Martin Weinelt

The adsorption structure of glycine adsorbed on Cu(110); comparison with formate and acetate/Cu(110)

The molecular orientation of an ordered monolayer of glycine adsorbed on Cu(110) has been studied using X-ray Photoelectron Spectroscopy (XPS), Near Edge X-ray Absorption Fine Structure (NEXAFS), X-ray Photoelectron Diffraction (XPD), Low-Energy Electron Diffraction (LEED) and theoretical calculations. In particular, the NEXAFS results are discussed in terms of the spectra of the related molecules ammonia (NH3), formate (HCOO), and acetate (CH3COO) on Cu(110). Whereas the latter two molecules chemisorb in similar geometries, glycine is found to assume a very different chemisorption geometry. Formate and acetate bond through two equivalent oxygen atoms with the molecular plane oriented nearly perpendicular to the surface, aligned along the [110]-azimuth. In the case of adsorbed glycine (NH2CH2COO), the azimuthal orientation is still present, i.e. the bonding oxygen atoms are aligned along the [110]-azimuth, but the molecule is found to bend towards the surface. A second chemisorption bond is formed at the nitrogen end of the molecule, involving copper atoms in the neighboring [110]-row. We therefore have the interesting case of a chemisorption bond involving different functional groups in the same molecule.

The electronic structure and surface chemistry of glycine adsorbed on Cu(110)

We present a combined density functional theory and x-ray emission spectroscopy study of the bonding and chemistry of glycine (NH2CH2COOH) chemisorbed on Cu(110). The amino acid deprotonates upon adsorption. The adsorbate exhibits a rich surface chemistry leading to several intermediate adsorption structures. The most stable geometry is found to involve both the carboxylic and amino functional end groups in the bond. This structure appears only after annealing to 400 K, which in the present work is attributed to a removal of surface or subsurface hydrogen from the metal. Comparison with experimental x-ray emission and near edge x-ray absorption fine structure (NEXAFS) spectra provide a detailed picture of the electronic structure for the most stable structure. This allows conclusions to be drawn regarding the covalent interaction of the adsorbate system. When combined with theoretical calculations addressing, e.g., the electrostatic adsorbate–substrate interaction, a complete picture of the surface chemic...

Time-resolved two-photon photoemission from metal surfaces

The Rydberg-like series of image-potential states is a prototype system for loosely bound electrons at a metal surface. The electronic structure and the femtosecond dynamics of these states is studied by high-resolution energy-and time-resolved two-photon photoemission spectroscopy. The electron trapped in the image potential moves virtually freely laterally to the surface where it is subject to inelastic and quasielastic scattering processes which cause decay of population and phase relaxation. The influence of surface corrugation on these processes has been investigated for adsorbates on Cu(001) and stepped Cu(117) and Cu(119) surfaces which are vicinal to Cu(001). The dynamics depend on both the distance of the electron in front of the surface and the parallel momentum. For CO molecules on Cu(001) inelastic scattering into bulk states and adsorbate-induced resonances determine the decay rate. For small numbers of Cu adatoms on Cu(001) and the vicinal surfaces the decay rate of image-potential states is significantly modified by interband and subsequent intraband scattering. On the vicinal surfaces the origin of the interband-scattering process is clarified as quasielastic scattering caused by disorder of the lateral superlattice. It occurs for electrons with group velocity perpendicular to the step edge and, moreover, exhibits a sizeable asymmetry. Electrons are mainly scattered into states with momentum in the upstairs direction. This asymmetry in quasielastic scattering explains the direction dependence of the lifetime of the first image-potential states on stepped Cu(119).

Hot-Electron-Driven Enhancement of Spin-Lattice Coupling in Gd and Tb 4 f Ferromagnets Observed by Femtosecond X-Ray Magnetic Circular Dichroism

Femtosecond x-ray magnetic circular dichroism was used to study the time-dependent magnetic moment of 4f electrons in the ferromagnets Gd and Tb, which are known for their different spin-lattice coupling. We observe a two-step demagnetization with an ultrafast demagnetization time of 750 fs identical for both systems and slower times which differ sizeably with 40 ps for Gd and 8 ps for Tb. We conclude that spin-lattice coupling in the electronically excited state is enhanced up to 50 times compared to equilibrium.

Structure and excitonic coupling in self-assembled monolayers of azobenzene-functionalized alkanethiols.

Optical properties and the geometric structure of self-assembled monolayers of azobenzene-functionalized alkanethiols have been investigated by UV/visible and near edge X-ray absorption fine structure spectroscopy in combination with density-functional theory. By attaching a trifluoro-methyl end group to the chromophore both the molecular tilt and twist angle of the azobenzene moiety are accessible. Based on this detailed structural analysis the energetic shifts observed in optical reflection spectroscopy can be qualitatively described within an extended dipole model. This substantiates sizable excitonic coupling among the azobenzene chromophores as an important mechanism that hinders trans to cis isomerization in densely packed self-assembled monolayers.

Femtosecond two-photon photoemission studies of image-potential states

Abstract High-resolution two-photon photoemission studies with femtosecond time resolution permit the accurate determination of decay and dephasing processes for image-potential states. The influence of adsorbates on the respective inelastic and quasielastic scattering processes is investigated for Cu on Cu(100) and Cu(111). The results are discussed in relation to previous work for CO on Cu(100).

