Ultracold Atoms as a Target: Absolute Scattering Cross-Section Measurements

We report on a new experimental platform for the measurement of absolute scattering cross-sections. The target atoms are trapped in an optical dipole trap and are exposed to an incident particle beam. The exponential decay of the atom number directly yields the absolute total scattering cross-section. The technique can be applied to any atomic or molecular species that can be prepared in an optical dipole trap and provides a large variety of possible scattering scenarios

Adiabatic loading of a Bose-Einstein condensate in a 3D optical lattice

We experimentally investigate the adiabatic loading of a Bose-Einstein condensate into an optical lattice potential. The generation of excitations during the ramp is detected by a corresponding decrease in the visibility of the interference pattern observed after free expansion of the cloud. We focus on the superfluid regime, where we show that the limiting time scale is related to the redistribution of atoms across the lattice by single-particle tunneling

Precision measurement of spin-dependent interaction strengths for spin-1 and spin-2 87Rb atoms

We report on precision measurements of spin-dependent interaction-strengths in the 87Rb spin-1 and spin-2 hyperfine ground states. Our method is based on the recent observation of coherence in the collisionally driven spin-dynamics of ultracold atom pairs trapped in optical lattices. Analysis of the Rabi-type oscillations between two spin states of an atom pair allows a direct determination of the coupling parameters in the interaction hamiltonian. We deduce differences in scattering lengths from our data that can directly be compared to theoretical predictions in order to test interatomic potentials. Our measurements agree with the predictions within 20%. The knowledge of these coupling parameters allows one to determine the nature of the magnetic ground state. Our data imply a ferromagnetic ground state for 87Rb in the f=1 manifold, in agreement with earlier experiments performed without the optical lattice. For 87Rb in the f=2 manifold the data points towards an antiferromagnetic ground state, however our error bars do not exclude a possible cyclic phase.Comment: 11 pages, 5 figure

In Situ Surface Studies of Site-Isolated Hydrogenation Catalysts – The Intermetallic Compound PdGa

Selective acetylene hydrogenation is an important method for removing traces of acetylene in the ethylene feed for the production of polyethylene. Typical catalysts, like Pd dispersed on metal oxides are widely used for this reaction and show a limited selectivity and long-term stability. This can be attributed to the presence of active-sites ensembles on the catalyst surface. This drawback can be overcome by using the intermetallic compound PdGa which possesses palladium atoms in the crystal structure well isolated from each other by a gallium shell. PdGa shows higher selectivity and increased long-term stability compared to the commercial catalysts, including PdAg alloys. In the present work the surface of the intermetallic compound PdGa was probed by in situ XPS as well as CO adsorption using FTIR spectroscopy. The XPS investigation before hydrogenation revealed a significant modification of the Pd electronic state in the intermetallic compound compared to Pd metal: the Pd3d5/2 peak is shifted by 1 eV to higher binding energy. In situ XPS measurements, performed at ~1 mbar pressure, showed a high stability of the Pd surface states without appearance of any additional components or significant shifts of the Pd3d5/2 peak when applying the reactive atmosphere and temperature (1.0 mbar of H2 + 0.1 mbar of C2H2 at 120 ºC). This is in contrast to Pd metal for which the formation of an additional Pd component during alkyne hydrogenation was detected. Investigation of carbon and palladium depth profiles for PdGa indicates the absence of a subsurface carbon-containing phase, distinguishing this material decidedly from metallic palladium catalysts. The adsorption of CO on the PdGa compound at room temperature results in the appearance of only one band with a maximum at 2047 cm-1, which corresponds to linear Pd–CO carbonyls. It should be mentioned that the observed band (2047 cm–1) is shifted to lower wavenumbers compared to the respective CO (on-top) species forming upon adsorption on metallic palladium (2100-2080 cm–1), which is an indication for the modification of the Pd electronic states by covalent bonding in the investigated intermetallic compound. The absence of bands due to bridged carbonyls in the observed spectra and the fact that the observed band is not coverage dependent indicated that the active sites in PdGa are really isolated. Characterization of PdGa by FTIR and in situ XPS revealed high surface stability during the reaction of acetylene hydrogenation and confirms the isolation of the active Pd site on the surface. In combination with modified electronic Pd states due to covalent bonding in the intermetallic compound it leads to superior catalytic properties like high selectivity and long-term stability during the partial hydrogenation of acetylene

