Characterization of EUV periodic multilayers

Nanometric Co/Mg, Co/Mg/B₄C, Al/SiC and Al/Mo/SiC periodic multilayers deposited by magnetron sputtering are studied in order to correlate their optical performances in the extreme ultraviolet (EUV) range to their structural quality. To that purpose, our recently developed methodology based on high‐resolution X‐ray emission spectroscopy (XES) and X‐ray and EUV reflectometry is now extended to nuclear magnetic resonance (NMR) spectroscopy and time‐of‐flight secondary ions mass spectrometry (ToF‐SIMS). The analysis of the Co Lαβ and Mg Kβ emission spectra shows that the Co and Mg atoms within the multilayers are in a chemical state equivalent to that of the atoms in the pure Co and Mg references, respectively. But NMR spectra give evidence for a reaction between Co atoms and B and/or C atoms from B₄C. The Al and Si Kβ emission spectra do not reveal the formation of an interfacial compound in Al/SiC and Al/Mo/SiC. Only the roughness limits the optical quality of Al/SiC. The comparative analysis of the ToF‐SIMS spectra of Al/SiC and Al/Mo/SiC indicates that the structural quality is enhanced when Mo is introduced within the stack. Copyright © 2011 John Wiley & Sons, Ltd.     

Donor‐Acceptor‐Functionalized Tetraethynylethenes with Nitrothienyl Substituents: Structure‐Property Relationships

Tetraethynylethenes (TEEs) functionalized with donor (4‐(dimethylamino)phenyl) and acceptor (5‐nitro‐2‐thienyl) groups were prepared by Pd⁰‐catalyzed Sonogashira cross‐coupling reactions (Schemes 1 – 6). The physical properties of these novel chromophores were examined and compared with those of analogous systems containing 4‐nitrophenyl instead of 5‐nitro‐2‐thienyl acceptor groups. X‐Ray crystal‐structure analyses showed the π‐conjugated frameworks of 2 , 11 , and 13 , including the TEE core and all aryl moieties, to be nearly perfectly planar (Figs. 1, 3, and 4). In contrast, one 4‐(dimethylamino)phenyl group in 10 is rotated almost 90° out of the molecular plane, presumably due to crystal‐packing effects (Fig. 2). The analysis of bond lengths and bond angles revealed little, if any, evidence of intramolecular ground‐state donor‐acceptor interactions. The electrochemical behavior of nitrothienyl‐substituted TEEs is similar to that of the corresponding nitrophenyl‐functionalized derivatives (Table 3). The nitrothienyl groups were reduced at −1.23 V (vs. the ferrocene/ferricinium couple, Fc/Fc⁺), regardless of the degree or pattern of other substitutions. For nonsymmetrical TEE 13 , the reduction of the nitrothienyl group at −1.23 V is followed by a reduction of the nitrophenyl group at −1.40 V, a potential typical for the reduction of other nitrophenyl‐substituted TEEs, such as 17 – 20 . UV/VIS Spectroscopy showed a consistently lower‐energy absorption cutoff for nitrothienyl derivatives compared with the analogous nitrophenyl‐substituted TEEs that confirms a lowering of the HOMO‐LUMO gap as a result of nitrothiophene substitution (Figs. 5 and 6). A comparison of the tetrakis‐arylated TEEs 11 , 13 , and 20 clearly showed a steady bathochromic shift of the longest‐wavelength absorption maximum and the end‐absorption upon sequential replacement of nitrophenyl by nitrothienyl groups. Quantum‐chemical computations were performed to explain a number of complex features of the electronic absorption spectra. All empirical features of relevance in the experimental UV/VIS spectra for 2 , 5 , 6 , and 17 – 19 were correctly reproduced by computation (Tables 4 and 5). The combination of theory and experiment was found to be very useful to explain the particular acceptor properties of the 5‐nitro‐2‐thienyl group.     

Structural Properties of Phenylalanine-Based Dimers Revealed Using IR Action Spectroscopy

Peptide segments with phenylalanine residues are commonly found in proteins that are related to neurodegenerative diseases. However, the self-assembly of phenylalanine-based peptides can be also functional. Peptides containing phenylalanine residues with different side caps, composition, and chemical alteration can form different types of nanostructures that find many applications in technology and medicine. Various studies have been performed in order to explain the remarkable stability of the resulting nanostructures. Here, we study the early stages of self-assembly of two phenylalanine derived peptides in the gas phase using IR action spectroscopy. Our focus lies on the identification of the key intra- and intermolecular interactions that govern the formation of the dimers. The far-IR region allowed us to distinguish between structural families and to assign the 2-(2-amino-2-phenylacetamido)-2-phenylacetic acid (PhgPhg) dimer to a very symmetric structure with two intermolecular hydrogen bonds and its aromatic rings folded away from the backbone. By comparison with the phenylalanine-based peptide cyclic L-phenylalanyl-L-phenylalanine (cyclo-FF), we found that the linear FF dimer likely adopts a less ordered structure. However, when one more phenylalanine residue is added (FFF), a more structurally organized dimer is formed with several intermolecular hydrogen bonds.     

Phenylpropargyl Radicals and Their Dimerization Products: An IR/UV Double Resonance Study

Two C(9)H(7) isomers, 1-phenylpropargyl and 3-phenylpropargyl, have been studied by IR/UV double resonance spectroscopy in a free jet. The species are possible intermediates in the formation of soot and polycyclic aromatic hydrocarbons (PAH). The radicals are generated by flash pyrolysis from the corresponding bromides and ionized at 255-297 nm in a one-color, two-photon process. Mid-infrared radiation between 500 and 1800 cm(-1) is provided by a free electron laser (FEL). It is shown that the two radicals can be distinguished by their infrared spectra. In addition, we studied the dimerization products originating from the phenylpropargyl self-reaction. We utilize the fact that the pyrolysis tube can be considered to be a flow reactor permitting us to investigate the chemistry in such a thermal reactor. Dimerization of phenylpropargyl produces predominately species with m/z = 228 and 230. A surprisingly high selectivity has been found: The species with m/z = 230 is identified to be para-terphenyl, whereas m/z = 228 can be assigned to 1-phenylethynyl-naphthalene. The results allow to derive a mechanism for the dimerization of phenylpropargyl and suggest hitherto unexplored pathways to the formation of soot and PAH.     

Simple test cases for validating a finite element unstructured grid fecal bacteria transport model

This paper presents analytical test cases for tracer advection–diffusion-decay problems. The test cases are used to validate a finite element, unstructured grid fecal bacteria transport model. The test cases include the following domains: one-dimensional infinitely long river, two-dimensional half plane and two-dimensional infinitely long channel. In this work the bacteria are considered to enter the domain only through point sources. Analytical solutions are derived using either a Dirac delta function or a variable-width Gaussian function as a point source. Both analytical derivations and numerical simulations suggest that the error is maximised at the source. We present formulae for estimating the error caused by replacing a Dirac source with a Gaussian function in the numerical model. Furthermore, numerical simulations suggest that the best approximation for a Dirac source is a Gaussian whose width parameter is one third of the local mesh size.     

Do Xylylenes Isomerize in Pyrolysis?

We report infrared spectra of xylylene isomers in the gas phase, using free electron laser (FEL) radiation. All xylylenes were generated by flash pyrolysis. The IR spectra were obtained by monitoring the ion dip signal, using a IR/UV double resonance scheme. A gas phase IR spectrum of para-xylylene  was recorded, whereas ortho- and meta-xylylene were found to partially rearrange to benzocyclobutene and styrene. Computations of the UV oscillator strength  for all molecules were carried out and provde an explanation for the observation of the isomerization products.