Anisotropy of electron structure at InAs(1 1 1) surfaces by laser pump-and-probe photoemission spectroscopy

The electronic structure and the electron dynamics of the clean InAs(1 1 1)A 2 × 2 and the InAs(1 1 1)B 1 × 1 surfaces have been studied by laser pump-and-probe photoemission spectroscopy. Normally unpopulated electron states above the valence band maximum (VBM) are filled on the InAs(1 1 1)A surface due to the conduction band pinning above the Fermi level (E_F). Accompanied by the downward band banding alignment, a charge accumulation layer is confined to the surface region creating a two dimensional electron gas (2DEG). The decay of the photoexcited carriers above the conduction band minimum (CBM) is originated by bulk states affected by the presence of the surface. No occupied states were found on the InAs(1 1 1)B 1 × 1 surface. This fact is suggested to be due to the surface stabilisation by the charge removal from the surface into the bulk. The weak photoemission intensity above the VBM on the (1 1 1)B surface is attributed to electron states trapped by surface defects. The fast decay of the photoexcited electron states on the (1 1 1)A and the (1 1 1)B surfaces was found to be τ_1 1 1 A ? 5 ps and τ_1 1 1 B ?  4 ps, respectively. We suggest the diffusion of the hot electrons into the bulk is the decay mechanism.     

Scaling behavior of island density in submonolayer growth of CaF2 on vicinal Si(1 1 1)

We have studied the scaling behavior of two-dimensional island density during submonolayer growth of CaF₂ on vicinal Si(1 1 1) surfaces using scanning tunneling microscopy. We have analyzed the morphology of the Si(1 1 1) surfaces where CaF₂ partial monolayers with coverages of about 0.1 monolayer are deposited at ∼600 °C. The number density of terrace nucleated islands increases with substrate terrace width l as ∼l⁴ in a low island density regime. This scaling behavior is consistent with predictions for the case of the irreversible growth of islands.     

Evidence for diffusion-limited kinetics during electromigration-induced step bunching on Si(1 1 1)

We have measured how the initial terrace width l₀ on vicinal Si(1 1 1) surfaces influences the rate of step bunching and the minimum terrace width within a bunch when direct-current heated at 940-1290 °C. A comparison of this data with analytic solutions and numerical simulations of the conventional “sharp-step” model give strong evidence that the kinetic length d is relatively small (d < ∼20 nm) in both temperature regime I (∼850-950 °C) and regime III (∼1200-1300 °C), in which step-down current is required for step bunching. This indicates that surface mass transport is diffusion-limited in both regimes I and III when l₀ > 20 nm, and hence that the adatom attachment- and terrace diffusion-hopping rates are of comparable magnitude. We also observe similar scaling with initial terrace width in temperature regime II (∼1040-1190 °C), in which step-up current is required for bunching, suggesting a similar step bunching mechanism in all three temperature regimes.     

Evidence for long-range surface state mediated interaction between sodium atoms on copper (0 0 1)

Inelastic helium atom scattering from sodium atoms on the Cu(0 0 1) surface at 50 K reveals a remarkable 15% increase in the frequency of the frustrated translational vibrations (T-mode) from ?ω=5.56 to 6.34 meV with increasing coverage from Θ_Na=0.008 to 0.125. The coverage dependence and the negligible dispersion of the frequency cannot be explained by the direct dipole-dipole coupling but are well-understood in terms of the Lau-Kohn effective long-range interaction via intrinsic surfaces states.     

Electronic properties of clean unreconstructed 6H-SiC(0 0 0 1) surfaces studied by angle resolved photoelectron spectroscopy

The properties of the clean and unreconstructed 6H-SiC(0 0 0 1) and 6H-SiC surfaces were investigated by means of angle-resolved photoelectron spectroscopy (ARPES). These highly metastable surfaces were prepared by exposing hydrogen terminated surfaces to a high flux of synchrotron radiation. On both surfaces we find a band of surface states with 1 × 1 periodicity assigned to unsaturated Si and C dangling bonds located at 0.8 eV and 0.2 eV above the valence band maximum, respectively. Both states are located below the Fermi level. The dispersion of the surface bands amounts to 0.2 eV for the Si derived band and 0.7 eV for C derived band. It is suggested that the electronic properties of these surfaces are governed by strong correlation effects (Mott-Hubbard metal insulator transition). The results for the (0 0 0 1) surface are directly compared to Si-rich (√3 × √3)R30° reconstructed surface. Distinct differences in electronic structure of the (√3 × √3)R30° and 1 × 1 surfaces are observed.     

