Electronically driven adsorbate excitation mechanism in femtosecond-pulse laser desorption

Brandbyge, Mads; Hedegård, Per; Heinz, Tony F.; Misewich, James A.; Newns, D. M.

doi:10.1103/physrevb.52.6042

Cited by 209 publications

(246 citation statements)

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“…Energy transfer from substrate to adsorbate: (a) Based on the two-temperature model [13], the calculated time profiles of the electron and the phonon temperature T el and T ph at the substrate surface are plotted for two laser pulses with a pulse separation of t 1 ps. Assuming a purely electronmediated energy transfer [11], the adsorbate temperatures T ads for an H and D saturation layer, respectively, result. In the lower section, the corresponding rates and their time integrals, the desorption yields, are displayed.…”

Section: P H Y S I C a L R E V I E W L E T T E R Smentioning

confidence: 99%

“…Relaxation occurs by electron transfer back to the substrate, and thereby the system returns to the ground state potential. This electronically nonadiabatic process causes coupling between electronic and nuclear degrees of freedom [19] and is also denoted as electronic friction [11,20]. Its multiple repetition eventually enables the adsorbed species to take up enough energy to react and desorb from the surface (desorption induced by multiple electronic transitions [17]).…”

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confidence: 99%

“…For theoretical modeling, the two-temperature model [13] in conjunction with the modified electronic friction model [11] accounting for the electron-mediated energy transfer from the Ru substrate to the H=D adsorbate is employed. In this model, the coupling time el between substrate and adsorbate is the inverse of the friction coefficient and scales proportionally with the mass of the adsorbate.…”

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confidence: 99%

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Electronic Excitation and Dynamic Promotion of a Surface Reaction

et al. 2003

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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...

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Section: P H Y S I C a L R E V I E W L E T T E R Smentioning

confidence: 99%

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confidence: 99%

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confidence: 99%

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Electronic Excitation and Dynamic Promotion of a Surface Reaction

et al. 2003

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“…To obtain the time-dependent width G͑t͒ and center frequency v CO ͑t͒ of the C-O vibration, used in the optical Bloch equations, we first calculate the transient substrate phonon temperature T ph following pulsed laser excitation by means of the two-temperature model [15]. Subsequently, a friction-model calculation allows the calculation of the time-dependent thermal occupation of the 47 cm 21 mode characterized by an adsorbate temperature T ads [19] by coupling this mode to the phonon heat bath with a coupling time of t ph 1.0 ps derived from previous experiments [2]. The temperature dependence of G͑T ads ͒ and v CO ͑T ads ͒, determined by anharmonic coupling (coupling parameter dv 23 cm 21 ) to the parallel frustrated translation and by the dipole-dipole coupling, are then calculated for all relevant temperatures [17].…”

Section: Femtosecond Surface Vibrational Spectroscopy Of Co Adsorbed mentioning

confidence: 99%

Femtosecond Surface Vibrational Spectroscopy of CO Adsorbed on Ru(001) during Desorption

