Electric field gradients in dilute aluminium alloys

Electric field gradients probed by¹¹¹In, which are produced in dilute PdAl alloys, were studied with the PAC technique. Four different configurations are found which are probably due to metastable aluminium-rich PdAl phases.     

Local magnetic properties of ultrathin ferromagnetic fcc-Fe-films on Cu(001)

Conversion electron Mössbayer spectroscopy (CEMS) on three monolayers (ML) thick metastable fcc-Fe(001) films grown epitaxially on a Cu(001) substrate under different conditions shows that these films are characterized by a distributionP(B _hf) of magnetic hyperfine fieldsB _hf. The vast majority of⁵⁷Fe nuclei experience relatively large hyperfine fields at low temperature. The temperature dependence of the most probable fieldB _peak was found to follow aT ^3/2 spin-wave law below 300 K. It is shown from the relative line intensities that preferential Fe spin orientation perpendicular to the film plane exists in films grown at 120 K, while preferential in-plane spin orientation is found for a growth temperature of 300 K. Coating a low-temperature grown Fe film by 2 ML of Cu(001) drastically reduces the hyperfine field, in contrast to the case of room-temperature grown Cu-coated films.Dedicated to Professor Ulrich Gonser on the occasion of his 70th birthday     

Synthesis of silver nanodisks using polystyrene mesospheres as templates

We report the synthesis of Ag nanodisk using polystyrene mesospheres as templates. TEM measurements show that the average disk thickness is 9.0 +/- 1.0 nm and the diameter is 36 +/- 8 nm. The particles exhibit an intense electronic spectrum that differs markedly from that for spheres (purple vs yellow solutions). We present electromagnetic theory calculations which exhibit a qualitative match with the observed spectrum, enabling us to assign several of the plasmon resonance features.     

Matrix isolation magnetic circular dichroism (MCD) spectrum of NH radical produced by a glow discharge technique

MCD and absorption spectra are reported for the X(³Σ^?)→A(³H_i transition of NH produced by a glow discharge and isolated in Ar and Xe matrices. The MCD is strongly temperature dependent and is detectable at concentrations well below those required in a conventional absorption measurement.     

Theory and method for calculating resonance Raman scattering from resonance polarizability derivatives

We present a method to calculate both normal Raman-scattering (NRS) and resonance Raman-scattering (RRS) spectra from the geometrical derivatives of the frequency-dependent polarizability. In the RRS case, the polarizability derivatives are calculated from resonance polarizabilities by including a finite lifetime of the electronic excited states using time-dependent density-functional theory. The method is a short-time approximation to the Kramers, Heisenberg, and Dirac formalism. It is similar to the simple excited-state gradient approximation method if only one electronic excited state is important, however, it is not restricted to only one electronic excited state. Since the method can be applied to both NRS and RRS, it can be used to obtain complete Raman excitation profiles. To test the method we present the results for the S2 state of uracil and the S4, S3, and S2 states of pyrene. As expected, the results are almost identical to the results obtained from the excited-state gradient approximation method. Comparing with the experimental results, we find in general quite good agreement which enables an assignment of the experimental bands to bands in the calculated spectrum. For uracil the inclusion of explicit waters in the calculations was found to be necessary to match the solution spectra. The calculated resonance enhancements are on the order of 10(4)-10(6), which is in agreement with experimental findings. For pyrene the method is also able to distinguish between the three different electronic states for which experimental data are available. The neglect of anharmonicity and solvent effects in the calculations leads to some discrepancy between theory and experiment.     

Capillary force in atomic force microscopy

Under ambient conditions, a water meniscus generally forms between a nanoscale atomic force microscope tip and a hydrophilic surface. Using a lattice gas model for water and thermodynamic integration methods, we calculate the capillary force due to the water meniscus for both hydrophobic and hydrophilic tips at various humidities. As humidity rises, the pull-off force rapidly reaches a plateau value for a hydrophobic tip but monotonically increases for a weakly hydrophilic tip. For a strongly hydrophilic tip, the force increases at low humidities (<30%) and then decreases. We show that mean-field density functional theory reproduces the simulated pull-off force very well.     

Electromagnetic fields around silver nanoparticles and dimers

We use the discrete dipole approximation to investigate the electromagnetic fields induced by optical excitation of localized surface plasmon resonances of silver nanoparticles, including monomers and dimers, with emphasis on what size, shape, and arrangement leads to the largest local electric field (E-field) enhancement near the particle surfaces. The results are used to determine what conditions are most favorable for producing enhancements large enough to observe single molecule surface enhanced Raman spectroscopy. Most of the calculations refer to triangular prisms, which exhibit distinct dipole and quadrupole resonances that can easily be controlled by varying particle size. In addition, for the dimer calculations we study the influence of dimer separation and orientation, especially for dimers that are separated by a few nanometers. We find that the largest /E/2 values for dimers are about a factor of 10 larger than those for all the monomers examined. For all particles and particle orientations, the plasmon resonances which lead to the largest E-fields are those with the longest wavelength dipolar excitation. The spacing of the particles in the dimer plays a crucial role, and we find that the spacing needed to achieve a given /E/2 is proportional to nanoparticle size for particles below 100 nm in size. Particle shape and curvature are of lesser importance, with a head to tail configuration of two triangles giving enhanced fields comparable to head to head, or rounded head to tail. The largest /E/2 values we have calculated for spacings of 2 nm or more is approximately 10(5).     

