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Carlos Javier Villagomez Tomaso Zambelli Sébastien Gauthier André Gourdon Sladjana Stojkovic Christian Joachim 《Surface science》2009,603(10-12):1526-1532
An isomer of the methylterrylene molecule was adsorbed both on Cu(111) and on a NaCl bilayer deposited on Cu(111) and imaged by ultra high vacuum scanning tunneling microscopy at low temperature (5 K). On the bare metal surface, the STM images do not reveal any intramolecular resolution and do not depend on the applied tunnel bias. On the contrary, the images acquired at specific bias voltages for the molecule on the salt layer show a striking similarity with the spatial distribution of the electronic probability density in the highest occupied molecular orbital (HOMO) and in the lowest unoccupied molecular orbital (LUMO) of free methylterrylene. They are well reproduced by elastic scattering quantum chemistry calculations. These data provide a direct view of the hyperconjugative interaction between the methyl group and the frontier orbitals of terrylene. 相似文献
885.
The authors propose a method for the extraction of material parameter and thickness information from sub-100-μm thin samples using non-differential transmission terahertz time domain spectroscopy. The approach relies on an additional Fourier transform of the frequency dependent material parameters to a quasi space regime. In this quasi space, periodic Fabry-Perot oscillations from the frequency domain, which originate from multiple reflections inside the sample, correspond to discrete peaks. By iterative minimization of these peaks, the highly precise thickness information along with the refractive index and absorption coefficient of the sample can be determined. Experimental verification of the approach is also provided. 相似文献
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Ellis AL Christian Mason J Lee HW Strekowski L Patonay G Choi H Yang JJ 《Talanta》2002,56(6):1099-1107
Intracellular calcium concentration in biological cells varies from 0.1 to 10 μM depending upon cell signaling and disease states. A direct estimate of calcium concentration in cell tissues within this range is possible with a novel calcium-selective reagent 15C5-774. The molecule of 15C5-774 consists of a near-infrared (NIR) chromophore (λmax=774 nm) and a metal complexing moiety of benzo-15-crown-5. The reagent shows a strong calcium binding affinity in a 1:1 ratio and metal selectivity in the order Ca2+>Mg2+>Sr2+≈K+≈Na+>Zn2+>Li+. The high sensitivity is achieved by conducting absorption measurements in the NIR region where background interference from the biological matrix is low. 相似文献
888.
Ampan P Lapanantnoppakhun S Sooksamiti P Jakmunee J Kradtap Hartwell S Jayasvati S Christian GD Grudpan K 《Talanta》2002,58(6):1327-1334
Three flow injection (FI) systems were investigated for the determination of trace iron in beer: an FI-in-valve column-flame atomic absorption spectrophotometry (FI-FAAS) system, a spectrophotometric FI system with a column placed at the detection point, and an FI-spectrophotometric system with bead injection (FI-BI). Cationic exchange resin Dowex 50W X8 and iminodiacetate chelating resin, Chelex-100, were employed for the FI-spectrophotometric and FI-FAAS systems, respectively. The FI-in-valve column, packed with the resin, enhances the FAAS performance. The spectrophotometric FI system with a column (packed with Chelex-100) placed at the detection point (in a cell holder of a spectrophotometer) is based on the formation of iron (II)–1,10-phenanthroline complex sorbed onto the resin. No eluent has been found to be suitable. The FI-BI for renewable microcolumn has been proven to be an alternative. The FI-FAAS and FI-BI procedures provide online sample preseparation and preconcentration for the determination of iron in beer. Both are simple, rapid, and economical. The procedures also involve sample preparation (decarbonation and suppression of tannin interference by adding ascorbic acid) and standard addition. The results obtained by FI-FAAS and FI-BI agree with those of AOAC spectrophotometric method. 相似文献
889.
Understanding and controlling solid-state morphologies and molecular conformations is the key to optimizing the properties of materials. As an example for the influence of small chemical changes on solid-state structures, we studied oligo(m-phenylene ethynylene) foldamers, where the introduction of an endo-methyl group induces a transition from an extended all-transoid to a helical all-cisoid conformation. The resulting structural changes were analyzed by X-ray diffraction (XRD), polarized optical microscopy (POM), and low-dose high-resolution electron microscopy (LD-HREM) over several length scales from the molecular to the mesoscopic level. The strong tendency of the endo-methyl oligomer 1 to form stable compact helices in solution resulted in round droplets with an ordered hexagonal columnar (Col(ho)) liquid crystalline structure, where shrinkage during the crystallization resulted in the formation of a banded texture. On the other hand, the endo-hydrogen oligomer 2 exhibited a very different morphology; its extended linear shape was maintained during crystallization and resulted in an extended lamellar structure, which was determined by a compromise between crystalline packing and minimization of the surface area. Another pronounced difference between both molecular structures was the ability of the extended lamellar "crystals" to bend, whereas the helices form either straight or disordered domains. In addition, both materials exhibit strong surface effects, which extend considerably inside the droplet and induce uniform bending of the supramolecular structures. 相似文献
890.
Calculating the energy absorption of atomic clusters as a function of the laser pulse length T we find a maximum for a critical T(*). We show that T(*) can be linked to an optimal cluster radius R(*). The existence of this radius can be attributed to the enhanced ionization mechanism originally discovered for diatomic molecules. Our findings indicate that enhanced ionization should be operative for a wide class of rare-gas clusters. From a simple Coulomb-explosion ansatz, we derive an analytical expression relating the maximum energy release to a suitably scaled expansion time which can be expressed with the pulse length T(*). 相似文献