The ferroelectric as well as multiferroic surface domain structures of BaTiO3(001) and BiFeO3(001... more The ferroelectric as well as multiferroic surface domain structures of BaTiO3(001) and BiFeO3(001) are imaged based on photoemission electron microscopy (PEEM) by femtosecond laser threshold excitation under UHV conditions [1]. For well-prepared BaTiO3(001), three ferroelectric domain types are clearly discriminable due to work function differences. At room temperature, the surface domains resemble the known ferroelectric domain structure of the bulk. Upon heating above the Curie point of 400 K, the specific surface domain pattern remains up to 500 K [2]. Ab-initio calculations explain this observation by a remaining tetragonal distortion of the topmost unit cells stabilized by a surface relaxation. The (001) surface of the single-phase multiferroic BiFeO3 which is ferroelectric and antiferromagnetic, shows clear ferroelectric work function contrast in PEEM. Additionally, the multiferroic domains show significant linear dichroism. The observation of a varying dichroism for different...
19th International Conference on Ultrafast Phenomena, 2014
ABSTRACT We demonstrate high-order harmonic generation driven by a compact fiber laser as a light... more ABSTRACT We demonstrate high-order harmonic generation driven by a compact fiber laser as a light source for efficient photoemission spectroscopy, which allows mapping of the dominant part of the valence band of Ag(100) within 10 seconds.
A time-resolving and energy-dispersive photoelectron detector for time-resolved experiments has b... more A time-resolving and energy-dispersive photoelectron detector for time-resolved experiments has been set up for pump–multiple-probe experiments with combined laser and synchrotron radiation. The time resolution of the detector of about 1 ns allows an assignment of the photoelectron signal to a specific synchrotron radiation pulse for any filling pattern of third-generation synchrotron storage rings. This leads to an overall temporal resolution given by the synchrotron radiation pulse width, which is 30 ps at the synchrotron radiation facility BESSY, Berlin. Application of the pump–multiple-probe technique is demonstrated for time-resolved photoemission experiments with combined laser and synchrotron radiation.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2001
A novel silicon bonding technique and its application for surface science studies in a wide tempe... more A novel silicon bonding technique and its application for surface science studies in a wide temperature range of 30–1400 K is presented. The silicon single crystal is bonded on a polished tantalum plate via thin silver and tungsten interlayers deposited by evaporation in high vacuum. Upon annealing the silicon sample is bonded with a strong mechanical and good thermal contact. Several problems common to alternative sample mounting techniques are solved by this procedure. Excellent temperature control and small temperature gradients across the surface well below 1 K are demonstrated by thermal desorption data for two different adsorbates: ethane and hydrogen on Si(001)-(2×1).
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1993
The adsorption of acetylene on the Si(100)-(2×1) surface has been studied by high-resolution elec... more The adsorption of acetylene on the Si(100)-(2×1) surface has been studied by high-resolution electron energy-loss spectroscopy (HREELS) and temperature programmed desorption. An energy loss peak at 1450 cm−1, which can be assigned to a C=C double bond, is observed unambiguously in the HREEL spectra. This observation provides direct evidence for the sp2 di-σ adsorption model. The unusually low frequency for the C=C vibration is most likely due to an interaction between the Si atoms and the rehybridized acetylene π orbital. As the surface temperature is increased, one of the C–H bonds cleaves to form a monohydride (Si–H). Most of the acetylene dissociates over a wide temperature range to produce surface carbon and hydrogen. The carbon atoms remain on the surface to form silicon carbide (SiC), while the hydrogen atoms recombine to form molecular hydrogen, which desorbs from the Si(100) surface.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1996
The adsorption of atomic hydrogen on acetylene-saturated Si(100)-(2×1) was studied using high-res... more The adsorption of atomic hydrogen on acetylene-saturated Si(100)-(2×1) was studied using high-resolution electron energy loss spectroscopy, temperature programmed desorption, Auger electron spectroscopy, and low-energy electron diffraction (LEED). With saturation coverage of preadsorbed acetylene, the initial adsorption probability of atomic hydrogen on Si is reduced to 40% of the initial adsorption probability of atomic hydrogen on clean Si(100)-(2×1), and the uptake of hydrogen to form the Si monohydride up to 0.65±0.05 of a monolayer is observed. Although the hydrogenation of acetylene is substantially slower than the chemisorption of hydrogen as the Si monohydride, molecular desorption of ethylene is observed in temperature programmed desorption following the hydrogenation of chemisorbed acetylene. With further postexposures of atomic hydrogen, the LEED pattern changes from (2×1) to (1×1) and the Si dihydride forms, confirming the conversion of ethylene to ethyl.
