J. Hasselström

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...

The bonding of CO to metal surfaces

The atom and symmetry specific properties of x-ray emission spectroscopy have been applied to the investigation of CO adsorbed on Ni(100) and Cu(100) surfaces. In comparison to ab initio electronic structure calculations, obtained in density functional theory, we develop a consistent electronic structure model of CO adsorption on transition and noble metals and extend to a conceptual model of the surface chemical bond. A strong CO–substrate interaction is found, characterized by significant hybridization of the initial CO orbitals and the metal bands. In the π system an allylic configuration is found as the result of orbital mixing between the CO 1π, 2π* and the metal dπ-band which is manifested experimentally in the observation of an oxygen lone-pair state. In the σ system experimental evidence of equally strong orbital mixing has been found. Energetically, the adsorbate–substrate complex is stabilized by the π-interaction but is destabilized by the σ-interaction. Furthermore, the internal C–O bond carri...

C 1s ionisation potential and energy referencing for solid C60 films on metal surfaces

We present a measurement of the C Is ionisation potential (IP) for solid C-60, as well as a reliable determination of the highest occupied molecular orbital (HOMO) IP. The difference between the gas phase and solid state measurements for the HOMO agrees w

Probing Chemical Bonding in Adsorbates using X-ray Emission Spectroscopy

Abstract When a molecule is adsorbed on a metal surface by chemical bonding new electronic states are formed. The direct observation and identification of these states has been an experimental challenge. Their signature is often obscured by bulk substrate states. In the following contribution we will show how X-ray emission spectroscopy (XES), in spite of its inherent bulk sensitivity, can be used to investigate adsorbed molecules. Due to the localization of the core-excited intermediate state, XE spectroscopy allows an atom specific separation of the valence electronic states. Thus the molecular contributions to the surface chemical bond can be separated from those of the substrate. Furthermore, angle dependent measurements make it possible to determine the symmetry of the molecular states, i.e. the separation of π and α type states. Density functional theory calculations in the frozen orbital approximation can describe the XE spectra with a good agreement with experiments. In all we can obtain an atomic view of the electronic states involved in the formation of the chemical bond to the surface. We will show how the electronic structure in simple atomic adsorbates on Cu and Ni surfaces can be related to the concept of less or more noble metals. We also show how new molecular states are formed in adsorbed N 2 and CO on Ni(100). The resulting strength of the adsorbate bond comes from a delicate balance between π bonding and σ repulsion. We can use an additional symmetry selection rule for adsorbed molecules with equivalent atoms where π and π * states can be selectively enhanced depending on the nature of the primary excited state. This will be demonstrated for ethylene and benzene adsorbed on Cu(110). The future prospect is illustrated by the adsorption of formate and ammonia, on Cu(110). These adsorbates represent the interaction of functional groups in amino acids, which are an important class of biological molecules involved in building proteins.

Ammonia adsorbed on Cu(110): An angle resolved x-ray spectroscopic and ab initio study

We present a study of a monolayer of ammonia (NH3) adsorbed on Cu(110) using core level spectroscopies in combination with ab initio calculations based on density functional theory. In particular, x-ray emission spectroscopy has been applied, providing an unsurpassed view of the electronic structure of NH3 upon adsorption. The saturated NH3 monolayer, Θ∼0.4 ML, is found to induce strong adsorbate–adsorbate interaction, causing the molecules to tilt on the surface. Based on the angular distribution of the x-ray emission (XE) spectra, we have been able to estimate a mean tilt angle from the surface normal of 40°–45° for the saturated monolayer; the accompanying theoretical calculations for up to three NH3 molecules on a Cu21 all-electron cluster model support a tilted structure due to adsorbate–adsorbate dipole, and possibly hydrogen bonding, interactions. Since the creation of a core hole on the nitrogen atom site in the intermediate state of the XE process does not affect the symmetry of the molecule, a s...

The bonding of simple carboxylic acids on Cu(110)

We present a study of the bonding of formate (HCOO) and acetate (CH3COO) chemisorbed on Cu(110) using core level spectroscopies in combination with theoretical calculations. For the first time, we apply x-ray emission spectroscopy (XES) to these systems. When XES is used in conjunction with x-ray absorption spectroscopy (XAS) and ab initio calculations, new information about the electronic interaction in the adsorbate–substrate system is provided. In particular, we have used the azimuthal orientation of the COO–surface bond on the (110) surface, to make a complete partition into x, y, and z orbital contributions. The surface bond is found to be predominantly ionic. For the case of adsorbed formate, the covalent bonding is dominated by 6a1/7a1, (σ)-type, frontier orbitals, interacting with the Cu valence band. The resulting hybrid orbitals form a distribution of states that cross the Fermi level. The contribution from adsorbate π-type orbitals is small. The chemical bond formation of adsorbed acetate is ve...

Franck–Condon breakdown in core-level photoelectron spectroscopy of chemisorbed CO

Abstract The photon energy dependence of the vibrational fine structure in the C1s and O1s X-ray photoelectron main lines of chemisorbed CO on Ni(100) and Ru(0001) has been measured from 6 to 150 eV above the core-level thresholds. Significant deviations from the behavior in gas-phase CO are found. A strong dominance of the adiabatic peak towards threshold is found for the C1s, but not the O1s, lines. In the C1s lines, we observe a broad maximum of vibrational excitation 5 eV above the shape resonance. At high photon energies, Franck–Condon behavior is observed in both the C1s and O1s lines. This behavior is discussed in terms of the adsorbate electronic structure and the dynamic metallic screening upon core ionization.

Vibrational fine structure in core level photoelectron lines of adsorbed molecules: System dependent effects

System dependent effects in resolving vibrational fine structure in the corn-level lines of adsorbates are discussed. The example is carbon monoxide on transition metals. The importance of dynamic ...

New final state in the autoionization decay detected for N2/(2×2)K/graphite: Relevance for the affinity level of NO on a K monolayer

We have observed a new type of charge transfer state in the autoionization spectrum for N2/(2×2)K/graphite. It can be viewed as a consequence of an important covalent component to the bonding with the surface in the core hole excited state for N2/(2×2)K/graphite or equivalently for the Z+1 system, “NO”/(2×2)K/graphite.