Born-Oppenheimer breakdown and non-adiabatic lifetimes of rovibrational levels of D2 lying near the n = 2 dissociation limit: Experiment and theory

The singlet gerade states of the hydrogen molecule are strongly affected by the breakdown of the Born-Oppenheimer approximation. This leads to strong non-adiabatic coupling resulting in large changes of the energies and lifetimes of the quantum levels compared to the values obtained in the Born-Oppenheimer or even the adiabatic levels of approximation. The non-adiabatic calculations of Quadrelli, Dressler, and Wolniewicz (1990) [7] (hereinafter QDW) for the three highest vibrational levels (υ = 44, 45, and 46) of the EF ¹Σ_g⁺ state of D₂ predicted an enormous increase of the lifetimes upon excitation of just one quantum of rotational motion. However, although our experimental results for these levels just below the n = 2 dissociation limit do show a strong increase in lifetime, the non-adiabatic lifetimes calculated by QDW are longer than experiment by as much as three orders of magnitude. In their work on isotopomers of hydrogen QDW and Yu and Dressler (1994) [5] published extensive summary tables of ab initio non-adiabatic coupling data. We present a technique which allows us to use their summary data to calculate approximate non-adiabatic ab initio lifetimes. The results reconcile our observed lifetimes with the non-adiabatic coupling from those previous ab initio calculations and also provide a detailed quantitative and qualitative understanding of the unusual rotational dependence of the lifetimes of these very highly excited levels. We also test the current technique by calculating the lifetimes of other levels involved in interactions with these EF levels and by calculating the lifetimes of the EF υ = 33 level of H₂, for which no corresponding level exists in the Born-Oppenheimer or adiabatic approximations.     

Anisotropy analysis of the surface energy of hcp (c/a < 1.633) metals

In this paper, anisotropy of the surface energy of 5 hcp metals Be, Hf, Ru, Ti and Y have been analyzed. The surface energies of three kinds of representative surfaces, (h 0 l), (h h l) and (h k 0) belong to [0 1 0], [] and [0 0 1] crystal band, respectively, have been calculated using the modified embedded atom method. For all 5 hcp metals, the (1 1 0) plane has the minimum surface energy in all 35 surfaces studied. Considering surface energy minimization solely, the (1 1 0) texture should be favorable in the hcp films. The fact that the short termination corresponds to much lower surface energy than long one implies the former is more stable for those surfaces having two possible terminations. Such as the prism plane (1 0 0), only the short termination was observed in experiment.     

Hopping motion of chlorine atoms on Si(1 0 0)-(2 × 1) surfaces induced by carrier injection from scanning tunneling microscope tips

Injection of tunneling electrons and holes from the probe tips of a scanning tunneling microscope was found to enhance the hopping motion of Cl atoms between neighboring dangling-bond sites of Si dimers on Si(1 0 0)-(2 × 1) surfaces, featured by the rate of hopping linearly dependent on the injection current. The hopping rate formed peaks at sample biases of V_S∼+1.25 and −0.85 V, which agree with the peaks in the local density of states spectrum measured by scanning tunneling spectroscopy. The Cl hopping was enhanced at Cl-adsorbed sites even remote from the injection point. The Cl hopping by hole injection was more efficiently enhanced by sweeping the tip along the Si dimer row than by tip-sweeping along the perpendicular direction. Such anisotropy, on the other hand, was insignificant in the electron injection case. All of these findings can be interpreted by the model that the holes injected primarily into a surface band originated from the dangling bonds of Si dimers propagate quite anisotropically along the surface, and become localized at Cl sites somehow to destabilize the Si-Cl bonds causing hopping of the Cl atoms. The electrons injected into a bulk band propagate in an isotropic manner and then get resonantly trapped at Si-Cl antibonding orbitals, resulting in bond destabilization and hopping of the Cl atoms.     

A DFT study the solvent effects of H2 adsorption on Cu(h k l) surface

Density functional theory (DFT) combined with conductor-like solvent model (COSMO) have been performed to study the solvent effects of H₂ adsorption on Cu(h k l) surface. The result shows H₂ can not be parallel adsorbed on Cu(h k l) surface in gas phase and only vertical adsorbed. At this moment, the binding energies are small and H₂ orientation with respect to Cu(h k l) surfaces is not a determining parameter. In liquid paraffin, when H₂ adsorbs vertically on Cu(h k l) surface, solvent effects not only influences the adsorptive stability, but also improves the ability of H₂ activation; When H₂ vertical adsorption on Cu(h k l) surface at 1/4 and 1/2 coverage, H-H bond is broken by solvent effects. However, no stable structures at 3/4 and 1 ML coverage are found, indicating that it is impossible to get H₂ parallel adsorption on Cu(h k l) surfaces at 3/4 and 1 ML coverages due to the repulsion between adsorbed H₂ molecules.     

