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The detection of intermediate species and the correlation of their ultrafast dynamics with the morphology and electronic structure of a surface is crucial to fully understand and control heterogeneous photoinduced and photocatalytic reactions. In this work, the ultrafast photodissociation dynamics of CH3Br molecules adsorbed on variable-size Au clusters on MgO/Mo(100) is investigated by monitoring the CH3+ transient evolution using a pump-probe technique in conjunction with surface mass spectrometry. Furthermore, extreme-UV photoemission spectroscopy in combination with theoretical calculations is employed to study the electronic structure of the Au clusters on MgO/Mo(100). Changes in the ultrafast dynamics of the CH3+ fragment are correlated with the electronic structure of Au as it evolves from monomers to small nonmetallic clusters to larger nanoparticles with a metallic character. This work provides a new avenue to a detailed understanding of how surface-photoinduced chemical reactions are influenced by the composition and electronic structure of the surface.

The reaction of free di-manganese oxide clusters Mn2Ox+ (x = 2-7) with hydrogen in a flow tube reactor results in a strongly cluster size dependent complex formation: Mn2O3+ and Mn2O4+ are most reactive and adsorb up to two H2 molecules, the reactivity is reduced for Mn2O2+/Mn2O5+, and Mn2O6+/Mn2O7+ appear to be non-reactive. Infrared multiple-photon dissociation (IR-MPD) spectra of the complexes Mn2O4H2+ and Mn2O5H2+ reveal one band that shifts upon isotopic labelling of hydrogen, whereas the spectra of Mn2O2H2+ and Mn2O3H2+ do not show any clear spectral shifts upon H2/D2 exchange. Detailed analysis of the IR-MPD spectra in conjunction with density functional theory (DFT) calculations strongly indicate the molecular η2-binding of H2 to one of the Mn atoms. H2 dissociation via hydroxylation of the cluster oxo bridges yielding H-Mn(MH)O(OH)O2+ or Mn2(OH)2O2+ appears to be thermodynamically favourable but kinetically hampered. [ABSTRACT FROM AUTHOR]

The interaction of free manganese oxide clusters, MnxOy+ (x = 1–9, y = 0–12), with formic acid was studied via infrared multiple-photon dissociation (IR-MPD) spectroscopy together with calculations using density functional theory (DFT). Clusters containing only one Mn atom, such as MnO2+ and MnO4+, bind formic acid as an intact molecule in both the cis- and trans-configuration. In contrast, all clusters containing two or more manganese atoms deprotonate the acid's hydroxyl group. The coordination of the resulting formate group is strongly cluster-size-dependent according to supporting DFT calculations for selected model systems. For Mn2O2+ the co-existence of two isomers with the formate bound in a bidentate bridging and chelating configurations, respectively, is found, whereas for Mn2O4+ the bidentate chelating configuration is preferred. In contrast, the bidentate bridging structure is energetically considerably more favorable for Mn4O4+. This binding motif stabilizes the 2D ring structure of the core of the Mn4O4+ cluster with respect to the 3D cubic geometry of the Mn4O4+ cluster core. [ABSTRACT FROM AUTHOR]

The bound unoccupied electronic state structure of an Ir(111)/graphene surface covered by differently sized and spaced Ir clusters was investigated by means of two-photon photoemission spectroscopy. The cluster lattice was found to affect the image potential states of the substrate to a surprisingly large extent. This effect can be related to the influence of the cluster lattice on the screening of the image state electron trapped in front of the surface. The symmetric arrangement of Ir clusters considerably reduces the lateral extension of graphene areas with a homogeneous local work function, and from a certain minimum area size, the excitation of an electron to a stable state in a Coulomb-like potential is not possible anymore. Furthermore, lateral confinement effects could be observed due to the decreasing extension of bare graphene areas. [ABSTRACT FROM AUTHOR]

Series of time-resolved still images of the explosion dynamics of micrometer-sized water droplets after femtosecond laser-pulse irradiation were obtained for different laser-pulse intensities. Amplified pulses centered around a wavelength of 805 nm with 1-mJ energy and 60-fs duration were focused onto the droplet to initiate the dynamics. Several effects, such as forward and backward plumes, jets, water films, and shock waves, were investigated. Additionally, the influence of different pulse durations produced by chirping the laser pulses was observed. OCIS codes: 320.0320, 190.3970, 320.7120.

The reactions of the small gas-phase gold cluster cations Au3+ and Au5+ with N2, H2, and O2 as well as mixtures thereof were investigated in an octopole ion trap under multicollision conditions. While gold cations are inert toward molecular oxygen, a distinct reaction time and temperature dependent reaction behavior was observed toward H2 and N2. Introducing mixtures of the reactive gases to the ion trap revealed both, competitive and cooperative coadsorption effects: (i) A competitive displacement reaction was detected for the coadsorption of H2 and N2 indicating the molecular adsorption of these molecules onto the gold clusters. The enthalpy of the displacement reaction determined from equilibrium mass spectra was found to be small (<10 kJ/mol). (ii) Preadsorption of N2 enabled the cooperative coadsorption of O2 under special experimental conditions (low temperature and very small amounts of N2). In this surprising cooperative reaction even AuxO2y+ were formed via elimination of the initially activating nitrogen molecules, whereas such complexes were never detected in reactions of gold cluster cations with pure molecular oxygen. [ABSTRACT FROM AUTHOR]

A novel experimental approach to the investigation of surface adsorbate reaction dynamics is presented. The direct time-resolved monitoring of the surface reaction transition state and product formation dynamics were accomplished via pump-probe mass spectrometry. As an example, methyl iodide molecules adsorbed at submonolayer coverage on an ultrathin magnesia film on Mo(100) were photoexcited to the A-band by ultrafast laser pulse irradiation. Employing time-delayed multiphoton ionization the dynamics of the dissociative methyl iodide transition state and of the emerging methyl photoproduct could be detected with femtosecond resolution. The reaction times deduced from the temporal evolution of the methyl ion mass signal indicate a strong interaction of the methyl fragment with the substrate surface prior to desorption. [ABSTRACT FROM AUTHOR]

Reactions of free silver anions Ag[sub n][sup -] (n=1–13) with O[sub 2], CO, and their mixtures are investigated in a temperature controlled radio frequency ion trap setup. Cluster anions Ag[sub n][sup -] (n=1–11) readily react with molecular oxygen to yield Ag[sub n]O[sub m][sup -] (m=2, 4, or 6) oxide products. In contrast, no reaction of the silver cluster anions with carbon monoxide is detected. However, if silver cluster anions are exposed to the mixture of O[sub 2] and CO, new reaction products and a pronounced, discontinuous size dependence in the reaction behavior is observed. In particular, coadsorption complexes Ag[sub n](CO)O[sub 2][sup -] are detected for cluster sizes with n=4 and 6 and, the most striking observation, in the case of the larger odd atom number clusters Ag[sub 7][sup -], Ag[sub 9][sup -], and Ag[sub 11][sup -], the oxide product concentration decreases while a reappearance of the bare metal cluster signal is observed. This leads to the conclusion that carbon monoxide reacts with the activated oxygen on these silver clusters and indicates the prevalence of a catalytic reaction cycle. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The measurement of temperature- and reaction-time-dependent ion intensities, obtained by ion trap mass spectrometry, has shown the promotion and activation of molecular oxygen by preadsorbed hydrogen. The results have provided insights into hydrogen and oxygen cooperative adsorption effects and consequent reaction mechanisms.