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. As a result, large Pt NP's were found at room temperature on TiO2 (110) surfaces with H adatoms. However, at ∼150 K the diffusion of Pt was kinetically hindered on all TiO2 (110) surfaces considered. STM data acquired after vacuum-annealing at 800 K showed comparable results on all TiO2 (110) surfaces because the diffusion of Pt is not influenced by surface defects at such high temperatures.

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. This channel was observed with at least a similar yield as events with stronger fragmentation, producing protons together with neutral fragments and showing an absolute cross section of (0.5 ± 0.2) × 10(-18) cm2.

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. A similar experiment performed on the Val5 angiopeptide also yielded a dissociation time shorter than 100 ns. We suggest dissociation mechanisms that account for the different number of photons required for the release of CO2.

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. Surprisingly, we find that an additional water molecule acts to promote all the elementary steps of the dissociation process and subsequent hydrogen migration, revealing the important assisting role of a water molecule in its own dissociation process on a metal oxide. Inspired by the experimental findings, we theoretically model the oxygen evolution reaction activity of cobalt oxide nanoislands and show that the nanoparticle metal edges also display favourable adsorption energetics for water oxidation under electrochemical 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.

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%.

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. The chromophore has a maximum absorption at 415 +/- 5 nm. When corrected for the presence of the charged group attached to the GFP model chromophore, the unperturbed neutral chromophore is predicted to have an absorption maximum at 399 nm in vacuum. This is very close to the corresponding absorption peak of the protein at 397 nm. Together with previous data obtained with an anionic GFP model chromophore, the present data show that the absorption of GFP is primarily determined by intrinsic chromophore properties. In other words, there is strong experimental evidence that, in terms of absorption, the conditions in the hydrophobic interior of this protein are very close to those in vacuum.

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. It is located approximately 1.2 eV above S1 for both chromophores. The S2 state was not visible in a solution measurement where only one highly blueshifted absorption band corresponding to the first excited state was visible. Knowledge of the position of the excited states in retinal is essential for the understanding of the fast photoisomerization in, for example, visual pigments.

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. Heating processes in the residual gas and by collisions with energetic electrons, as well as cooling owing to interaction with cold electrons, were observed in long-time storage experiments, using the low-energy dissociative recombination rate coefficient as a probe, and their consistency with the recent cold H3+ measurements is discussed.

High-resolution dissociative recombination of cold H3+and first evidence for nuclear spin effects.

The energy-resolved rate coefficient for the dissociative recombination (DR) of H(3)(+) with slow electrons has been measured by the storage-ring method using an ion beam produced from a radiofrequency multipole ion trap, employing buffer-gas cooling at 13 K. The electron energy spread of the merged-beams measurement is reduced to 500 microeV by using a cryogenic GaAs photocathode. This and a previous cold- measurement jointly confirm the capability of ion storage rings, with suitable ion sources, to store and investigate H(3)(+) in the two lowest, (J,G) = (1,1) and (1,0) rotational states prevailing also in cold interstellar matter. The use of para-H(2) in the ion source, expected to enhance para-H(3)(+) in the stored ion beam, is found to increase the DR rate coefficient at meV electron energies.

Two- and three-body kinematical correlation in the dissociative recombination of H(3)(+).

Fragmentation patterns for dissociative recombination of the triatomic hydrogen molecular ion H(3)(+) in the vibrational ground state have been measured using the storage ring technique and molecular fragment imaging. A broad distribution of vibrational states in the H(2) fragment after two-body dissociation and a large predominance of nearly linear momentum geometries after three-body dissociation are found. The fragmentation results are directly contrasted with Coulomb explosion imaging data on the initial H(3)(+) geometry, compared to existing wave-packet calculations, and considered in the light of a simple physical picture.

Evidence for subthermal rotational populations in stored molecular ions through state-dependent dissociative recombination.

We demonstrate that the dissociative recombination of D2H+ with low-energy electrons depends on the rotational energy of the molecular ion such that highly excited ions have a larger rate coefficient than colder ones. Observations on an ion beam continuously interacting with electrons at low relative velocity indicate that excited rotational levels are preferentially depleted which, in competition with radiative heating due to blackbody radiation, provides an opportunity for controlling the rotational temperature of stored molecules.