U. Höfer

Desorption kinetics of hydrogen from the Si(111)7×7 surface

Thermal desorption of hydrogen from the Si(111)7×7 surface was investigated using optical second‐harmonic generation to monitor the hydrogen coverage from 0.2 monolayer to below 0.01 monolayer. The results of isothermal desorption measurements are found to be compatible neither with simple first nor second‐order kinetic behavior. It is suggested that different binding sites available for Si–H monohydride states on the Si(111)7×7 surface give rise to the apparent intermediate reaction order.

Nonlinear optical investigations of the dynamics of hydrogen interaction with silicon surfaces

Optical second-harmonic generation (SHG) from silicon surfaces may be resonantly enhanced by dangling-bond-derived surface states. The resulting high sensitivity to hydrogen adsorption combined with unique features of SHG as an optical probe has been exploited to study various kinetical and dynamical aspects of the adsorption system H2/Si. Studies of surface diffusion of H/Si(111)7×7 and recombinative desorption of hydrogen from Si(111)7 × 7 and Si(100)2 × 1 revealed that the covalent nature of hydrogen bonding on silicon surfaces leads to high diffusion barriers and to desorption kinetics that strongly depend on the surface structure. Recently, dissociative adsorption of molecular hydrogen on Si(100)2×1 and Si(111)7×7 could be observed for the first time by heating the surfaces to temperatures between 550 K and 1050 K and monitoring the SH response during exposure to a high flux of H2 or D2. The measured initial sticking coefficients for a gas temperature of 300K range from 10−9 to 10−5 and strongly increase as a function of surface temperature. These results demonstrate that the lattice degrees of freedom may play a decisive role in the reaction dynamics on semiconductor surfaces.

Time-Resolved Investigation of Coherently Controlled Electric Currents at a Metal Surface

Studies of current dynamics in solids have been hindered by insufficiently brief trigger signals and electronic detection speeds. By combining a coherent control scheme with photoelectron spectroscopy, we generated and detected lateral electron currents at a metal surface on a femtosecond time scale with a contact-free experimental setup. We used coherent optical excitation at the light frequencies ωa and ωa/2 to induce the current, whose direction was controlled by the relative phase between the phase-locked laser excitation pulses. Time- and angle-resolved photoelectron spectroscopy afforded a direct image of the momentum distribution of the excited electrons as a function of time. For the first (n = 1) image-potential state of Cu(100), we found a decay time of 10 femtoseconds, attributable to electron scattering with steps and surface defects.

Vibrationally assisted electronic desorption: Femtosecond surface chemistry of O2/Pd(111)

The process of desorption for the system of O2/Pd(111) under excitation by 100 fs pulses of visible light has been examined. Molecular desorption is found to occur with high efficiency and a nonlinear dependence on laser fluence. Direct time‐domain measurements using a two‐pulse correlation scheme reveal a dominant subpicosecond response together with a weaker, but significant correlation persisting for tens of picoseconds. These results imply a desorption process driven by the high electron temperatures produced by the femtosecond laser radiation. The slower component of the correlation response is interpreted as an enhancement of the desorption rate by adsorbate vibrational excitation.

Effect of beam energy and surface temperature on the dissociative adsorption of H2 on Si(001)

Dissociative adsorption of H2 from a high-flux supersonic molecular beam on flat and vicinal Si(001) surfaces was investigated by means of optical second harmonic generation (SHG). The initial sticking coefficients for terrace adsorption varied between 10−8 and 10−4. They revealed a strongly activated dissociation process, both with respect to the kinetic energy of the incident molecules (70 meV⩽Ekin⩽380 meV) and the surface temperature (440 K⩽Ts⩽670 K). The results indicate that dynamical distortions of Si surface atoms can lower the effective adsorption barriers from 0.8±0.2 eV to almost negligible values. Previously proposed defect-mediated processes can be ruled out as a major adsorption channel.