Reversible photoisomerization of an azobenzene-functionalized self-assembled monolayer probed by sum-frequency generation vibrational spectroscopy

Sum-frequency generation (SFG) vibrational spectroscopy is employed to investigate the reversible, photoinduced trans/cis isomerization of an azobenzene-functionalized self-assembled monolayer (SAM) on a gold substrate. A C[triple bond]N marker group at the outer phenyl ring is used as a direct measure of the switching state. The azobenzene unit is connected to the surface by a tripodal linker system with an adamantane core, which results in both a sufficient decoupling of the functional azobenzene unit from the metallic substrate and a free volume to prevent steric hindrance, thus allowing the isomerization process. Optical excitation at 405 nm induces the trans-->cis isomerization, whereas light exposure at 470 nm leads to the back reaction. The effective cross sections for the reactions are sigma(eff)(cis) = 4 +/- 1 x 10(-18) cm(2) at 405 nm (trans-->cis) and sigma(eff)(trans) = 2.5 +/- 0.9 x 10(-19) cm(2) at 470 nm (cis-->trans). We propose that the photoisomerization is driven by a direct (intramolecular) electronic excitation of the azobenzene conjugate, analogous to the free molecules in solution.

Azimuthal reorientation of adsorbed molecules induced by lateral interactions: benzene/Ni(110)

Abstract The adsorption of benzene on the Ni(110) surface was studied by TPD, LEED, NEXAFS and ARUPS. At initial coverages below θC6D6 = 0.10 ML (relative to Ni) benzene completely decomposes upon heating. At higher coverages molecular benzene desorption is also observed. At the saturation coverage of 0.23 ± 0.04 ML a sharp c(4 × 2) LEED pattern is observed with weak additional reflexes indicating a larger unit mesh. The ARUPS measurements for a dilute layer (θdil ≃ 0.5θsat) suggest an orientation of the molecular plane parallel to the surface with C2v symmetry of the adsorption complex benzene/Ni(110) and the molecules azimuthally oriented with their corners along [001]. For the saturated layer the molecular plane is still parallel to the surface but the symmetry of the adsorption complex is lowered to C1, due to an azimuthal rotation of the benzene molecules induced by strong lateral interactions in the densely packed saturated layer. The 2a{lg} band shows strong dispersion (0.8 eV) with the periodicity of the 2D bandstructure corresponding to a c(4 × 2) structure. For the coadsorption of benzene + CO a sharp c(4 × 4) LEED structure is observed with identical CO and benzene coverages of ~ 0.12 ML. The electronic structure, orientation and symmetry of benzene in the coadsorbed layer is identical to that in the dilute pure layer. The photon energy dependence of the photoionization cross section of the 2e{lu} level of benzene in the saturated layer shows a maximum at a photon energy of 25 eV, similar to benzene on Ni(111); the corresponding electron emission is centered along the surface normal. The NEXAFS spectra for the dilute and the saturated layer at normal incidence are also indicative of a flat-lying benzene molecule.

The electronic structure of ethylene on Ni(110): an experimental and theoretical study

Abstract The electronic structure of ethylene adsorbed on Ni(110) has been studied by ARUPS using linearly polarized synchrotron radiation, and by LCGTO-LDF model cluster calculations. The adsorption system has also been characterized by TPD and LEED. The ARUPS measurement were performed for a dilute ethylene layer (θ ≈ 1 2 θ SAT ) , where lateral interactions are not important. From the detailed analysis of the polarization, polar angle and azimuthal dependence of the ARUPS spectra we deduce an orientation of the ethylene molecules with the molecular plane coplanar to the surface and the C-C axis preferentially aligned along the [1 1 0] azimuth. The symmetry of the adsorption complex is determined as C 1 . The ethylene π-orbital 1b 2u exhibits a differential shift of 1.1 eV to higher binding energy as compared to the free molecule. The corresponding theoretical value is found to be ∼ 1.7 eV. In the cluster calculations a partial optimization of the adsorbate geometry has been carried out assuming C 2 symmetry. The adsorption bonding is found to be quite similar in π and di-σ coordination, for both cases the π donation to the substrate being stronger than the π ∗ backdonation. The π donation is mediated mainly by Nis and p contributions, gp ∗ backdonation mostly by Ni d orbitals. The optimized geometry parameters for the π-bonded species are: CC: 1.42 A: NiC: 2.01 A tilting of a CH 2 group relative to the (110) crystal plane: 23°.

Sulphur dioxide adsorption on the Ni(110) surface

Abstract The adsorption of SO 2 on Ni(110) has been studied in detail by temperature programmed desorption (TPD), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and angle resolved UV photoelectron spectroscopy (ARUPS) using linearly polarized synchrotron radiation. At 100 K SO 2 is found to chemisorb in molecular form on Ni(110) with a saturation coverage of 0.5 ML (1 ML = 1 molecule Ni-atom ) . The saturated chemisorbed layer exhibits a c(2 × 2) LEED pattern. At initial coverages below 0.25 ML, SO 2 completely decomposes upon heating to 750 K; for higher coverages molecular SO 2 desorption is also observed in a single peak between 300 and 400 K. From the angle resolved UPS spectra and symmetry selection rules we deduce the symmetry of the adsorbed SO 2 molecules for low coverages as C s (σ yz ), with the molecular O-S-O plane oriented perpendicular to the surface and aligned along the [001] azimuth (perpendicular to the close-packed substrate rows). Within the plane the molecule is tilted, which is attributed to an additional Ni-O interaction. In the saturated layer the symmetry is further reduced to C 1 indicating an additional tilting and/or rotation of the molecular plane with respect to the substrate.