P. Jakob

Chemical etching of vicinal Si(111): Dependence of the surface structure and the hydrogen termination on the pH of the etching solutions

Infrared spectroscopy is used to study the etching process of stepped Si(111)9° surfaces as a function of the pH of the etching HF solutions. This process results in complete H termination of the silicon surface, including terraces, steps, and defects; the surface structure can therefore be well studied using infrared (IR) spectroscopy. Polarized IR absorption spectra of the Si–H stretching vibrations (i.e., in the region 2060–2150 cm−1) vary dramatically as the pH of the etching solutions increases from 2.0 to 7.8. In general, higher pH solutions yield sharper bands and more easily assigned spectra, making it possible to identify the step and terrace species and thus to infer the surface structure and step morphology (i.e., to investigate the etching process). The data are explained by a model involving different etching rates for each individual surface species: The highest rate of removal is for isolated adatom defects located on (111) planes and the lowest is for the ideally H‐terminated (111) planes ...

Vibrational energy transfer on hydrogen-terminated vicinal Si(111) surfaces : interadsorbate energy flow

We report measurements of excited‐state lifetimes for Si–H stretching vibrational modes of steps and terraces on chemically prepared, hydrogen‐terminated vicinal Si(111) surfaces using picosecond pump–probe surface spectroscopy. The steps present on these vicinal surfaces are shown to play an important role in the vibrational energy relaxation pathways. Three types of vicinal Si(111) surfaces are studied, all having monohydride‐terminated terraces but differing in step termination or in step density. Two surfaces are cut along the 〈112〉 direction, 9° and 5° from the (111) plane, respectively. Both of these surfaces have dihydride‐terminated steps. A third surface is cut 9° from the (111) plane along the 〈112〉 direction and has monohydride‐terminated steps. Two normal modes of the dihydride‐terminated steps show vibrational energy relaxation times of ∼100 ps [≤80 ps and 130(20) ps, uncertainty in parentheses], while the monohydride‐terminated steps relax 10 times more slowly with an 1100(120) ps lifetim...

Structural properties and site specific interactions of Pt with the graphene/Ru(0001) moiré overlayer

The coherence of graphene layers on Ru(0001) over extended distances has been employed to identify fcc and hcp regions of the associated moire superstructure. These findings can be used as a straightforward method to discriminate between fcc and hcp hollow sites of Ru(0001). Our approach thereby makes use of the magnifying lens characteristics of the graphene/Ru(0001) overlayer and its coherence across several monatomic steps of the substrate. We demonstrate that the individual regions of the graphene/Ru(0001) overlayer exhibit pronounced variations in interaction strengths with deposited metal atoms. Specifically, Pt clusters have been grown at 140-180 K and they are found to organize in a well-ordered periodic array defined by the moire superlattice. Their preferred location within the graphene/Ru(0001) moiré unit cell is identified to be the fcc region.

Observation of a novel high density 3O(2 × 2) structure on Ru(001)

Abstract High-resolution electron energy loss spectroscopy and scanning tunneling microscopy have been applied to identify and characterize a novel high density phase of oxygen adsorbed on Ru(001). Three oxygen atoms per (2 × 2) unit cell are arranged such that a (2 × 2) vacancy superstructure is formed. Atomically resolved STM images clearly establish the existence of such a phase. The ordered layer is characterized by a sharp (2 × 2) superstructure as seen by low energy electron diffraction. Two dipole active energy losses were found for the 3O(2 × 2) layer at 0.75 ML and interpreted as the in-phase oxygen to metal stretching vibration and the totally symmetric eigenmode consisting of frustrated translations of three next-neighbor oxygen atoms towards each other. The vibrational modes of this novel structure are compared to data for oxygen coverages in the range 0.5–1 ML.

Infrared spectroscopy of overtones and combination bands

We present a detailed discussion on infrared spectroscopy of vibrational combination bands and overtones of adsorbate systems. For the case that the (dynamical) lateral coupling between the adsorbates is dominated by dipole coupling, we present general results for the absorption spectra which can be used to analyze experimental data and deduce the bond anharmonicity δω. The theoretical results are used to analyze experimental line shape data for the combination band of the C–O and the Ru–CO stretch vibrational modes of CO adsorbed on Ru(001), as well as the overtone of the C–O stretch vibration for the same adsorbate system. It is found that for strong lateral coupling (and weak anharmonicities) asymmetric line shapes are common; strong anharmonicities lead to the formation of localized two-phonon bound states besides a continuum of delocalized two-phonon states. However, even then the extraction of anharmonic parameters can be severely impeded by dynamic line shifts of the localized overtone band through...

