Search

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]

Abstract: Diffusion of atomic hydrogen on silicon serves as a model system for the investigation of thermally activated diffusion processes of covalently bound adsorbates on semiconductor surfaces. Over the past two decades, a detailed understanding of the hopping mechanisms for H/Si(001) and H/Si(111) has been obtained using a variety of experimental and theoretical methods. Hydrogen diffusion on silicon is in general characterized by energy barriers that are substantially larger than for adsorbate diffusion on metal surfaces, by the occurrence of different pathways on one surface, as well as by a strong participation of the underlying lattice in the hopping process. In the case of the flat Si(001) surface, three diffusion pathways were identified: site exchange within one Si dimer, hopping along dimer rows, and hopping across dimer rows, with barriers of 1.4, 1.7 and 2.4eV, respectively. These barriers correlate with the distances of the involved adsorption sites of 2.4, 3.8 and 5.2Å. While hydrogen diffusion on Si(001) is strongly anisotropic at surface temperatures below 700K, the measurement of high hopping rates by means of a combination of pulsed laser heating and scanning tunneling microscopy reveals similar jump frequencies around 108 s−1 at 1400K. Diffusion across steps is found to occur with similar speed as diffusion along dimer rows. Hydrogen diffusion on Si(111) 7×7 involves 4.4-Å-long jumps between restatom and adatom sites, accompanied by strong distortions of the adatom backbonds. Crossing the unit-cell boundaries via a 6.7-Å-long migration pathway between two adatoms is the rate limiting process for diffusion on macroscopic length scales, which has an activation energy of 1.5eV. [Copyright &y& Elsevier]

Abstract: The dissociative adsorption of molecular hydrogen on silicon is considered to be the prototype for an activated chemical reaction at a semiconductor surface. The covalent nature of the silicon–silicon and silicon–hydrogen bonds lead to large lattice distortion in the transition state of the reaction. As a result, the apparently simple reaction exhibits relatively complex pathways and surprisingly rich dynamics. The report reviews, among others, experiments using optical second-harmonic generation, molecular beam techniques and scanning tunnelling microscopy which, in close connection with state-of-the-art density functional theory, have led to a detailed microscopic understanding of H2 adsorption on Si(001) and Si(111) surfaces. On the dimerized Si(001) surface, dissociative adsorption as well as recombinative desorption of H2 is shown to involve the dangling bonds of two neighbouring dimers. Preadsorption of atomic hydrogen or thermal excitation is able to substantially alter the adsorption barrier. As a consequence, the reactivity strongly depends on coverage and surface temperature. In contrast to activated adsorption of hydrogen at metal surfaces, even the most basic description of the dynamics has to include phonon excitation of the silicon substrate. [Copyright &y& Elsevier]

Abstract. Ultrafast electronic coherence phenomena of metallic image-potential states and of silicon dangling-bond states are discussed. The image states were investigated on a Cu(100) surface by means of time-resolved two-photon photoemission (2PPE). Spectroscopical and dynamical information about states with high quantum numbers (n Is greater than or equal to 4) were obtained by the coherent excitation of several eigenstates of the Rydberg series and detection of the resulting quantum beats. Electron wave packets that describe the quasi-classical periodic motion of weakly bound electrons at metal surfaces could be created by the superposition of several eigen-functions around n = 7. A surface-sensitive purely optical technique is introduced for the investigation of rapid dephasing processes of excited dangling-bond states of Si(111)7 x 7. The measurement of the diffracted second-harmonic signal from a transient population grating yielded decoherence times of about 5 fs. [ABSTRACT FROM AUTHOR]

Abstract. The dissociative adsorption of molecular hydrogen on vicinal Si(001) and Si(111) surfaces was investigated by means of optical second-harmonic generation (SHG). For temperatures up to 600 K the measured initial sticking coefficients could be decomposed into contributions from step/defect and terrace sites. Whereas the presence of steps on Si(111) leads only to a moderate increase of reactivity, the sticking coefficients for step adsorption on misoriented Si(001) are many orders of magnitude higher than for terrace adsorption and reach values up to 10[sup -4]. Neither the absolute values nor the temperature dependence of adsorption on the terraces are affected by the misorientation. [ABSTRACT FROM AUTHOR]

The adsorption dynamics of the datively bonded trimethylamine (TMA) on Si(001) was investigated by means of molecular beam techniques. The initial sticking probability s 0 of TMA on Si(001) was measured as a function of kinetic energy at two different surface temperatures (230 and 550 K). At given surface temperature, s 0 was found to decrease with increasing kinetic energy (0.1 to 0.6 eV) indicating a non-activated reaction channel. At increased surface temperature, s 0 is reduced due to the onset of desorption into the gas phase. The energy dependence of s 0 is compared to the results for the adsorption of tetrahydrofuran (THF) on Si(001), which reacts via a datively bonded intermediate into a covalently bound final state. As s 0 follows the same energy dependence both for TMA and THF, the datively bonded intermediate state is concluded to dominate the reaction dynamics in the latter case as well. [ABSTRACT FROM AUTHOR]

Abstract: Adsorption of cyclooctyne on Si(001) was investigated by means of scanning tunneling microscopy. Two different adsorption configurations were identified for the chemisorbed molecules which exhibit a tendency for clustering along the dimer rows. At high coverage well ordered adsorbate structures are formed. The adsorption behavior was observed to be similar at room temperature and at low surface temperature of 50K. Moreover, it was found to be insensitive to distorted, hydrogen induced dangling-bond configurations. Our results suggest a direct adsorption pathway for cyclooctyne on Si(001) most likely governed by the molecule’s triple bond in combination with its high ringstrain. [Copyright &y& Elsevier]