Influence of Xe adlayer morphology and electronic structure on image-potential state lifetimes of Ru(0001)

Abstract The dynamics of image-potential states of clean and Xe-covered Ru(0001) was investigated by means of time-resolved two-photon photoemission. Xe adsorption is found to cause a substantial increase of the lifetime of the first image-potential state from 11 fs for the clean surface to 125 fs for a bilayer of Xe. The lifetimes in the presence of the commensurate and the incommensurate monolayer phases differ distinctly; 36 fs and 52 fs, respectively, were obtained on the two phases. The lifetime of the second image-potential state shows a weaker dependence on Xe coverage. A simple tunneling model and a dielectric continuum model are used to estimate the reduced coupling of the wave function of the image-potential states with the metallic substrate which is responsible for the increased lifetime in the presence of Xe layers.

Structural Evolution of Perfluoro-Pentacene Films on Ag(111): Transition from 2D to 3D Growth

The structural evolution and thermal stability of perfluoro-pentacene (PF-PEN) thin films on Ag(111) have been studied by means of low-temperature scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy (TDS). Well-defined monolayer films can be prepared by utilizing the different adsorption energy of mono- and multilayer films and selectively desorbing multilayers upon careful heating at 380 K, whereas at temperatures above 400 K, a dissociation occurs. In the first monolayer, the molecules adopt a planar adsorption geometry and form a well-ordered commensurate (6 × 3) superstructure where molecules are uniformly oriented with their long axis along the <110> azimuth. This molecular orientation is also maintained in the second layer, where molecules exhibit a staggered packing motif, whereas further deposition leads to the formation of isolated, tall islands. Moreover, on smooth silver surfaces with extended terraces, growth of PF-PEN onto beforehand prepared long-range ordered monolayer films at elevated temperature leads to needle-like islands that are uniformly aligned at substrate steps along <110> azimuth directions.

Electronic structure at the perylene-tetracarboxylic acid dianhydride/Ag(111) interface studied with two-photon photoelectron spectroscopy

The electronic structure of the prototype metal/organic contact 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) on a Ag(111)-surface has been investigated using time- and angle-resolved two-photon photoelectron spectroscopy (2PPE). Our analysis addresses particularly the nature of the interface state (IS) emerging at the interface due to the substrate-adsorbate interaction [C. H. Schwalb, S. Sachs, M. Marks et al., Phys. Rev. Lett. 101, 146801 (2008)]. Its free-electron-like dispersion and a possible backfolding at the surface Brillouin zone boundaries are discussed. Time-resolved pump-probe experiments reveal the inelastic electron lifetime along the dispersion parabola and show its decrease for increasing parallel momentum. The temperature dependence of the peak linewidth indicates a coupling of the IS to molecular vibrations. Moreover, additional aspects are addressed, such as the determination of the electron attenuation length of photoelectrons for low kinetic energy originating from the IS and the work function change of the sample upon PTCDA adsorption with very high energy resolution.

Momentum-resolved dynamics of Ar/Cu(1 0 0) interface states probed by time-resolved two-photon photoemission

The electron dynamics of buried Ar/Cu(1 0 0) image-potential states was investigated by time-resolved two-photon photoemission (2PPE) as a function of parallel momentum. The first interface state shows a parabolic dispersion with an effective mass of 0.6. Its lifetime of 110 fs at the ¯ � -point decreases with increasing parallel momentum. The momentum dependence of the decay can be understood by intra- and inter-band decay processes mediated by Cu electrons, just as the decay of image-potential states on the clean Cu(1 0 0) surface.

Optical second-harmonic investigations of the isothermal desorption of SiO from the Si(100) and Si(111) surfaces

The isothermal desorption of SiO from the Si(100) and Si(111) surfaces was investigated by means of optical second-harmonic generation (SHG). Due to the high adsorbate sensitivity of this method, desorption rates could be measured over a wide range from 10’1 to 10’6 ML s’1. From their temperature dependence between 780 and 1000 K, activation energies of E A =3.4±0.2 eV and E A =4.0±0.3 eV and pre-exponential factors of n 0 =1016±1 s’1 and n 0 =1020±1 s’1 for SiO desorption were obtained for Si(100) and Si(111), respectively. In the case of the Si(100) surface, a pronounced decrease of the first-order rate constants was observed upon increasing the initial coverage from 0.02 to 0.6 ML. The results are interpreted in terms of coverage-dependent oxygen-binding configurations, which influence the stability of the oxide layer.