Heating of a ruthenium surface on which carbon monoxide and atomic oxygen are coadsorbed leads exclusively to desorption of carbon monoxide. In contrast, excitation with femtosecond infrared laser pulses enables also the formation of carbon dioxide. The desorption is caused by coupling of the adsorbate to the phonon bath of the ruthenium substrate, whereas the oxidation reaction is initiated by hot substrate electrons, as evidenced by the observed subpicosecond reaction dynamics and density functional calculations. The presence of this laser-induced reaction pathway allows elucidation of the microscopic mechanism and the dynamics of the carbon monoxide oxidation reaction.

Effect of polydispersity, bimodality, and aspect ratio on the phase behavior of colloidal platelet suspensions J. Chem. Phys. 137, 134906 (2012) Autonomous colloidal crystallization in a galvanic microreactor J. Appl. Phys. 112, 074905 (2012) Colloidal cluster crystallization dynamics J. Chem. Phys. 137, 134901 (2012) Position-displacement correlations in QELSS spectra of non-dilute colloids J. Chem. Phys. 137, 124901 (2012) Photoluminescence dynamics in solid formulations of colloidal PbSe quantum dots: Three-dimensional versus two-dimensional films ZnS nanoparticles were prepared by chemical precipitation of Zn 2ϩ with sulfur ions in aqueous solution. The ultraviolet-excited samples reveal detailed structure in the luminescence spectra. A doublet pattern observed in the long wavelength region is attributed to the coexistence of the two crystalline forms in ZnS particles. The visible luminescent radiation at 590.1 nm is due to Mn impurities. The dominant emission band at short wavelengths exhibits a quadruple fine structure with peaks located at 416.1, 423.9, 430.1, and 437.8 nm which are identified with optical transitions arising from vacancy and interstitial sites for both Zn and S atoms.

Irradiation of a Ru͑001͒ surface covered with CO using intense femtosecond laser pulses ͑800 nm, 130 fs͒ leads to desorption of CO with a nonlinear dependence of the yield on the absorbed fluence ͑100-380 J/m 2 ͒. Two-pulse correlation measurements reveal a response time of 20 ps ͑FWHM͒. The lack of an isotope effect together with the strong rise of the phonon temperature ͑2500 K͒ and the specific electronic structure of the adsorbate-substrate system strongly indicate that coupling to phonons is dominant. The experimental findings can be well reproduced within a friction-coupled heat bath model. Yet, pronounced dynamical cooling in desorption, found in the fluence-dependence of the translational energy, and in a non-Arrhenius behavior of the desorption probability reflect pronounced deviations from thermal equilibrium during desorption taking place on such a short time scale.

The mechanism of recombinative desorption of hydrogen from a Ru(0001) surface induced by femtosecond-laser excitation has been investigated and compared to thermally initiated desorption. For the laser-driven process, it is shown that hot substrate electrons mediate the reaction within a few hundred femtoseconds resulting in a huge isotope effect between H 2 and D 2 in the desorption yield. In mixed saturation coverages, this ratio crucially depends on the proportions of H and D. Deviations from second order desorption kinetics demonstrate that the recombination is dynamically promoted by excitation of neighboring, but nonreacting adatoms. A concentration dependent rate constant which accounts for the faster excitation of H versus D is proposed. DOI: 10.1103/PhysRevLett.91.226102 PACS numbers: 82.65.+r, 68.43.Mn, 78.90.+t, 82.53.-k Chemical reactions involving species adsorbed on a metal surface are mediated through excitation of electrons and/or phonons of the substrate. Since thermal equilibration between these excitations occurs on a femtosecond (fs) to picosecond (ps) time scale, the rate normally may be described to a very good approximation within the framework of transition state theory [1] in terms of the temperature dependent rate constant and as a function of the surface concentrations. Rapid absorption of a fs-laser pulse by the conduction electrons of the substrate may, however, trigger the onset of a surface reaction before equilibration between the heat baths of electrons and phonons is reached, as has been exemplified in the reaction between O and CO adsorbed on a Ru(0001) surface yielding the release of CO 2 into the gas phase [2]. For adsorption on thin metal films with Schottky contact, it was recently demonstrated that nonadiabatic coupling to electron-hole pairs plays an important role in surface reactions and is not negligible even in low-energy processes in which phononic excitations are thought to dominate [3]. Hot electrons were proposed to routinely participate in substrate-mediated reactions contrary to the traditional picture of a thermal surface reaction, in which phonons solely drive the system over the reaction barrier in the electronic ground state.Coadsorbed species on a metal substrate can modify the electronic structure and hence influence the surface reactivity. Altering the height of the reaction barrier in the electronic ground state and/or energetic shifts of the potential energy surface in electronically excited states are typical consequences. In the case of catalytic promotion (e.g., by alkali atoms), these static changes in the electronic potential energy landscape result in an enhanced reaction rate [4]. In this Letter, we report on dynamic promotion of a prototype surface reaction, H ad H ad ! H 2;gas on Ru(0001). Hydrogen recombination may be initiated thermally (i.e., under conditions of thermal equilibrium between all degrees of freedom), but if induced by fs-laser excitation characteristic differences are observed: (i) The hydrogen molecules coming off the surf...

An experimental scheme ͑double pump/reflectivity probe using femtosecond laser pulses͒ enables the investigation of nonequilibrium electron dynamics at metal surfaces by measuring the equilibrated surface temperature. The competition between electron-phonon coupling and hot-electron transport gives rise to a reduced equilibrated temperature when the two pump pulses overlap in time, and provides a way of accurately determining the electron-phonon coupling constant. These observations have important consequences for femtosecond photochemical investigations.

Absorption of femtosecond near-infrared laser pulses in the surface near region of Ru(001) covered with atomic hydrogen induces recombinative desorption of its molecular species. The ultrafast time scale of this surface reaction as evidenced by two-pulse correlation measurements (fwhm of ∼1 ps) together with a nonlinear dependence of the reaction yield on the absorbed laser fluence reveals an electron-mediated reaction pathway involving nonadiabatic coupling between adsorbate vibrational degrees of freedom and transient electronhole pair excitations in the substrate. A pronounced isotope effect in the H 2 vs D 2 yield exhibits a strong fluence dependence (H 2 /D 2 ratio changes from 5:1 at 120 J/m 2 to ∼20:1 at 50 J/m 2 ). All experimental findings can be well described within the framework of electronic friction between the substrate and adsorbate degrees of freedom, with an effective activation energy of 1.35 eV and coupling times of 180 and 360 fs for H 2 and D 2 , respectively, as parameters. A pronounced coverage dependence of the desorption yield underlines that adsorbate-adsorbate interactions play a crucial role in the hydrogen recombination reaction.

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