum in the barrier towards dissociative adsorption. This effect is not observed if the molecular adsorption well is located at the same site as the minimum energy pathway to dissociative adsorption.

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