Calculation of the surface energy of hcp metals by using the modified embedded atom method

With MEAM, the surface energies of three kinds of representative surfaces, (h 0 l), (h h l) and (h k 0) belong to [0 1 0], [] and [0 0 1] crystal band, respectively, have been calculated for 13 closed-packed hexagonal (hcp) metals Co, Dy, Er, Gd, Ho, Mg, Nd, Pr, Re, Sc, Tb, Tl and Zr. For all 13 hcp metals, the basal plane (0 0 1) has the minimum surface energy. So from surface energy minimization, the (0 0 1) texture should be favored in the hcp films, this is consistent with the experimental results. The fact that the short termination corresponds to much lower surface energy than long one implies the former is more stable for those surfaces having two possible terminations. Such as the prism plane (1 0 0), only the short termination was observed in experiment.     

Transient desorption of HD and D2 molecules from the D/Si(1 0 0) surfaces exposed to a modulated H-beam

We have studied the direct and indirect abstraction of D adatoms by H on the Si(1 0 0) surfaces by employing a pulsed H-beam. Desorptions of HD molecules is found to occur promptly as a result of direct abstraction at the beam on-cycles. In contrast, we find that D₂ desorption induced by adsorption of H atoms, i.e., the so-called adsorption-induced desorption (AID), occurs even at the beam off-cycles. The D₂ rate curves measured with the pulsed-H beam are decomposed into four components characterized with the reaction lifetimes of ?0.005, 0.06 ± 0.01, 0.8 ± 0.1, and 30 ± 5 s. We propose that the fastest and the second fastest AID channels are related to the thermodynamical instability of (1 × 1) dihydride domains locally formed on the (3 × 1) monodeuteride/dideuteride domains. The 0.8 s AID channel is attributed to the desorption occurring at the stage when (3 × 1) monodeuteride/dideuteride domains are built up upon H adsorption onto the (2 × 1) monohydride surface. The 30 s AID path is attributed to the thermal desorption accompanied by the shrinkage of the (3 × 1) domains which were excessively formed during the beam on-cycles on the (2 × 1) monohydride surface. Atomistic mechanisms are proposed for these three AID pathways.     

Structural and electronic properties of graphite layers grown on SiC(0 0 0 1)

Photoelectron spectroscopy, low-energy electron diffraction, and scanning probe microscopy were used to investigate the electronic and structural properties of graphite layers grown by solid state graphitization of SiC(0 0 0 1) surfaces. The process leads to well-ordered graphite layers which are rotated against the substrate lattice by 30°. On on-axis 6H-SiC(0 0 0 1) substrates we observe graphitic layers with up to several 100 nm wide terraces. ARUPS spectra of the graphite layers grown on on-axis 6H-SiC(0 0 0 1) surfaces are indicative of a well-developed band structure. For the graphite/n-type 6H-SiC(0 0 0 1) layer system we observe a Schottky barrier height of ?_B,n = 0.3 ± 0.1 eV. ARUPS spectra of graphite layers grown on 8° off-axis oriented 4H-SiC(0 0 0 1) show unique replicas which are explained by a carpet-like growth mode combined with a step bunching of the substrate.     

Eigenvalues, eigenfunctions, and surface states in finite periodic systems

Using a simple approach that requires neither the Bloch functions nor the reciprocal lattice, new, compact, and rigorous analytical formulas are derived for an accurate evaluation of resonant energies, resonant states, energy eigenvalues and eigenfunctions of open and bounded n-cell periodic systems with arbitrary 1D potential shapes, provided the single cell transfer matrix is given. These formulas are applied to obtain the energy spectra and wave functions of a number of simple but representative open and bounded superlattices. We solve the fine structure in bands and exhibit unambiguously that the true eigenfunctions do no not fulfill the periodicity property |Ψ_μ,ν (z + l_c)|² = |Ψ_μ,ν (z)|², with l_c the single cell length. We show that the well known surface states and surface energy levels come out naturally. We analyze the surface repulsion effect and calculate exactly the surface energy levels for different potential discontinuities an the ends.     

A valence band photoemission study of Pb adsorption on Rh(1 0 0) and Rh(1 1 0)

We have studied the adsorption of Pb on the Rh(1 0 0) and (1 1 0) surfaces by photoemission and low energy electron diffraction (LEED), and tested the chemical properties by adsorption of CO. Pb forms two distinct c(2 × 2) phases on Rh(1 0 0), according to the temperature of the substrate. The phase formed below about 570-620 K, denoted α-c(2 × 2), reduces the coverage of adsorbed CO but does not affect the valence band spectrum of the molecule. The phase formed above this temperature, denoted β-c(2 × 2), also reduces the coverage of adsorbed CO but the valence band spectrum of the adsorbed CO is strongly affected. The two phases are also characterised by a slightly different binding energy of the Pb 5d_5/2 level, 17.54 eV for the α phase and 17.70 for the β phase. The Pb/Rh(1 1 0) surface shows two ordered Pb induced phases, c(2 × 2) and p(3 × 1). CO adsorbs on the first with reduced heat of adsorption and with a valence band spectrum that is strongly altered with respect to CO adsorbed on clean Rh(1 1 0), but does not adsorb on the p(3 × 1) structure at 300 K. We compare the present results with previous results from related systems.     

