Electronic Excitation and Dynamic Promotion of a Surface Reaction

Abstract

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...

Plasmonic hot carrier dynamics in solid-state and chemical systems for energy conversion

“…Surface photochemistry, chemical reactions driven by photoexcited carriers at metal surfaces, has been well studied in many contexts including solar energy conversion and atmospheric chemistry [37,[96][97][98]. The possibility of efficiently absorbing light and generating hot carriers in metal nanostructures using plasmon resonances has driven a lot of research in plasmon-driven photocatalysis [33,45,[99][100][101][102][103].…”

Solar-Powered Plasmon-Enhanced Heterogeneous Catalysis

“…In both cases, the plasmonic material can selectively deposit the energy of photons into specific adsorbate electronic states thus activating the desired chemical bonds [116,117]. The mechanism is also known as desorption induced by electronic transition and consists of three fundamental steps: (i) charge carriers injection into the targeted molecular orbitals; (ii) coupling between the excited electronic state of the metallic NP and the excited vibrational state of the adsorbate; (iii) formation of the transient negative ion and promotion to a more energetic potential energy surface ( Figure 6B) [118][119][120][121][122][123]. By forcing the adsorbate-metal system to move to a different PES, the energy of hot carriers is converted into kinetic energy of the complex and specific chemical bonds can be activated.…”

Hydrogen Oxidation-Driven Hot Electron Flow Detected by Catalytic Nanodiodes

“…Usually only a small fraction of the reaction heat is transferred to an electron-hole pair. A few chemical processes on high work-function metal surfaces have been recently discovered that transfer a substantial fraction of the reaction heat to the kinetic energy of individual metal electrons [1][2][3][4][5][6][7][8][9][10][11][12] . There is growing evidence which indicates that these energetic electrons, often called hot electrons, influence surface reactivity 13,14 .…”