Onset of crater formation during short pulse laser ablation

We report morphologic changes of metallic surfaces at the onset of ablation, starting from gentle ablation to the emergence of ablation craters. The evolution of both observed melting zones and of ablation craters therein are investigated in dependence of the ablation laser fluence for nanosecond ultraviolet laser pulses. Further, consequences of crater formation for cluster synthesis within the released atomic vapor are pointed out. PACS 52.38.Mf; 79.20.Ds; 65.40.De     

SIACON—Equipment for the on-line determination of the silicon content of coal

The paper describes an automatic equipment which allows to determine the silicon content of coal by neutron activation analysis. The technique is based on the measurement of gamma-ray intensity of the²⁸Al radionuclide, which is the result of the fast neutron reaction²⁸Si(n, p)²⁸ Al. The Activation part includes a Pu-238/Be neutron source which is located in the middle of the sample container; the measuring part includes two NaI(Tl)-scintillation counters. The total time of the anlysis of one sample amounts to a few minutes; the standard deviation of a single determination is 0.2% SiO₂.     

Spatially resolved ultra-trace analysis of elements combining resonance ionization with a MALDI-TOF spectrometer

A combined setup for spatially resolved mass analysis of trace amounts of elements and macromolecules is presented. Using a MALDI-TOF mass spectrometer, a laser spectroscopic setup for resonant ionization of neutral atoms has been implemented. This allows for an efficient and selective detection of trace elements by means of resonance ionization mass spectrometry (RIMS). The instrumental scheme is described, and methodological developments are presented. In a first application pure, laser desorption/ionization with TOF-MS was used to measure mass distributions of cosmic nanodiamonds. For further applications regarding the spatially resolved ultra-trace analysis of elements in solid samples, an implanted target was used to characterize both laser desorption/ionization and laser desorption/resonance ionization for the detection of trace elements within. A perspective of the setup is given and future investigations are outlined.     

Post-synthetic modification of epitaxially grown, highly oriented functionalized MOF thin films

Thin films of MOFs grown on solid substrates offer a huge potential with regard to tailoring the properties of a surface, in particular if used in connection with post-synthesis modification (PSM). Here, we report on the PSM of surface-supported crystalline MOFs, with target molecules using an amine-based coupling strategy.     

Electronic excitations of glycine, alanine, and cysteine conformers from first-principles calculations

The electronic and optical properties are studied for three conformers of amino acid molecules using gradient-corrected (spin-) density functional theory within a projector-augmented wave scheme and the supercell method. We investigate single-particle excitations such as ionization energies and electron affinities as well as pair excitations. By comparing eigenvalues resulting from several local and nonlocal energy functionals, the influence of treatment of exchange and correlation is demonstrated. The excitations are described within the Delta-self-consistent field method with an occupation number constraint to obtain excitation energies and Stokes shifts. The results are used to also discuss the optical absorption properties. In contrast to the lowest single- and two-particle excitation energies, remarkable changes are found in absorption spectra in dependence on the conformation of the molecule geometry.     

Non-Gaussian range profiles in amorphous solids

Range profiles of boron in amorphous silicon exhibit pronounced deviations from Gaussian at energies above about 40 keV due to increasing electronic stopping. A detailed comparison with computed profiles allows a semiempirical determination of the electronic stopping cross section (S_e ≈ E^0.4).     

Application of LC and GC hyphenated with mass spectrometry as tool for characterization of unknown derivatives of isoflavonoids

Polyphenols belonging to the class of secondary metabolites of plants and microorganisms play an important role as bioactive food constituents as well as contaminants. Structure elucidation of polyphenols in plant extracts or polyphenol metabolites, especially those arising during biotransformation, still represents a challenge for analytical chemistry. Various approaches have been proposed to utilize fragmentation reactions in connection with mass spectrometry (MS) for structural considerations on polyphenolic targets. We compiled and applied specific liquid chromatography (LC)–electrospray ionization in positive mode [ESI(+)]–tandem MS (MS/MS) and gas chromatography (GC)–(electron impact, EI)–MS/MS fragmentation reactions with a special focus on the analysis of isoflavones, whereby this technique was also found to be extendable to determine further polyphenols. For ESI(+)-MS the basic retro-Diels–Alder (rDA) fragmentation offers information about the substitution pattern in the A- and B-rings of flavonoids and the elimination of a protonated 4-methylenecyclohexa-2,5-dienone (m/z = 107) fragment can be used as a diagnostic tool for many isoflavanones. For GC-(EI)-MS/MS analysis after derivatization of the analytes to their trimethylsilyl ethers, the elimination of methyl radicals, tetramethylsilane groups or the combined loss of two methyl groups can be shown to be specific for certain substitution patterns in polyphenols. The applicability of the fragmentation reactions presented is demonstrated exemplarily for three derivatives of the isoflavone irilone. With the help of these fragmentation reactions of the two MS techniques combined, a reliable identification of polyphenols is possible. Especially in such cases where NMR cannot be utilized owing to low analyte amounts being available or prior to purification, valuable information can be obtained. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Ronald Maul and Nils Helge Schebb contributed equally to this work.     

DFT studies using supercells and projector-augmented waves for structure, energetics, and dynamics of glycine, alanine, and cysteine

A large variety of gas phase conformations of the amino acids glycine, alanine, and cysteine is studied by numerically efficient semi-local gradient-corrected density functional theory calculations using a projector-augmented wave scheme and periodic boundary conditions. Equilibrium geometries, conformational energies, dipole moments, vibrational modes, and IR optical spectra are calculated from first principles. A comparison of our results with values obtained from quantum-chemistry methods with localized basis sets and nonlocal exchange-correlation functionals as well as with experimental data is made. For conformations containing strong intramolecular hydrogen bonds deviations in their energetic ordering occur, which are traced back to different treatments of spatial nonlocality in the exchange-correlation functional. However, even for these structures, the comparison of calculated and measured vibrational frequencies shows satisfying agreement.