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The reaction dynamics of methanol and water on Si(001) were investigated by means of molecular beam techniques. The initial sticking probability s0 was determined as a function of the kinetic energy of the incoming molecules, Ekin, and surface temperature, Ts. For both, methanol and water, a nonactivated reactional channel was observed; the dynamics were found to be determined by the reaction into the datively bonded intermediate state. A low conversion barrier was deduced for the conversion from this intermediate into the final state. It is attributed to the reaction mechanism, which proceeds via proton transfer from the OH-group of the datively bonded molecules to a Si surface atom. Despite this low conversion barrier, adsorption into the intermediate and further reaction via proton transfer were found to be largely decoupled. [ABSTRACT FROM AUTHOR]

We present an investigation of the electronic structure and excited state dynamics of optically excited 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) thin films adsorbed on Ag(111) using two-photon photoemission spectroscopy (2PPE). 2PPE allows us to study both occupied and unoccupied electronic states, and we are able to identify signals from the highest occupied and the two lowest unoccupied electronic states of the PTCDA thin film in the 2PPE spectra. The energies for occupied states are identical to values from ultraviolet photoelectron spectroscopy. Compared to results from inverse photoelectron spectroscopy (IPES), the 2PPE signals from the two lowest unoccupied electronic states, LUMO and LUMO+1, are found at 0.8 eV and 1.0 eV lower energies, respectively. We attribute this deviation to the different final states probed in 2PPE and IPES and the attractive interaction of the photoexcited electron and the remaining hole. Furthermore, we present a time-resolved investigation of the excited state dynamics of the PTCDA film in the femtosecond time regime. We observe a significantly shorter inelastic excited state lifetime compared to findings from time-resolved photoluminescence spectroscopy of PTCDA single crystals which could originate from excitation quenching by the metal substrate. [ABSTRACT FROM AUTHOR]

The ultrafast charge-carrier dynamics at the buried heterointerface of gallium phosphide on silicon(001) are investigated by means of time-resolved optical second-harmonic generation. Photon energy dependent measurements reveal the existence of electronic interface states in the bandgap of both materials. Charge carriers excited via these interface states are efficiently injected within a few hundred femtoseconds from the GaP/Si interface into the Si substrate, resulting in the build-up of an electric field perpendicular to the interface on a picosecond time scale. [ABSTRACT FROM AUTHOR]

The dynamics of ethylene adsorption on the Si(001) surface was investigated by means of molecular beam techniques. A constant decrease of initial sticking probability s0 was observed with increasing kinetic energy indicating a non-activated adsorption channel. With increasing surface temperature, s0 decreases as well, pointing towards adsorption via a precursor state. Quantitative evaluation of the temperature dependence of s0 via the Kisliuk model was possible for surface temperatures above 250 K; below that value, the temperature dependence is dominated by the adsorption dynamics into the precursor state. Maximum surface coverage was found to be reduced with increasing surface temperature, which is discussed on the basis of a long lifetime of the precursor state at low temperatures. [ABSTRACT FROM AUTHOR]

The influence of molecular vibrations on the reaction dynamics of H2 on Si(001) as well as isotopic effects have been investigated by means of optical second-harmonic generation and molecular beam techniques. Enhanced dissociation of vibrationally excited H2 on Si(001)2×1 has been found corresponding to a reduction of the mean adsorption barrier to 390 meV and 180 meV for ν=1 and ν=2, respectively. The adsorption dynamics of the isotopes H2 and D2 show only small differences in the accessible range of beam energies between 50 meV and 350 meV. They are traced back to different degrees of vibrational excitation and do not point to an important influence of quantum tunneling in crossing the adsorption barrier. The sticking probability of H2 on the 7×7-reconstructed Si(111) surface was found to be activated both by H2 kinetic energy and surface temperature in a qualitatively similar fashion as H2/Si(001)2×1. Quantitatively, the overall sticking probabilities of H2 on the Si(111) surface are about one order of magnitude lower than on Si(001), the influence of surface temperature is generally stronger.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Thermal desorption of hydrogen from the Si(111)7×7 surface was investigated using optical second-harmonic generation to monitor the hydrogen coverage from 0.2 monolayer to below 0.01 monolayer. The results of isothermal desorption measurements are found to be compatible neither with simple first nor second-order kinetic behavior. It is suggested that different binding sites available for Si–H monohydride states on the Si(111)7×7 surface give rise to the apparent intermediate reaction order. [ABSTRACT FROM AUTHOR]

