Rudolf David
Forschungszentrum Jülich
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Surface Science Reports | 1985
George Comsa; Rudolf David
Abstract The aim to uncover fundamental details of the desorption process as well as the needs of more applied research, like fusion, led recently to a growing interest in the study of the parameters of desorbing molecules. The main experimental methods and the most significant experimental results obtained so far are reviewed and discussed. The investigation of the parameters of desorbing molecules being one of the oldest fields of surface science, a number of pre- and misconceptions still hamper the mutual understanding and hereby impede progress. Accordingly, a relatively large pace is dedicated to the historical development, to definitions, and even to semantics.
Surface Science | 1982
George Comsa; Rudolf David
Abstract Velocity distributions of H2 and D2 molecules desorbing from Cu(100) and (111) surfaces were measured as a function of desorption angle ϑ and sulphur coverage by the time-of-flight (TOF) cross-correlation method. The H and D atoms were supplied to the surface by permeation through the bulk of the Cu crystal. The TOF spectra are very narrow and correspond to mean energies of about 8kTs. i.e. four times larger than the energy corresponding to the surface temperature Ts = 1000 K. The velocity distributions of H2 and D2 from Cu appear to be the first known case in which all desorbing molecules belong to the group of “fast” molecules. This provides an opportunity to study the desorption of “fast” molecules in their pure form. The present data on Cu as well as former ones on Ni and Pd may be tentatively explained by assuming that hydrogen atoms permeating through the bulk to the sample surface have a much higher probability to equilibrate in some subsurface potential well, e.g. in the recently proposed subsurface state of H, than in the chemisorption well. The excess kinetic energy of the desorbing molecules resulting from recombination of atoms in this state originates from the potential barrier separating the subsurface state from the chemisorption well. The height of this barrier appears to correlate with the activation barrier for the bulk permeation of hydrogen.
Review of Scientific Instruments | 1981
George Comsa; Rudolf David; B. J. Schumacher
The advantages of the cross‐correlation time of flight (TOF) method when applied to surface scattering experiments under ultrahigh vacuum conditions are demonstrated in comparison with the conventional TOF method. The principle of the cross‐correlation TOF technique including the deconvolution procedure and statistical accuracy given by this method is discussed. The main parts of the spectrometer: chopper blade, magnetic rotor suspension and drive, and data acquisition system are described followed by the test of the TOF spectrometer, and by an illustration of the efficiency of this technique by means of TOF distributions measured for hydrogen molecules desorbing from metal surfaces.
Surface Science | 1979
George Comsa; Rudolf David; Bernd-Josef Schumacher
Abstract Experimental data are presented for the angular dependence of the relative flux, the mean energy and the speed ratio of deuterium molecules desorbing from a Ni(111) crystal surface at a surface temperature of Ts = 1143 K and at sulphur coverages ranging between 30% and less than 2% of a monolayer.The angular flux distribution is sharply peaked in the forward direction (cosdθwith 3 ⩽ d ⩽ 5) and the mean energy 〈E〉 of the desorbate depends strongly on the desorption angle θ. For normal desorption (θ = 0°) 〈E〉 2k is about 700 K higher than Ts and for glancing angles (θ = 80°) it decreases to about 400 K below Ts The results obtained on sulphur free and sulphur covered Ni(111) surfaces are compared with our former data on polycrystalline nickel. The main differences in the kinetic features can be ascribed to the surface roughness. Accordingly, the angular distributions of flux, mean energy, and speed ratio, which deviate strongly from the Knudson and Maxwellian law, do not seem to depend considerably on sulphur coverage and surface structure. A qualitative explanation for these deviations is presented using the principle of detailed balancing.
Surface Science | 1980
George Comsa; Rudolf David; Bernd-Josef Schumacher
Abstract The velocity distribution of D 2 desorbing from Pd(100) reveals the existence of two distinct groups of molecules, fast and slow ones. A new mechanism is proposed for the desorption of the molecules belonging to the fast group: after diffusing to the surface, absorbed D atoms recombine directly, without equilibrating in the chemisorption well. The fast molecules observed in the D 2 desorption spectra from Cu and Ni seem to desorb through the same mechanism. In the case of Pd this mechanism is controlled by adsorbed sulphur.
Chemical Physics Letters | 1977
George Comsa; Rudolf David
Abstract A model is presented which describes features of the “hot”, non-cosine and non-maxwellian velocity distributions of H 2 molecules desorbing from Ni surfaces. The model, based on detailed balancing, assumes that incident molecules encounter an activation barrier with holes. The model also correlates with results of kinetic measurements: sticking coefficient ≈0.1, non-activated adsorption at low temperatures and activated adsorption at high temperatures.
Surface Science | 1988
Klaus Kern; Rudolf David; Peter Zeppenfeld; George Comsa
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).
Review of Scientific Instruments | 1986
Rudolf David; Klaus Kern; Peter Zeppenfeld; George Comsa
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...
Solid State Communications | 1987
Klaus Kern; Rudolf David; Peter Zeppenfeld; Robert L. Palmer; George Comsa
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.
Review of Scientific Instruments | 1994
Sebastian Horch; Peter Zeppenfeld; Rudolf David; George Comsa
We report on the construction of an ultrahigh vacuum (UHV) scanning tunneling microscope (STM) specially designed for operation in the entire range of sample temperatures between 10 and 400 K. The sample is cooled by means of a liquid helium continuous‐flow cryostat, while the supporting manipulator and the surrounding devices remain at room temperature. This allows rapid variation of the sample temperature. The standard instruments for surface preparation and analysis and the STM are contained in a single UHV chamber. By rotation of the manipulator the sample can be positioned in front of any of these instruments without changing the sample temperature. The performance of the microscope is demonstrated by two examples of images of xenon adsorbed on platinum(111) showing: (a) the evolution of the morphology of a submonolayer of xenon from adsorption at 17K up to desorption at about 90 K and (b) atomically resolved images of the hexagonal incommensurate rotated phase for xenon at monolayer completion.