Spectroscopic probing of the acid-base properties and photosensitization of a fluorinated phthalocyanine in organic solutions and liposomes

A perfluorinated derivative of phthalocyanine was synthesized as the free base, hexadeca-(2,2,2-trifluoroethoxy) phthalocyanine (H2F48Pc), and as a zinc complex, hexadeca-(2,2,2-trifluoroethoxy)-phthalocyaninatozinc (ZnF48Pc), and their spectroscopic and photochemical properties were studied. The absorption bands are shifted bathochromically relative to simple phthalocyanines, exhibiting the longest wavelength band near 735 nm (H2F48Pc) and 705 (ZnF48Pc). The solvatochromism of both compounds was modeled by Reichardt's ET(30) parameter and Kamlet, Abboud and Taft multiparameter approach. The former, simpler, model was found to be adequate. We found that H2F48Pc undergoes unique basic and acidic titrations in organic solvents. These titration processes are accompanied by spectral changes that are explained on the basis of the chromophore's symmetry. Singular value decomposition was employed to resolve the spectra into the contributions of the species at various stages of protonation and to obtain the equilibrium constants. Nuclear magnetic resonance spectra (1H, 19F and 13C) for the free base were obtained in a tetrahydrofurand8 solution. The carbon spectrum, taken as a function of temperature, provided evidence for the presence of a tautomerization process, which switches the two internal hydrogens between the four central nitrogen atoms. As far as we know, this is the first report of the measurement of the free energy of activation for such process (delta G = 10.6-11.4 kcal mol-1 between 217 and 330 K) for a phthalocyanine, in solution. Like most other phthalocyanines these two compounds also act as photosensitizers and as generators of singlet molecular oxygen. The absolute quantum yields (phi delta) for ZnF48Pc was 0.58 +/- 0.01 in benzene and 0.35 +/- 0.01 in lipid vesicles. H2F48Pc had lower yields, 0.16 and 0.005, respectively. Either protonation or deprotonation of the pyrrole nitrogens in H2F48Pc lowered the phi delta.     

Über die Reaktion von Δ9, 12-Linolsäureäthylester in Wasser, 4. Mitt.: Bildung wasserlöslicher Fettsäureester-Hydroperoxyde mit atmungs- und gärungshemmender Wirkung auf Tumorzellen und Hefe

Zusammenfassung Die weitere Untersuchung der Wasserreaktion von Äthyllinolat ergab, daß es sich zweifellos um eine, wenn auch weitgehend modifizierte Autoxydation handelt. Als interessantestes Reaktionsprodukt werden wasserlösliche C₁₈-Monohydroperoxyde gefunden, deren chemische Konstitution von der der bisher bekannten Linolmonohydroperoxyde wesentlich abweichen dürfte.Wenn die gefundenen Fettsäureester-Hydroperoxyde auch noch nicht in Substanz dargestellt und hinsichtlich ihrer Konstitution aufgeklärt werden konnten, so war es doch bereits möglich, sie als Träger einer bemerkenswerten biologischen Wirkung zu identifizieren: der Hemmung von aerober und anaerober Glykolyse und der Atmung vonEhrlich-Ascites-Tumorzellen. Da die aktiven Hydroperoxydgruppen des Präparates durch Katalase nicht angegriffen werden, ist auch ihre Hemmwirkung katalaseresistent. Die Glykolysehemmung setzt sich aus einem verhältnismäßig geringen unspezifischen Lipideffekt sowie aus einem spezifischen Hydroperoxydeffekt zusammen, der z. B. über eine Hemmung der Triosephosphatde-hydrogenase zustande kommen könnte. Die Halbwertsdosis für die gleich starke Atmungs- und Gärungshemmung liegt bei den derzeit zur Verfügung stehenden Mischpräparaten bei 10^–3 m/l.Erwartungsgemäß hemmt Präp. LHPO infolge seiner Resistenz gegen Katalase die Gärung der Hefe (Halbwertkonzentration <5·10^–4 m/l), während H₂O₂ verständlicherweise unwirksam ist.Ergänzende Versuche mit dem reduzierten Präparat zeigten, daß auch bei Hefe die Hemmwirkung von LHPO aus einem spezifischen Hydroperoxydeffekt und einem schwächeren unspezifischen sogen. Lipideffekt besteht.Die hier mitgeteilten Ergebnisse lassen als reine in-vitro-Befunde noch keinerlei Rückschlüsse zu, ob Präp. LHPO in gleicher Weise auch im lebenden Organismus wirkt. Diese Frage bildet den Gegenstand umfangreicher Untersuchungen vonM. Ratzenhofer undJ. Zangger, worüber in einer gemeinsamen Arbeitsreihe a. a. O. berichtet werden wird.Mit 12 Abbildungen