Quantum-well resonances and image states in the Ar/Cu(1 0 0) system

A theoretical study of the effect of an atomically thin rare gas layer on the dynamics of excited electronic states at metal surfaces is presented for the case of a few mono-layers of Ar on a Cu(1 0 0) surface. We develop a 3D-microscopic model with predictive capabilities of the interaction of an electron with an Ar layer physisorbed on a metal surface. It takes into account the 3D structure of the Ar layer as well as its dielectric character. The dynamics of the excited electron on the surface is treated within a wave-packet propagation approach. The calculations show that two different types of excited states are present at the Ar/Cu(1 0 0) surface. (i) Image states that are repelled into vacuum as compared to their position on clean Cu(1 0 0) surfaces, leading to a decrease of their binding energies and to an increase of their lifetimes. (ii) Quantum-well resonances, corresponding to quasi-stationary states localised inside the Ar layer; they are associated with the quantisation of the conduction band in the finite size Ar layer. The present results on image states nicely agree with very recent time-resolved two-photon-photo-emission experiments by Berthold, Feulner and Höfer.     

Structural and statistical analysis of Yb/Si(1 1 1) and Eu/Si(1 1 1) reconstructions

We have performed the structural and statistical analysis of Yb/Si(1 1 1) and Eu/Si(1 1 1) surfaces in the submonolayer regime utilizing low-energy electron diffraction and scanning tunneling microscopy (STM). The almost identical series of one-dimensional chain structures (e.g., 3 × 2/3 × 1, 5 × 1, 7 × 1, 9 × 1, and 2 × 1 phases) are found in order of increasing metal coverage for both adsorbed systems, however, only the Eu/Si system reveals the ‘√3’-like reconstruction before the 2 × 1 endpoint phase. The atomic models of chain structures are proposed and discussed. In particular, our results suggest the odd-order n×1 (n=5,7,9,…) intermediate reconstructions to incorporate the Seiwatz chains and honeycomb chains with the proportion of m:1, where . The statistical analysis of STM images is carried out to examine the correlation of atomic rows on Eu/Si and Yb/Si surfaces. It is found that Eu stabilizes more ordered row configuration compared to Yb, which can be explained in terms of indirect electronic interaction of atomic chains or/and different magnetic properties of adsorbed species.     

Three- and two-photon absorption in HCl and DCl: identification of Ω = 3 states and state interaction analysis

High resolution (3 + 1) and (2 + 1)REMPI spectra of HCl and DCl for total current detection at room temperature or TOF mass detection after jet cooling were recorded for the spectral region 89 000-89 600 cm⁻¹. Analysis of the (3 + 1)REMPI spectra by use of three-photon absorption modeling allowed, for the first time, identification and characterization of the l (³Φ₃) states. Consistent anomalies in spectral structures due to transitions to the j (³Σ⁻) (0⁺) state are interpreted as being due to interactions with the V (¹Σ⁺) ion-pair states. Interaction strengths are evaluated. Simulation analyses and determination of isotope shifts allowed evaluation of vibrational and rotational spectroscopic parameters for the l (³Φ₃) and the j (³Σ⁻) (0⁺) states for both molecules.     

Neutralization of Li ions scattered by the Cu (1 0 0) and (1 1 1) surfaces: A localized picture of the atom-surface interaction

Large and face dependent neutral fractions have been found recently in the scattering of Li⁺ by Cu(1 0 0) and Cu(1 1 1) surfaces. These results for high work function surfaces are unexpected within the ‘traditional’ picture of a Li⁺ ion departing from a jellium surface model. In the present work the Li⁺/Cu(1 0 0) and Li⁺/Cu(1 1 1) interacting systems are described by a previously developed bond-pair model based on the localized interactions between the projectile ion and the atoms of the surface, and on the extended features of the electronic band structure through the surface local density of states. By only including the resonant neutralization to the Li atom ground state we explained the face and energy dependences of the measured neutral fractions for large outgoing energy values. We found that the downward shift of the Li ionization level below the Fermi level caused by the short range chemical interactions, is the main responsible of a high neutralization by the resonant mechanism. The remaining differences between theory and experiment values can be explained in terms of the energy gaps and image potential states appearing in these surfaces. The calculated distance behaviours of the energy levels corresponding to the first excited (Li-1s²2p) and the negative (Li-1s²2s²) atomic configurations indicate that they can also participate in the ion-surface charge exchange process.