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Ultrafast charge carrier dynamics in graphite has been investigated by time-resolved terahertz spectroscopy. Analysis of the transient dielectric function and model calculations show that more than 90% of the initially deposited excitation energy is transferred to a few strongly coupled lattice vibrations within 500 fs. These hot optical phonons also substantially contribute to the striking increase of the Drude relaxation rate observed during the first picosecond after photoexcitation. The subsequent cooling of the hot phonons yields a lifetime estimate of 7 ps for these modes.

Electron solvation dynamics in photoexcited anion clusters of I-(D2O)n=4-6 and I-(H2O)4-6 were probed by using femtosecond photoelectron spectroscopy (FPES). An ultrafast pump pulse excited the anion to the cluster analog of the charge-transfer-to-solvent state seen for I- in aqueous solution. Evolution of this state was monitored by time-resolved photoelectron spectroscopy using an ultrafast probe pulse. The excited n = 4 clusters showed simple population decay, but in the n = 5 and 6 clusters the solvent molecules rearranged to stabilize and localize the excess electron, showing characteristics associated with electron solvation dynamics in bulk water. Comparison of the FPES of I-(D2O)n with I-(H2O)n indicates more rapid solvation in the H2O clusters.

The femtosecond dynamics of localization and solvation of photoinjected electrons in ultrathin layers of amorphous solid H2O and D2O have been studied by time- and angle-resolved two-photon-photoelectron spectroscopy. After electron transfer from the metal substrate into the conduction band of ice, the excess electron localizes within the first 100 fs in a state at 2.9 eV above E(F), which is further stabilized by 300 meV on a time scale of 0.5-1 ps due to molecular rearrangements in the adlayer. A pronounced change of the solvation dynamics at a coverage of approximately 2 bilayers is attributed to different rigidity of the solvation shell in the bulk and near the surface of ice.

A three dimensional model based on molecular dynamics with electronic frictions is developed to describe the femtosecond laser induced associative desorption of H2 from Ru(0001)(1 x 1)H. Two molecular coordinates (internuclear separation d and center of mass distance to surface z) and a single phonon coordinate are included in the dynamics. Both the potential energy surface and the electronic friction tensor are calculated by density functional theory so that there are no adjustable parameters in the comparison of this model with the wide range of experiments available for this system. This "first principles" dynamic model gives results in semiquantitative agreement with all experimental results; nonlinear fluence dependence of the yield, isotope effect, two pulse correlation, and energy partitioning. The good agreement of theory with experiment supports a description of this surface femtochemistry in terms of thermalized hot electron induced chemistry with coupling to nuclear coordinates through electronic frictions. By comparing the dynamics with the analytical one dimensional frictional model used previously to fit the experiments for this system, we show that the success of the one dimensional model is based on the rapid intermixing of the z and d coordinates as the H-H climbs out of the adsorption well. However, projecting the three dimensional dynamics onto one dimension introduces a fluence (adsorbate temperature) dependent "entropic" barrier in addition to the potential barrier for the chemistry. This implies that some caution must be used in interpreting activation energies obtained in fitting experiments to the one dimensional model.

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...

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.

The ultraviolet photolysis of HBr molecules and (HBr)n clusters with average size around n̄=9 is studied at three different wavelengths of 243, 205, and 193 nm. Applying polarized laser light, the kinetic energy distribution of the hydrogen photofragment is measured with a time-of-flight mass spectrometer with low extraction fields. In the case of HBr monomers and at 243.1 nm, an almost pure perpendicular character (β=−0.96±0.05) of the transitions is observed leading to the spin–orbit state Br(2P3/2). The dissociation channel associated with the excited state Br*(2P1/2) is populated by a parallel transition (β*=1.96±0.05) with a branching ratio of R=0.20±0.03. At the wavelength of 193 nm, about the same value of R=0.18±0.03 is found, but both channels show a mainly perpendicular character with β=−0.90±0.10 for Br and β*=0.00±0.10 for Br*. The results for 205 nm are in between these two cases. For the clusters at 243 nm, essentially three different groups appear which can be classified according to their kinetic energy: (i) A fast one with a very similar behavior as the monomers, (ii) a faster one which is caused by vibrationally and rotationally excited HBr molecules within the cluster, and (iii) a slower one with a shoulder close to the fast peak which gradually decreases and ends with a peak at zero velocity. The zero energy fragments are attributed to completely caged H atoms. The angular dependence of the group (iii) is isotropic, while that of the other two is anisotropic similar to the monomers. At 193 nm only the fast and the slow part is observed without the peak at zero energy. Apparently the kinetic energy is too large to be completely dissipated in the cluster.