Visualizing the gas-sensitive structure of the CuZn surface in methanol synthesis catalysis.

Methanol formation over Cu/ZnO catalysts is linked with a catalytically active phase created by contact between Cu nanoparticles and Zn species whose chemical and structural state depends on reaction conditions. Herein, we use variable-temperature scanning tunneling microscopy at elevated pressure conditions combined with X-ray photoelectron spectroscopy measurements to investigate the surface structures and chemical states that evolve when a CuZn/Cu(111) surface alloy is exposed to reaction gas mixtures. In CO hydrogenation conditions, Zn stays embedded in the CuZn surface, but once CO gas is added to the mixture, the Zn segregates onto the Cu surface.

Origin of hydroxyl pair formation on reduced anatase TiO(101).

The interaction of water with metal oxide surfaces is of key importance to several research fields and applications. Because of its ability to photo-catalyze water splitting, reducible anatase TiO (a-TiO) is of particular interest. Here, we combine experiments and theory to study the dissociation of water on bulk-reduced a-TiO(101).

The interface ofgrown single-layer epitaxial MoSon SrTiO(001) and (111).

SrTiO(STO) is a versatile substrate with a high dielectric constant, which may be used in heterostructures with 2D materials, such as MoS, to induce interesting changes to the electronic structure. STO single crystal substrates have previously been shown to support the growth of well-defined epitaxial single-layer (SL) MoScrystals. The STO substrate is already known to renormalize the electronic bandgap of SL MoS, but the electronic nature of the interface and its dependence on epitaxy are still unclear.

Atomic-Scale View of the Oxidation and Reduction of Supported Ultrathin FeO Islands.

By means of scanning tunneling microscopy (STM) measurements, we studied the oxidation and reduction of FeO bilayer islands on Au(111) by oxygen (O) and hydrogen (H), respectively. The FeO islands respond very dynamically toward O, with the coordinatively unsaturated ferrous (CUF) sites at the island edges being essential for O dissociation and O atom incorporation. An STM movie obtained during oxidation reveals how further O molecules can dissociate after the consumption of all initially existing CUF sites through the formation of new CUF sites.

Supramolecular Corrals on Surfaces Resulting from Aromatic Interactions of Nonplanar Triazoles.

Interaction forces between aromatic moieties, often referred to as π-π interactions, are an important element in stabilizing complex supramolecular structures. For supramolecular self-assembly occurring on surfaces, where aromatic moieties are typically forced to adsorb coplanar with the surface, the possible role of intermolecular aromatic interactions is much less explored. Here, we report on unusual, ring-shaped supramolecular corral surface structures resulting from adsorption of a molecule with nonplanar structure, allowing for intermolecular aromatic interactions.

Edge reactivity and water-assisted dissociation on cobalt oxide nanoislands.

Transition metal oxides show great promise as Earth-abundant catalysts for the oxygen evolution reaction in electrochemical water splitting. However, progress in the development of highly active oxide nanostructures is hampered by a lack of knowledge of the location and nature of the active sites. Here we show, through atom-resolved scanning tunnelling microscopy, X-ray spectroscopy and computational modelling, how hydroxyls form from water dissociation at under coordinated cobalt edge sites of cobalt oxide nanoislands.

Revealing the structural detail of individual polymers using a combination of electrospray deposition and UHV-STM.

A phenylene vinylene polymer derivative is deposited onto a Au(111) surface under Ultra-High Vacuum (UHV) conditions using electrospray ionisation deposition and characterised using Scanning Tunnelling Microscopy (STM). High resolution STM images reveal the polymer structure on the monomeric scale, allowing the identification of regioisomerism, the intricate isomerisations of the polymer side-chains, as well as the larger-scale topologies of the polymer strands.

Symmetry-Driven Band Gap Engineering in Hydrogen Functionalized Graphene.

Band gap engineering in hydrogen functionalized graphene is demonstrated by changing the symmetry of the functionalization structures. Small differences in hydrogen adsorbate binding energies on graphene on Ir(111) allow tailoring of highly periodic functionalization structures favoring one distinct region of the moiré supercell. Scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements show that a highly periodic hydrogen functionalized graphene sheet can thus be prepared by controlling the sample temperature (T) during hydrogen functionalization.

Unravelling Site-Specific Photo-Reactions of Ethanol on Rutile TiO2(110).