The coadsorption of CO and benzene on Ru(001) at 4:1 ratio: structure and mutual influences

Abstract The coadsorption system benzene + CO on Ru(001) at a CO: benzene mixture of 4:1 has been investigated with HREELS, LEED, Δφ and TDS. At this stoichiometry an ordered ( 13 × 13 )R13.9° superstructure containing one benzene and four CO molecules per unit cell is formed at 250 K. The mutual distortion of both coadsorbates is only weak as deduced from frequency shifts in the vibrational spectra. The work function change of the coadsorption layer is considerably smaller than the sum of those of the individual adsorbates at the corresponding coverage on the clean crystal. Thermal desorption shows that there is a net attraction between the two molecules in the mixed layer, since the molecular desorption temperatures of both CO and benzene are higher than from the pure, equally crowded layers. The desorption spectra of D 2 resulting from dissociation of C 6 D 6 show a strong enhancement of the peak at 494 K compared to the pure benzene layer which is explained by the sudden availability of sites vacated by desorbing CO, whereas the other desorption features remain essentially unchanged. As in the pure benzene layer a kinetic isotope effect is observed in the dehydrogenation of benzene in the coadsorbate layer.

Transient vibrational mode renormalization in dipole‐coupled adsorbates at surfaces

Dipole interactions among adsorbates at solid surfaces can strongly affect the intensities, positions, and line shapes of vibrational resonances. An understanding of these effects has been important in spectroscopic investigations of surface structure. Here, the adsorbate dipole interactions are shown to create transient spectral intensity and resonance position changes when vibrational modes are excited in ultrafast pump–probe laser experiments at surfaces. The spectral changes occur because the intensities and positions of vibrational resonances are dependent upon the magnitude of interadsorbate dipole interactions, and vibrational excitation modifies the effective oscillator dynamic dipoles that determine these interactions. The vibrational modes are different (renormalized) after excitation because of the change in coupling. The effects account for unusual spectral transients observed in recent pump–probe experiments on the Si–H stretching modes of vicinal H/Si(111) surfaces [K. Kuhnke, M. Morin, P. Jakob, N. J. Levinos, Y. J. Chabal, and A. L. Harris, J. Chem. Phys. 99, 6114 (1993)]. The predicted effects serve as a novel time‐resolved probe of the strength of dipolar interactions in adsorbate layers, and will arise in any adsorbate layer where the vibrational dynamic dipole interactions are large enough to cause spectral intensity borrowing among different adsorption sites or different adsorbates.

Electron–Vibron Coupling at Metal–Organic Interfaces from Theory and Experiment

We study the significance and characteristics of interfacial dynamical charge transfer at metal-organic interfaces for the organic semiconductor model system 1,4,5,8-naphthalene-tetra-carboxylic dianhydride (NTCDA) on Ag(111) quantitatively. We combine infrared absorption spectroscopy and dispersion-corrected density functional theory calculations to analyze dynamic dipole moments and electron-vibron coupling at the interface. We demonstrate that interfacial dynamical charge transfer is the dominant cause of infrared activity in these systems and that it correlates with results from partial charge and density of states analysis. Nuclear motion generates an additional dynamic dipole moment but represents a minor effect except for modes with significant out-of-plane amplitudes.

Adsorption geometry and interface states: Relaxed and compressed phases of NTCDA/Ag(111)

The theoretical modelling of metal-organic interfaces represents a formidable challenge, especially in consideration of the delicate balance of various interaction mechanisms and the large size of involved molecular species. In the present study, the energies of interface states, which are known to display a high sensitivity to the adsorption geometry and electronic structure of the deposited molecular species, have been used to test the suitability and reliability of current theoretical approaches. Two well-ordered overlayer structures (relaxed and compressed monolayer) of NTCDA on Ag(111) have been investigated using two-photon-photoemission to derive precise interface state energies for these closely related systems. The experimental values are reproduced by our DFT calculations using different treatments of dispersion interactions (optB88, PBE-D3) and basis set approaches (localized numerical atomic orbitals, plane waves) with remarkable accuracy. This underlines the trustworthiness regarding the description of geometric and electronic properties.