The ferroelectric as well as multiferroic surface domain structures of BaTiO3(001) and BiFeO3(001... more The ferroelectric as well as multiferroic surface domain structures of BaTiO3(001) and BiFeO3(001) are imaged based on photoemission electron microscopy (PEEM) by femtosecond laser threshold excitation under UHV conditions [1]. For well-prepared BaTiO3(001), three ferroelectric domain types are clearly discriminable due to work function differences. At room temperature, the surface domains resemble the known ferroelectric domain structure of the bulk. Upon heating above the Curie point of 400 K, the specific surface domain pattern remains up to 500 K [2]. Ab-initio calculations explain this observation by a remaining tetragonal distortion of the topmost unit cells stabilized by a surface relaxation. The (001) surface of the single-phase multiferroic BiFeO3 which is ferroelectric and antiferromagnetic, shows clear ferroelectric work function contrast in PEEM. Additionally, the multiferroic domains show significant linear dichroism. The observation of a varying dichroism for different...
19th International Conference on Ultrafast Phenomena, 2014
ABSTRACT We demonstrate high-order harmonic generation driven by a compact fiber laser as a light... more ABSTRACT We demonstrate high-order harmonic generation driven by a compact fiber laser as a light source for efficient photoemission spectroscopy, which allows mapping of the dominant part of the valence band of Ag(100) within 10 seconds.
A time-resolving and energy-dispersive photoelectron detector for time-resolved experiments has b... more A time-resolving and energy-dispersive photoelectron detector for time-resolved experiments has been set up for pump–multiple-probe experiments with combined laser and synchrotron radiation. The time resolution of the detector of about 1 ns allows an assignment of the photoelectron signal to a specific synchrotron radiation pulse for any filling pattern of third-generation synchrotron storage rings. This leads to an overall temporal resolution given by the synchrotron radiation pulse width, which is 30 ps at the synchrotron radiation facility BESSY, Berlin. Application of the pump–multiple-probe technique is demonstrated for time-resolved photoemission experiments with combined laser and synchrotron radiation.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2001
A novel silicon bonding technique and its application for surface science studies in a wide tempe... more A novel silicon bonding technique and its application for surface science studies in a wide temperature range of 30–1400 K is presented. The silicon single crystal is bonded on a polished tantalum plate via thin silver and tungsten interlayers deposited by evaporation in high vacuum. Upon annealing the silicon sample is bonded with a strong mechanical and good thermal contact. Several problems common to alternative sample mounting techniques are solved by this procedure. Excellent temperature control and small temperature gradients across the surface well below 1 K are demonstrated by thermal desorption data for two different adsorbates: ethane and hydrogen on Si(001)-(2×1).
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1993
The adsorption of acetylene on the Si(100)-(2×1) surface has been studied by high-resolution elec... more The adsorption of acetylene on the Si(100)-(2×1) surface has been studied by high-resolution electron energy-loss spectroscopy (HREELS) and temperature programmed desorption. An energy loss peak at 1450 cm−1, which can be assigned to a C=C double bond, is observed unambiguously in the HREEL spectra. This observation provides direct evidence for the sp2 di-σ adsorption model. The unusually low frequency for the C=C vibration is most likely due to an interaction between the Si atoms and the rehybridized acetylene π orbital. As the surface temperature is increased, one of the C–H bonds cleaves to form a monohydride (Si–H). Most of the acetylene dissociates over a wide temperature range to produce surface carbon and hydrogen. The carbon atoms remain on the surface to form silicon carbide (SiC), while the hydrogen atoms recombine to form molecular hydrogen, which desorbs from the Si(100) surface.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 1996
The adsorption of atomic hydrogen on acetylene-saturated Si(100)-(2×1) was studied using high-res... more The adsorption of atomic hydrogen on acetylene-saturated Si(100)-(2×1) was studied using high-resolution electron energy loss spectroscopy, temperature programmed desorption, Auger electron spectroscopy, and low-energy electron diffraction (LEED). With saturation coverage of preadsorbed acetylene, the initial adsorption probability of atomic hydrogen on Si is reduced to 40% of the initial adsorption probability of atomic hydrogen on clean Si(100)-(2×1), and the uptake of hydrogen to form the Si monohydride up to 0.65±0.05 of a monolayer is observed. Although the hydrogenation of acetylene is substantially slower than the chemisorption of hydrogen as the Si monohydride, molecular desorption of ethylene is observed in temperature programmed desorption following the hydrogenation of chemisorbed acetylene. With further postexposures of atomic hydrogen, the LEED pattern changes from (2×1) to (1×1) and the Si dihydride forms, confirming the conversion of ethylene to ethyl.
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Papers by Wolf Widdra