Finding the active sites of catalysts and photo-catalysts is crucial for an improved fundamental understanding and the development of efficient catalytic systems. Here we have studied the photo-activated dehydrogenation of ethanol on reduced and oxidized rutile TiO2(110) in ultrahigh vacuum conditions. Utilizing scanning tunnelling microscopy, various spectroscopic techniques and theoretical calculations we found that the photo-reaction proceeds most efficiently when the reactants are adsorbed on regular Ti surface sites, whereas species that are strongly adsorbed at surface defects such as O vacancies and step edges show little reaction under reducing conditions.

Characterization of a new electrostatic storage ring for photofragmentation experiments.

We describe the design of and the first commissioning experiments with a newly constructed electrostatic storage ring named SAPHIRA (Storage Ring in Aarhus for PHoton-Ion Reaction Analysis). With an intense beam of Cu(-) at 4 keV, the storage ring is characterized in terms of the stored ion beam decay rate, the longitudinal spreading of an injected ion bunch, as well as the direct measurements of the transverse spatial distributions under different conditions of storage. The ion storage stability in SAPHIRA was investigated systematically in a selected region of its electrical configuration space.

Direct Visualization of Catalytically Active Sites at the FeO-Pt(111) Interface.

Within the area of surface science, one of the "holy grails" is to directly visualize a chemical reaction at the atomic scale. Whereas this goal has been reached by high-resolution scanning tunneling microscopy (STM) in a number of cases for reactions occurring at flat surfaces, such a direct view is often inhibited for reaction occurring at steps and interfaces. Here we have studied the CO oxidation reaction at the interface between ultrathin FeO islands and a Pt(111) support by in situ STM and density functional theory (DFT) calculations.

Unraveling the edge structures of platinum(111)-supported ultrathin FeO islands: the influence of oxidation state.

We used high-resolution scanning tunneling microscopy to study the structure of ultrathin FeO islands grown on Pt(111). Our focus is particularly on the edges of the FeO islands that are important in heterogeneous catalysis, as they host the active sites on inversed catalysts. To imitate various reaction environments we studied pristine, oxidized, and reduced FeO islands.

Nucleation and growth of Pt nanoparticles on reduced and oxidized rutile TiO₂ (110).

The nucleation and growth of Pt nanoparticles (NP's) on rutile TiO2 (110) surfaces with O on-top atoms (oxidized TiO2), surface O vacancies, and H adatoms, respectively (reduced TiO2), was studied by means of scanning tunneling microscopy (STM) experiments and density functional theory calculations. At room temperature, Pt was found to be trapped at O on-top atoms and surface O vacancies, leading to rather small Pt NP's. In contrast, on surfaces with H adatoms the mobility of Pt was much larger.

Formation and sintering of Pt nanoparticles on vicinal rutile TiO₂ surfaces.

By means of scanning tunnelling microscopy (STM) the nucleation, growth and sintering of platinum nanoparticles (Pt NP's) was studied on vicinal and flat rutile titanium dioxide (TiO2) surfaces. Utilising physical vapour deposition, the nucleation of Pt NP's on TiO2 surfaces at room temperature (RT) was found to be random and invariant towards different surface morphologies and reduction states. Thus, the nucleation of Pt on TiO2 at RT is rather insensitive to the surface structure and surface defects.

Fragmentation pathways of H+(H2O)2 after extreme ultraviolet photoionization.

Photofragmentation of the protonated water dimer H+(H2O)_{2}, a fundamental system both in aqueous solutions and gas-phase water clusters, has been studied at 13.8 nm using the Free Electron Laser FLASH in Hamburg. In a crossed-beam experiment using time-resolved, single-molecule fragment imaging, the two-body breakup into H2O++H3O+ was found as a prominent fragmentation channel with a kinetic energy release of up to 10 eV.

Rotational cooling of HD+ molecular ions by superelastic collisions with electrons.

Merging an HD+ beam with velocity matched electrons in a heavy ion storage ring we observed rapid cooling of the rotational excitations of the HD+ ions by superelastic collisions (SEC) with the electrons. The cooling process is well described using theoretical SEC rate coefficients obtained by combining the molecular R-matrix approach with the adiabatic nuclei rotation approximation. We verify the DeltaJ=-2 SEC rate coefficients, which are predicted to be dominant as opposed to the DeltaJ=-1 rates and to amount to (1-2)x10;{-6} cm;{3} s;{-1} for initial angular momentum states with J< or =7, to within 30%.