et al. 2000

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Using time-resolved sum-frequency generation spectroscopy, the C-O stretch vibration of carbon monoxide adsorbed on a single-crystal Ru(001) surface is investigated during femtosecond near-IR laser excitation leading to desorption. A large transient redshift, a broadening of the resonance, and a strong decrease in intensity are observed. These originate from coupling of the C-O stretch to low-frequency modes, especially the frustrated rotation, that are highly excited in the desorption process. PACS numbers: 68.35.Ja, 33.70.Jg, 78.47. + p, 82.20.Rp The dynamics of the interaction between molecules and metal surfaces is of fundamental importance in surface science, since these determine key physical and chemical properties-essential in, e.g., catalysis-such as energy transfer and molecular reactivity [1] [6] at surfaces. These experiments were limited to temperatures below those at which desorption of the adsorbate occurs. However, when one is interested in surface reactions, the more relevant situation occurs at higher temperatures, where higher-lying vibrational modes get thermally occupied that play an important role in the surface chemistry.We present here femtosecond time-resolved vibrational sum-frequency generation (fs-SFG) spectra of CO molecules adsorbed on a Ru(001) surface, taken while a significant number (ϳ50%) of these molecules is desorbing due to fs laser excitation. With this fs-SFG method, snapshots of the (C-O) stretch vibration can be taken, under conditions where desorption is occurring (at lattice temperatures transiently exceeding the desorption temperature by over 500 K), shedding new light on the dynamics of the desorption process and the coupling of vibrational modes at the surface.The experiments were carried out in an ultrahigh vacuum chamber (base pressure 1 3 10 210 mbar) equipped with standard surface science tools. Our commercial laser system produces 800 nm, 110 fs pulses of 4.5 mJ͞pulse at 400 Hz, chopped down to 20 Hz for these experiments. One-third of the energy is used to excite the surface ("pump" pulse, typical absorbed fluence 55 J͞m 2 [7]), and the remaining energy is used to pump an optical parametric generator/amplifier (OPG͞OPA) providing tunable (l 2 10 mm, bandwidth ϳ150 cm 21 ) IR pulses with an energy of typically 10 mJ and a duration of 150 fs. The portion of the 800 nm pulse which is not converted into IR in the OPG͞OPA-process is spectrally narrowed down to 4 cm 21 , and is used in the SFG experiments as the visible (VIS) up-conversion pulse. An extensive description of the complete experimental setup as well as the surface cleaning, preparation, and characterization procedures can be found in Ref. [8].The surface sensitivity of SFG relies on the fact that the second-order nonlinear susceptibility is nonvanishing at interfaces [9]. This enables the generation of an electric field E SFG out of two incidents fields E VIS and E IR , where energyhv and parallel momentum k k must be con-For frequencies v IR within the IR bandwidth resonant with the vibrational tra...

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“…Optical laser pulses initiate surface reactions, which for CO=Ruð0001Þ lead to desorption [9,10,[26][27][28] and for 2O-CO=Ruð0001Þ include both CO desorption and a minor contribution from oxidation to CO 2 [29][30][31]. The temperature profile of the electron and phonon subsystems induced by ultrafast optical lasers can be described using the two-temperature model [16,32,33]. The absorption of visible photons in a metal substrate first gives rise to a nonequilibrium distribution of hot electrons which thermalizes on a time scale of a couple of hundred fs [34] into a quasiequilibrium distribution with a peak temperature of several thousand kelvin.…”

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Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics on Ru(0001) Probed by Ultrafast X-Ray Spectroscopy andAb InitioSimulations

et al. 2015

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We show that coadsorbed oxygen atoms have a dramatic influence on the CO desorption dynamics from Ru(0001). In contrast to the precursor-mediated desorption mechanism on Ru(0001), the presence of surface oxygen modifies the electronic structure of Ru atoms such that CO desorption occurs predominantly via the direct pathway. This phenomenon is directly observed in an ultrafast pump-probe experiment using a soft x-ray free-electron laser to monitor the dynamic evolution of the valence electronic structure of the surface species. This is supported with the potential of mean force along the CO desorption path obtained from density-functional theory calculations. Charge density distribution and frozen-orbital analysis suggest that the oxygen-induced reduction of the Pauli repulsion, and consequent increase of the dative interaction between the CO 5σ and the charged Ru atom, is the electronic origin of the distinct desorption dynamics. Ab initio molecular dynamics simulations of CO desorption from Ru(0001) and oxygen-coadsorbed Ru(0001) provide further insights into the surface bond-breaking process.

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Electronically driven adsorbate excitation mechanism in femtosecond-pulse laser desorption

Cited by 209 publications

References 27 publications

Electronic Excitation and Dynamic Promotion of a Surface Reaction

Electronic Excitation and Dynamic Promotion of a Surface Reaction

Femtosecond Surface Vibrational Spectroscopy of CO Adsorbed on Ru(001) during Desorption

Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics on Ru(0001) Probed by Ultrafast X-Ray Spectroscopy andAb InitioSimulations

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