Die Invarianten von Streifen auf Flächen in der Bewegungsgeometrie

Ohne Zusammenfassung     

DNA as helical ruler: exciton-coupled circular dichroism in DNA conjugates

The structure and properties of oligonucleotide conjugates possessing stilbenedicarboxamide chromophores at both ends of a poly(dA):poly(dT) base-pair domain of variable length have been investigated using a combination of spectroscopic and computational methods. These conjugates form capped hairpin structures in which one stilbene serves as a hairpin linker and the other as a hydrophobic end-cap. The capping stilbene stabilizes the hairpin structures by ca. 2 kcal/mol, making possible the formation of a stable folded structure containing a single A:T base pair. Exciton coupling between the stilbene chromophores has little effect on the absorption bands of capped hairpins. However, exciton-coupled circular dichroism (EC-CD) can be observed for capped hairpins possessing as many as 11 base pairs. Both the sign and intensity of the EC-CD spectrum are sensitive to the number of base pairs separating the stilbene chromophores, as a consequence of the distance and angular dependence of exciton coupling. Calculated spectra obtained using a static vector model based on canonical B-DNA are in good agreement with the experimental spectra. Molecular dynamics simulations show that conformational fluctuations of the capped hairpins result in large deviations of the averaged spectra in both the positive and negative directions. These results demonstrate for the first time the ability of B-DNA to serve as a helical ruler for the study of electronic interactions between aligned chromophores. Furthermore, they provide important tests for atomistic theoretical models of DNA.     

Ion current calculations based on three dimensional Poisson-Nernst-Planck theory for a cyclic peptide nanotube

Ion current calculations based on Poisson-Nernst-Planck (PNP) theory are performed for a synthetic cyclic peptide nanotube that consists of eight or ten cyclo[(-L-Trp-D-Leu-)4] embedded in a lipid bilayer membrane to investigate the ion transport properties of the nanotube. To explore systems with arbitrary geometries, three-dimensional PNP theory is implemented using a finite difference method. The influence of dipolar lipid molecules on the ion currents is also examined by turning on or off the charges of the lipid dipoles in dipalmitoylphosphatidylcholine (DPPC). Comparisons between the calculated and experimentally measured ion currents show that the PNP approach agrees well with the measurements at low ion concentrations but overestimates the currents at higher concentrations. Concentration profiles reveal the selectivity of the peptide nanotube to cations, which is attributed to the negatively charged carbonyl oxygens inside the nanotube. The dominant cation and the minimum anion concentrations inside the cyclic peptide nanotube suggest that these cyclic peptide nanotubes can be employed as ion sensors. In the case of the polar DPPC bilayer, smaller currents are obtained in the calculation. The variation of current with polarity of the lipids implies that both polar and nonpolar lipid bilayer membranes can be utilized to regulate ion currents in the peptide nanotube and other ion channels. Strengths and limitations of the PNP theory are also discussed.     

Gold nanoparticle dimer plasmonics: finite element method calculations of the electromagnetic enhancement to surface-enhanced Raman spectroscopy

Finite element method calculations were carried out to determine extinction spectra and the electromagnetic (EM) contributions to surface-enhanced Raman spectroscopy (SERS) for 90-nm Au nanoparticle dimers modeled after experimental nanotags. The calculations revealed that the EM properties depend significantly on the junction region, specifically the distance between the nanoparticles for spacings of less than 1 nm. For extinction spectra, spacings below 1 nm lead to maxima that are strongly red-shifted from the 600-nm plasmon maximum associated with an isolated nanoparticle. This result agrees qualitatively well with experimental transmission electron microscopy images and localized surface plasmon resonance spectra that are also presented. The calculations further revealed that spacings below 0.5 nm, and especially a slight fusing of the nanoparticles to give tiny crevices, leads to EM enhancements of 10¹⁰ or greater. Assuming a uniform coating of SERS molecules around both nanoparticles, we determined that regardless of the separation, the highest EM fields always dominate the SERS signal. In addition, we determined that for small separations less than 3% of the molecules always contribute to greater than 90% of the signal. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Hyperfine-interaction studies of surfaces

Applications of hyperfine-interaction techniques, like NMR, PAC and Mößbauer spectroscopy, to well-characterized surfaces are discussed and the present knowledge of surface hyperfine fields is reviewed. Measurements of nuclear spin relaxation permit to extract the local density of electron states at the Fermi level of adsorbed alkali atoms. From the observed electric-field-gradient properties surface probe sites and diffusion processes can be inferred; the experimentally determined magnetic hyperfine fields give access to the electron-spin behaviour at magnetic surfaces.     