In Situ Studies of Site-Isolated Hydrogenation Catalysts – The Intermetallic Compound PdGa

Selective acetylene hydrogenation is an important method for removing traces of acetylene in the ethylene feed for the production of polyethylene. Typical catalysts show a limited selectivity and long-term stability. This can be attributed to the presence of active-site ensembles. This drawback can be overcome by using the intermetallic compound PdGa which possesses palladium atoms in the crystal structure well isolated by a gallium shell. PdGa shows higher selectivity and increased long-term stability compared to commercial catalysts. The XPS investigation before the reaction revealed a significant modification of the Pd electronic state in the intermetallic compound compared to Pd metal: the Pd3d5/2 peak is shifted by 1 eV to higher binding energy. In situ XPS measurements showed a high stability of the Pd surface states without appearance of any additional components or significant shifts of the Pd3d5/2 peak when applying the reactive atmosphere and temperature (1.0 mbar H2 0.1 mbar C2H2 at 120 ºC). This is in contrast to Pd metal for which the formation of an additional Pd component during alkyne hydrogenation was reported recently. The adsorption of CO on PdGa at room temperature results in the appearance of only one band with a maximum at 2047 cm-1, which should correspond to linearly bound CO (Pd–CO). It should be mentioned that the observed band (2047 cm–1) is shifted to lower wavenumbers compared to the respective CO (on-top) species forming upon adsorption on metallic palladium (2100-2080 cm–1), which may be an indication for the modification of the Pd electronic states by covalent bonding in the investigated intermetallic compound. The absence of bands due to bridged carbonyls in the spectra and the fact that the observed band is not coverage dependent indicates that the active sites in PdGa are really isolated. Characterization of PdGa revealed high surface stability during the hydrogenation of acetylene and confirms the isolation of the active Pd sites on the surface. In combination with the modified electronic Pd states – perhaps due to the covalent bonding – it leads to superior catalytic properties high selectivity and long-term stability

Towards the pressure and material gap in heterogeneous catalysis: hydrogenation of acrolein over silver catalysts