The process of desorption for the system of O2/Pd(111) under excitation by 100 fs pulses of visible light has been examined. Molecular desorption is found to occur with high efficiency and a nonlinear dependence on laser fluence. Direct time-domain measurements using a two-pulse correlation scheme reveal a dominant subpicosecond response together with a weaker, but significant correlation persisting for tens of picoseconds. These results imply a desorption process driven by the high electron temperatures produced by the femtosecond laser radiation. The slower component of the correlation response is interpreted as an enhancement of the desorption rate by adsorbate vibrational excitation. [ABSTRACT FROM AUTHOR]

Using synchrotron radiation, Auger electron, and H+/D+-ion yields have been studied at and above the O 1s excitation energies for condensed H2O/D2O layers of varying thickness, and for two reproducible adsorbate layers (so-called bilayers and monolayers) on Ru(001). Decay electron spectra as well as polarization dependences, angular distributions, and energy distributions of desorbing ions have been investigated. For polarizations with sufficient E component perpendicular to the surface, a sharp peak in the H+ NEXAFS spectrum is seen for all layers which has no direct counterpart in the Auger NEXAFS spectra, and whose intensity maximizes for E oriented in the detection direction. This observation is interpreted as due to the 1a1→4a1 core-to-bound transition of the surface molecules whose final state decays electronically and dissociates on comparable time scales. This appears to have the consequence that the symmetry of the coupled excitation is different from that expected for the primary photoabsorption process. There appears also to be an influence of hydrogen bonding on these effects. Similarities and differences between the various layers investigated are also analyzed. [ABSTRACT FROM AUTHOR]

We report on the direct mapping of electron transfer in the momentum space of bulk MoS2 by means of time- and angle-resolved two-photon photoemission with a high-harmonic probe. For this purpose, we have combined a high-repetition rate high-harmonic source with tunable femtosecond pump pulses and a 3D (kx, ky, E) electron spectrometer. We show that optical excitation slightly above the A exciton resonance results in an immediate occupation of the conduction band at ... followed by an ultrafast transfer (<50 fs) to the conduction band minimum at ... Both signals, at ... and ..., do not vanish over the observed period of 400 fs. The technique described here enables direct access to the charge transfer dynamics in k-space and allows the study of decay times and decay channels in various systems with dependence on the excess energy or helicity of the excitation. [ABSTRACT FROM AUTHOR]

The reaction dynamics of allyl methyl ether (AME) on Si(001) was studied by means of molecular beam techniques. The reaction of this bifunctional molecule comprising an ether and an alkene group was found to proceed via an intermediate state as deduced from the temperature dependence of the initial sticking probability s 0 . At constant surface temperature T s , s 0 decreases continuously with increasing kinetic energy E kin , indicating a non-activated adsorption channel. Qualitatively and quantitatively, the energy dependence is almost identical to the adsorption dynamics of diethyl ether on Si(001). We attribute this to a similar nature of the intermediate state, which largely determines the adsorption dynamics. In consequence, this indicates a predominant role of the ether group and a minor influence of the C=C double bond on the adsorption dynamics of AME on Si(001).

The electronic structure of the prototype metal/organic contact 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) on a Ag(111)-surface has been investigated using time- and angle-resolved two-photon photoelectron spectroscopy (2PPE). Our analysis addresses particularly the nature of the interface state (IS) emerging at the interface due to the substrate-adsorbate interaction [C. H. Schwalb, S. Sachs, M. Marks et al., Phys. Rev. Lett. 101, 146801 (2008)]. Its free-electron-like dispersion and a possible backfolding at the surface Brillouin zone boundaries are discussed. Time-resolved pump-probe experiments reveal the inelastic electron lifetime along the dispersion parabola and show its decrease for increasing parallel momentum. The temperature dependence of the peak linewidth indicates a coupling of the IS to molecular vibrations. Moreover, additional aspects are addressed, such as the determination of the electron attenuation length of photoelectrons for low kinetic energy originating from the IS and the work function change of the sample upon PTCDA adsorption with very high energy resolution.