Dissociation lifetime studies of doubly deprotonated angiotensin peptides.

The doubly deprotonated [Asn,Val5] angiopeptide, in the gas phase, was irradiated with 266 nm photons. The time of flight (TOF) of the products formed following photoabsorption, namely, the monoanion and neutral fragments, was recorded with submicrosecond time resolution. Monte Carlo simulations of the TOF of the neutral fragments indicate that the dissociation occurs faster than 100 ns.

Photodissociation pathways of gas-phase photoactive yellow protein chromophores.

The absorption dynamics of two model chromophores of the photoactive yellow protein were studied in gas-phase experiments. Using different time-resolving techniques with an overall sensitivity ranging from seconds down to a few nanoseconds, complex dynamics were revealed for the p -coumaric acid anion, involving both fragmentation and electron detachment as possible photoresponse channels. For the trans-thiophenyl-p-coumarate model, despite its more complex molecular structure, simpler decay dynamics showing only fragmentation were observed.

Measured lifetime of SF6-.

The lifetime of the SF6 anion was measured at the electrostatic ion storage ring ELISA, where decays in the time span from 100 mus to a few seconds were recorded. We find a nonexponential decay with an approximate t(-1.5) power-law dependence.

Probing the sub-microsecond photodissociation dynamics in gas-phase retinal chromophores.

The photoinduced fragmentation of a retinal model chromophore (all-trans-n-butyl protonated Schiff-base retinal) was studied in vacuo using a new experimental technique. The apparatus is able to record the photodissociation yield of gas-phase biomolecular ions in the first microseconds after absorption. Together with the existing ion storage ring ELISA, which operates on the millisecond to second time scale, the complete decay dynamics of such molecules can now be followed.

Absorption studies of neutral retinal Schiff base chromophores.

The neutral retinal Schiff base is connected to opsin in UV sensing pigments and in the blue-shifted meta-II signaling state of the rhodopsin photocycle. We have designed and synthesized two model systems for this neutral chromophore and have measured their gas-phase absorption spectra in the electrostatic storage ring ELISA with a photofragmentation technique. By comparison to the absorption spectrum of the protonated retinal Schiff base in vacuo, we found that the blue shift caused by deprotonation of the Schiff base is more than 200 nm.

The gas-phase absorption spectrum of a neutral GFP model chromophore.

We have studied the gas-phase absorption properties of the green fluorescent protein (GFP) chromophore in its neutral (protonated) charge state in a heavy-ion storage ring. To accomplish this we synthesized a new molecular chromophore with a charged NH(3) group attached to a neutral model chromophore of GFP. The gas-phase absorption cross section of this chromophore molecule as a function of the wavelength is compared to the well-known absorption profile of GFP.

Effects of molecular rotation in low-energy electron collisions of H3+.

Measurements on the energetic structure of the dissociative recombination rate coefficient in the millielectronvolt range are described for H3+ ions produced in the lowest rotational levels by collisional cooling and stored as a fast beam in the magnetic storage ring TSR (Test Storage Ring). The observed resonant structure is consistent with that found previously at the storage ring facility CRYRING in Stockholm, Sweden; theoretical predictions yield good agreement on the overall size of the rate coefficient, but do not reproduce the detailed structure. First studies on the nuclear spin symmetry influencing the lowest level populations show a small effect different from the theoretical predictions.

Novel retinylidene iminium salts for defining opsin shifts: synthesis and intrinsic chromophoric properties.

Retinal Schiff bases serve as chromophores in many photoactive proteins that carry out functions such as signalling and light-induced ion translocation. The retinal Schiff base can be found as neutral or protonated, as all-trans, 11-cis or 13-cis isomers and can adopt different conformations in the protein binding pocket. Here we present the synthesis and characterisation of isomeric retinylidene iminium salts as mimics blocked towards isomerisation at the C11 position and conformationally restrained.

S1 and S2 excited States of gas-phase Schiff-base retinal chromophores.

Photoabsorption studies of 11-cis and all-trans Schiff-base retinal chromophore cations in the gas phase have been performed at the electrostatic ion storage ring in Aarhus. A broad absorption band due to the optically allowed excitation to the first electronically excited singlet state (S1) is observed at around 600 nm. A second "dark" excited state (S2) just below 400 nm is reported for the first time.