Generation of far-infrared pulses by use of a passively mode-locked high-pressure CO2 laser

Summary Our present study demonstrates for the first time generation of frequency-tunable intense infrared radiation pulses in the nanosecond time regime by mode-locking a high pressure CO₂ laser using p-doped germanium as a saturable absorber. These pulses were transferred into the FIR region via stimulated Raman scattering of the CO₂ laser radiation in CH₃F resulting in subnanosecond FIR laser pulses.     

Compound formation at Pd(100)/In interfaces

PAC spectroscopy has been used to study the compound formation at the Pd(100)/In interface. Indium films of various thicknesses have been deposited onto Pd(100) single crystals. The¹¹¹In PAC probes are either deposited on the Pd(100) surface before covering it with In or introduced into the In metal before producing the interface. Pd/In compounds with polycrystalline structure of various stoichiometry are identified from the detection of significant electric field gradient tensors after isochronal as well as isothemal annealing. In particular we find an ordered structure at the interface directly after In deposition.     

The electronic absorption and magnetic circular dichroism spectra of IrBr6 2- in several host crystals

The absorption and magnetic circular dichroism (M.C.D.) spectrum of the IrBr₆ ^2- ion at room and liquid helium temperature has been studied in the host crystals (NH₄)₂SnBr₆, K₂SnBr₆ and (C₂H₅NH₃)₂SnBr₆ in the region ～11 000–21 000 cm^-1. An interpretation of the spectrum is presented which differs significantly from those suggested previously. In order of increasing energy the allowed bands are assigned to the following ligand-to-metal charge-transfer transitions: E_g ″(² T _2g )→ U_u ′(² T _1u ) (13–14 000 cm^-1), E_g ″(² T _2g )→ E_u ″(² T _2u ) (16 800 cm^-1), and E_g ″(² T _2g )→ U_u ′(² T _2u ) (～ 18 300 cm^-1). Both our absorption and M.C.D. data strongly suggest a Jahn-Teller splitting of the U_u ′(² T _1u ) state but contradict a previous suggestion of such a splitting of the U_u ′(² T _2u ) state. Consideration of σ—π mixing in the t _1u (π + σ) molecular orbital suggests that the ～17 300 cm^-1 band arises from the orbitally-forbidden E_g ″(² T _2g )→ E_u ′(² T _1u ) transition. Bands in the 11 000–13 000 cm^-1 region are assigned to parity-forbidden charge-transfer transitions to states generated by the t _1g (π)→ t _2g excitation. The fine structure seen at liquid helium temperature in K₂SnBr₆ : Ir⁴⁺ both in the 14 500 cm^-1 band and overlying the E_g ″→ U_u ′(² T _2u ) band appears to be associated with parity-forbidden transitions.     

Elastic alpha-12C scattering and the 12C(alpha,gamma)16O E2 S factor

Angular distributions of 12C(alpha,alpha)12C have been measured for E(alpha) = 2.6-8.2 MeV, at angles from 24 to 166, yielding 12 864 data points. R-matrix analysis of the ratios of elastic scattering yields a reduced width amplitude of gamma12 = 0.47 +/- 0.06 MeV(1/2) for the Ex = 6.917 MeV (2+) state in 16O(a = 5.5 fm). The dependence of the chi2 surface on the interaction radius a has been investigated and a deep minimum is found at a = 5.42(+0.16)(-0.27) fm. Using this value of gamma12, radiative alpha capture and 16N beta-delayed alpha-decay data, the S factor is calculated at E(c.m.) = 300 keV to be S(E2)(300) = 53(+13)(-18) keV b for destructive interference between the subthreshold resonance tail and the ground state E2 direct capture.     

Response of a Bloch oscillator to a THz-field

In this paper we report on the observation of response of a Bloch oscillator at room temperature to a THz-field of a frequency larger than the Bloch frequency. The oscillator consisted of a semiconductor superlattice structure, with an applied dc voltage giving rise to a dc electron drift current. Submitting the oscillator to a field at a frequency of 3.3 THz caused a sizeable reduction of the current; the THz-field was generated by use of intense THz-radiation pulses focused on an antenna coupled to the superlattice. We attribute the THz-field induced reduction of the current to a frequency modulation of the Bloch oscillations of electrons at the frequency of the THz-field, leading to reduction of the electron drift velocity and, consequently, of the current.