Klaus Kern

Relaxations in the missing-row structure of the (1 × 2) reconstructed surfaces of Au(110) and Pt(110)

Abstract The atomic structure of the (1 × 2) reconstructed (110) surfaces of Au and Pt has been investigated in detail using X-ray diffraction. For both surfaces the reconstruction involves several layers of atoms. The top layer spacing is contracted and pairing occurs in the second and fourth layers. There have been enough investigations of these surfaces with other experimental methods that a detailed comparison can now be made. Though there is full agreement on the missing-row model, this is not true for the magnitude of the relaxations occurring at the surfaces. In general the X-ray diffraction results compare better with electron diffraction data than with ion scattering experiments. Many theoretical approaches fail to predict the correct sign and/or magnitude of some of the relaxations.

Registry effects in the thermodynamic quantities of Xe adsorption on Pt(111)

Abstract We report here on thermodynamic studies of Xe adsorption on Pt(111) using high resolution He scattering. From the data we extracted isosteric heats, partial molar entropies and lateral compressibilities in the adsorbed layer. Near completion of the commensurate ( 3 × 3 ) R30° phase, i.e. at the commensurate-incommensurate phase transition, we observe a substantial drop in the heat of adsorption of about 30 meV and an increase in the entropy of the adsorbed layer. The difference in adsorption heat Δq st ≈30 meV between the commensurate and the relaxed uniaxially compressed incommensurate phase reflects the binding energy difference between the lowest energy level occupied by the Xw atoms in the commensurate phase and the average of the energies of the sites occupied in the incommensurate phase; thus, Δ q st is a lower bound for the lateral corrugation of the substrate holding potential. The increase of the entropy of the adlayer in the incommensurate phase may be attributed to intrinsic properties of this phase. The results are discussed, and compared with recent Xe-adsorption studies on Ag(111) and Pd(100).

High‐resolution He‐scattering apparatus for gas–surface interaction studies

A high‐resolution apparatus designed for the study of elastic and inelastic scattering of thermal helium atoms from crystal surfaces is presented. The highly expanded He nozzle beam has an energy spread ΔE/E of about 1.4% and is collimated to 0.2°. The angle subtended by the detector opening as seen from the sample is also 0.2°. Beam intensities as low as 10−6 of the specular beam intensity from a low‐temperature clean Pt(111) surface are detectable. Pseudorandom chopping with a resolution of 2.5 μs (flight path 790 mm) is used for time‐of‐flight (TOF) analysis of the scattered helium. The base pressure in the sample chamber is in the low 10−11 mbar. The capabilities of the apparatus are demonstrated for physisorbed Xe adlayers on Pt(111). The results presented are obtained by using He scattering in various modes: coherent inelastic, coherent elastic, and incoherent (diffuse) elastic. This technique allows for a nondestructive nearly exhaustive characterization of the thermodynamics, structure, and dynami...

Thermally induced disorder and conformational defects of alkane monolayers on graphite

Abstract The two-dimensional molecular lamellar arrangement of long chain alkanes adsorbed on the basal plane of graphite and its thermally induced disordering has been studied by variable temperature scanning tunneling microscopy. Atomic scale resolution of the disordered phase is achieved by studying the quenched high temperature phase. Overall as well as conformational disorder, in particular molecular kink defects, have been observed. High temperature snapshots show that the thermal evolution towards molecular disorder proceeds via small longitudinal fluctuations of molecules or blocks of molecules around their average positions. For increasing temperatures, molecular excursions along the long axis of the molecules become larger until eventually the lamellae lose their individuality, accompanied by substantial loss of lateral order.

Symmetry breaking commensurate-incommensurate transition of monolayer Xe physisorbed on Pt(111)

Abstract We report a high resolution He-diffraction study of the commensurate-incommensurate transition of monolayer xenon physisorbed on Pt(111). The experimental results show that we have been able to observe for the first time a (√3x√3)R300 commensurate (C) striped incommensurate (SI) transition. The striped domain walls are found to run into the ΓK -direction, i.e. the uniaxial compression is in the ΓM -direction. The C-SI transition appears to be continuous within the experimental accuracy and the incommensurability in the weakly incommensurate phase follows a 1 2 power law versus reduced temperature.

Debye-Waller factor for He/Pt(111)

Abstract We report accurate measurements of the dependence of the specular reflectivity on surface temperature and on incident angle, for He scattering off Pt(111). The usual Debye-Waller formula with a Beeby correction fits the data, provided that the effective well depth is adjusted. We show that a full calculation of the Debye-Waller exponent also reproduces the data. The inelastic atom-surface interaction and the surface phonon spectrum used in the calculation are the ones that reproduce the time-of-flight data obtained in separate experiments. No further adjustable parameters are needed.

Structure and complete chemical passivation of Pt(997)

Abstract We report on scanning tunneling microscopy and infrared reflection absorption spectroscopy investigations of the stepped Pt(997) surface. The surface shows a regular terrace-step-structure with only moderate variations of the terrace widths with respect to the nominal structure. The adsorption of CO as well as the coadsorption of hydrogen and CO is studied. It is shown that covering the surface with a full monolayer of hydrogen at 165 K provides its complete passivation against adsorption from the residual gas atmosphere, without affecting the terrace-step-configuration. Thus the surface can be kept clean in UHV over months, a necessary precondition to serve as diffractive element in atom beam optics.

Thermodynamic measurements of Xe-adsorption on Pt(111)

Abstract The adsorption of xenon on a nearly defect free Pt(111) surface has been studied by elastic and inelastic He-scattering. With decreasing temperature the sequential of 2D-gas, monolayer, bilayer and multilayer films are observed. The thermodynamic phase diagram and the latent heats of adsorption are determined.

Low‐energy helium nozzle beam

A high‐intensity supersonic nozzle beam source is described, whose beam energy can be varied between 2 and 170 meV (helium). Experimental performance and time‐of‐flight measurements on cold helium beams are reported. Condensation in the free jet expansions of helium has been studied by mass spectrometric detection of He+2. A correlation between the maximum speed ratio and the onset of clustering is observed.

Vibrational spectroscopy of rare gas adlayers

Abstract An overview of the state of the art of the vibrational Spectroscopy of physisorbed rare gas adlayers is given and some recent developments in this field are reported. High resolution thermal He atom scattering offers a unique means of probing low energy surface phonons. At present, an overall energy resolution of typically 0.3 meV can be achieved at a primary He beam energy of about 18 meV. This allows the rare gas adlayer vibration energies of the order of a few meV to be measured with high accuracy. In addition, small perturbations of the phonon dispersion and linewidth broadening originating from the coupling of the adlayer vibrations to the substrate phonons can be accessed. Recently, the effects of anharmonicity in thin rare gas films have been revealed by studying the temperature dependence of the phonon linewidths. At the same time, the theoretical modeling of the dynamics of physisorbed adlayers has also evolved considerably. In its most advanced form, the theory uses realistic rare gas pair potentials, includes the dynamical coupling between the adlayer and the substrate, and takes into account anharmonic coupling terms between the rare gas atoms within the adlayer. As a result, an overall quantitative agreement between experiment and theory is obtained in the case of the rare gases Ar, Kr, and Xe physisorbed on Pt(111).