Introduction In recent time, increasing effort has been undertaken in order to answer the question, whether it is justified to transfer results from surface science studies, mostly obtained with idealised surfaces under UHV conditions, to "real" catalysis, i.e. high pressures and complex materials (the so-called pressure and material gaps). The DFG (German research foundation) has initialised a priority program (SPP 1091) in order to bring together experts from surface science, materials science, catalysis and theory with the aim of bridging the two gaps in catalysis. Within this priority program, we are currently studying the hydrogenation of acrolein over silver. Acrolein, an ,-unsaturated aldehyde, can be hydrogenated either to propanal (product of C=C-bond hydrogenation) or to allyl alcohol (product of C=O-bond hydrogenation. Whereas typical hydrogenation catalysts like Pt, Ru or Ni mainly produce the saturated aldehyde, selectivities to allyl alcohol of up to 53 % can be obtained when using monometallic silver (or gold) catalysts [ , ]. The aim of our studies is to clarify the influence of reaction pressure and material on the selectivity distribution in the acrolein hydrogenation. Catalytic experiments have been carried out with differently structured samples from single crystals to disperse Ag/support catalysts in a broad pressure range (few mbar up to 20 bar). Various methods like in situ-XAS and XPS, flow-adsorption calorimetry, infrared spectroscopy, and transient analysis of products (TAP) are performed in order to gain insight into the modes of interaction of acrolein and hydrogen with differently structured silver samples. Effects of particle size and shape are also considered as well as the influence of the support material. Experimental and Results would add a few words about the composition (Ag loading, different supports) / preparation of the catalysts and/or a reference Gas phase hydrogenation of acrolein has been carried out in a flow reaction system allowing a pressure in the range from 150 mbar up to 20 bar. When using silica supported silver catalysts, clear relations can be drawn concerning the pressure and material dependence of the selectivity to allyl alcohol: increasing partial pressure of either reactant (hydrogen or acrolein) leads to increased selectivity to allyl alcohol, also, smaller particles favour its formation. However, when using ZnO-supported catalysts, the situation becomes more complex. Catalysts prepared with the same catalyst loading and the same catalyst preparation technique but with different ZnO support materials yielded different selectivities to allyl alcohol at the same conversion. On the other hand, catalysts prepared from different precursors, but with the same support, lead to different activities but similar selectivities to allyl alcohol. TEM investigations of the Ag/ZnO and Ag/SiO2 catalysts reveal, that the particle sizes of the silica-supported catalysts are much smaller (2 nm and 15 nm in average for the two most intensively studied catalysts) whereas the silver particles in the Ag/ZnO catalysts are surprisingly large (50 nm up to several hundreds of nm). This is even more surprising since the activities of the catalysts are in the same order of magnitude, with the SiO2 catalysts however, being a bit more active. All these results indicate, that the product distribution at supported silver catalysts is governed by a complex interplay between particle size (and/or shape), pressure, and, as the obviously most important factor, the support and the interactions between silver and support. To gain more insight into the reasons for the catalytic behaviour of the Ag/support catalysts, the interaction of hydrogen alone with various silver samples has been studied. TAP (temporal analysis of products) indicates, that hydrogen interacts with nanodisperse Ag/SiO2 samples, but not with larger unsupported silver particles (several mm in size) like those from electrolyte silver. However, as monitored by transmission infrared spectroscopy, not only the Ag nanoparticles but also the SiO2 support interacts with hydrogen. SiO2 and Ag/SiO2 samples, after reduction and exposure to 100 mbar D2, show a reversible H-D-exchange, as monitored by the Si-O-H(D) bands. Time resolved IR spectra indicate, that this H-D-exchange is faster at silver-containing samples. From temperature-dependent measurements, activation energies for the H-D-exchange of ca. 28 kJ/mol for Ag/SiO2 and ca. 38 kJ/mol for SiO2 have been calculated. The interaction of acrolein with silver single crystals as well as with supported catalysts has been studied with in-situ-XAS and in-situ XPS. For both techniques the samples were contacted with mixtures of H2/acrolein in the mbar pressure range. Angular dependent XAS measurements on a Ag(111) single crystal indicated that acrolein is in the lying-down orientation. For all the samples measured the 1π* “C=O” transition is clearly increased compared to the 1π* “C=C”. Consequently, the surface concentration of C=O bonds relative to C=C bonds is higher, which is in line with concomitantly measured mass spectrometric data showing high selectivity towards C=C hydrogenation. In-situ XPS revealed that while silver foil is partly oxidic (~5%) the supported silver particles are completely reduced, as Ag is in the zero valence state. Data indicate also small amount of oxygen removal from the ZnO supported samples during the contact with hydrogen. The combination of different results suggests that metal-support interaction plays an important role in the reaction. The major difference in hydrogen activation between supported catalysts and pure silver/support provides us a hint that the so-called “adlineation sites” (the perimeter interface between silver and support) are the key sites in the mechanism

In-situ electron-beam lithography of deterministic single-quantum-dot mesa-structures using low-temperature cathodoluminescence spectroscopy

We report on the deterministic fabrication of sub-um mesa structures containing single quantum dots by in-situ electron-beam lithography. The fabrication method is based on a two-step lithography process using a low-temperature cathodoluminescence (CL) spectroscopy setup. In the first step the position and spectral features of single InGaAs quantum dots (QDs) are detected by CL. Then circular sub-um mesa-structures are exactly defined by high-resolution electron-beam lithography and subsequent etching in the second step. CL spectroscopy and micro-photoluminscence spectroscopy demonstrate the high optical quality of the single-QD mesa-structures with emission linewidths below 15 ueV and g(2)(0) = 0.04. Our lithography method allows for an alignment precision better than 100 nm which paves the way for a fully-deterministic device technology using in-situ CL lithography.Comment: 4 pages, 4 figure

In situ Surface Characterization of the Intermetallic Compound PdGa – A Highly Selective Hydrogenation Catalyst

The structurally well-defined intermetallic compound PdGa – a highly selective catalyst for the semi-hydrogenation of acetylene – was characterized by Fourier transform infrared spectroscopy (FTIR) in situ X-ray photoelectron spectroscopy and in situ Prompt gamma activation analysis. A strong modification of the electronic states in PdGa compared to elemental Pd was revealed as well as the complete isolation of the Pd atoms on the surface of PdGa. In situ investigations proved the high stability of the surface, thus excluding segregation phenomena (common for alloys) or sub-surface chemistry involving C and/or H atoms (known for elemental Pd). By suppressing the sub-surface chemistry, the electronic modification as well as the site isolation lead to the high selectivity and long-term stability of PdGa in the semi-hydrogenation of acetylene