No abstract

Coherent diffractive imaging for the reconstruction of a two-dimensional (2D) finite crystal structure with a single pulse train of free-electron laser radiation at 7.97 nm wavelength is demonstrated. This measurement shows an advance on traditional coherent imaging techniques by applying it to a periodic structure. It is also significant that this approach paves the way for the imaging of the class of specimens which readily form 2D, but not three-dimensional crystals. We show that the structure is reconstructed to the detected resolution, given an adequate signal-to-noise ratio.

The electronic structure of several arene-based monomolecular films on Au(111), with and without an alkyl linker between the aromatic unit and thiol headgroup, has been investigated by photoemission and inverse photoemission. While the HOMO–LUMO gap in these films was found to depend on the aromatic backbone, the molecular band offset of the electronic states was strongly affected by the interfacial dipole. The smallest HOMO–LUMO gap was found for the strongly conjugated anthracene moiety, intermediate for terphenyl, and widest for the perfluorinated terphenyl. The perfluorinated terphenyl-based films appear to be more n-type as a semiconductor than the terphenyl or anthracene-based monolayers, as indicated by the placement of the Fermi level (chemical potential) relative to the conduction or lowest molecular orbital band edge. Accordingly, the occupied electronic states related to the aromatic rings sink to greater binding energies, well below those for the alkyl linker, and thus for the perfluorinated terphenyl, the aromatic orbital contribution is not to the HOMO but the HOMO-1 orbital (one occupied molecular orbital away from the HOMO). This placement of the perfluorinated terphenyl aromatic orbital contribution is in drastic contrast to the nonfluorinated systems in our study, in which both HOMO and LUMO orbitals are extended throughout the aromatic moieties.

Abstract:The imaging of hydrated biological samples -especially in the energy window of 284-540 eV, where water does not obscure the signal of soft organic matter and biologically relevant elements -is of tremendous interest for life sciences. Free-electron lasers can provide highly intense and coherent pulses, which allow single pulse imaging to overcome resolution limits set by radiation damage. One current challenge is to match both the desired energy and the intensity of the light source. We present the first images of dehydrated biological material acquired with 3rd harmonic radiation from FLASH by digital in-line zone plate holography as one step towards the vision of imaging hydrated biological material with photons in the water window. We also demonstrate the first application of ultrathin molecular sheets as suitable substrates for future free-electron laser experiments with biological samples in the form of a rat fibroblast cell and marine biofouling bacteria Cobetia marina. ©2011 Optical Society of America References and links1. C. A. Brau, "Free-electron lasers," Science 239(4844), 1115-1121 (1988). 2. J. Feldhaus, J. Arthur, and J. Hastings, "X-ray free-electron lasers," J. Phys. B: At. Mol. Opt. Phys. 38(9), S799-S819 (2005

We report on a resonant magnetic scattering experiment using soft x-ray pulses generated from a freeelectron laser ͑FEL͒. The free-electron laser was operated at a fundamental wavelength of 7.97 nm and radiation at the fifth harmonic originating from self-amplified stimulated emission at 1.59 nm with an average energy of 4 nJ per pulse was detected. We demonstrate the feasibility of resonant magnetic scattering at FEL sources by using a Co/Pd multilayer as prototype sample that was illuminated with 20-fs-long soft x-ray pulses tuned to the Co L 3 absorption edge at 778.1 eV ͑1.59 nm͒.

Femtosecond vacuum ultraviolet (VUV) radiation provided by the free-electron laser FLASH was used for digital in-line holographic microscopy and applied to image particles, diatoms and critical point dried fibroblast cells. To realize the classical in-line Gabor geometry, a 1 microm pinhole was used as spatial filter to generate a divergent light cone with excellent pointing stability. At a fundamental wavelength of 8 nm test objects such as particles and diatoms were imaged at a spatial resolution of 620 nm. In order to demonstrate the applicability to biologically relevant systems, critical point dried rat embryonic fibroblast cells were for the first time imaged with free-electron laser radiation.

With the development of novel fluorescence techniques, high resolution light microscopy has become a challenging technique for investigations of the three-dimensional (3D) micro-cosmos in cells and sub-cellular components. So far, all fluorescence microscopes applied for 3D imaging in biosciences show a spatially anisotropic point spread function resulting in an anisotropic optical resolution or point localization precision. To overcome this shortcoming, micro axial tomography was suggested which allows object tilting on the microscopic stage and leads to an improvement in localization precision and spatial resolution. Here, we present a miniaturized device which can be implemented in a motor driven microscope stage. The footprint of this device corresponds to a standard microscope slide. A special glass fiber can manually be adjusted in the object space of the microscope lens. A stepwise fiber rotation can be controlled by a miniaturized stepping motor incorporated into the device. By means of a special mounting device, test particles were fixed onto glass fibers, optically localized with high precision, and automatically rotated to obtain views from different perspective angles under which distances of corresponding pairs of objects were determined. From these angle dependent distance values, the real 3D distance was calculated with a precision in the ten nanometer range (corresponding here to an optical resolution of 10-30 nm) using standard microscopic equipment. As a proof of concept, the spindle apparatus of a mature mouse oocyte was imaged during metaphase II meiotic arrest under different perspectives. Only very few images registered under different rotation angles are sufficient for full 3D reconstruction. The results indicate the principal advantage of the micro axial tomography approach for many microscopic setups therein and also those of improved resolutions as obtained by high precision localization determination.

Digital in-line soft x-ray holography (DIXH) was used to image immobilized polystyrene and iron oxide particles and to distinguish them based on their different x-ray absorption cross sections in the vicinity of the carbon K-absorption edge. The element-specific information from the resonant DIXH images was correlated with high-resolution scanning electron microscopy (SEM) pictures. We also present DIXH images of a cell nucleus and compare the contrast obtained for nuclear components with the appearance in optical microscopy.