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51.
Chen LX 《Angewandte Chemie (International ed. in English)》2004,43(22):2886-2905
Photoexcited molecules are quintessential reactants in photochemistry. Structures of these photoexcited molecules in disordered media in which a majority of photochemical reactions take place remained elusive for decades owing to a lack of suitable X-ray sources, despite their importance in understanding fundamental aspects in photochemistry. As new pulsed X-ray sources become available, short-lived excited-state molecular structures in disordered media can now be captured by using laser-pulse pump, X-ray pulse-probe techniques of third-generation synchrotron sources with time resolutions of 30-100 ps, as demonstrated by examples in this review. These studies provide unprecedented information on structural origins of molecular properties in the excited states. By using other ultrafast X-ray facilities that will be completed in the near future, time-resolution for the excited-state structure measurements should reach the femtosecond time scales, which will make "molecular movies" of bond breaking or formation, and vibrational relaxation, a reality. 相似文献
52.
Gopal Shenoy 《Structural chemistry》2003,14(1):3-14
Many advances have been made in chemical structure research over the past three decades using synchrotron radiation. Synchrotron radiation has a number of unique properties. They include high brightness, high collimation, broad energy spectrum, variable polarization, coherent power, and subnanosecond pulse width. The third-generation storage rings with wiggler and undulator sources and lower electron beam dimensions are delivering over 1012 times higher brightness than laboratory-based sources. The future of synchrotron sources looks very promising with the development of energy recovery linac sources and free-electron laser sources. These will permit dynamic studies of chemical structure with subpicosecond time resolution. Commensurate with the development of X-ray sources, major progress has occurred in optical schemes to meet the challenging needs of chemical structure research. High-resolution monochromatization and submicron focusing of X rays present new avenues for the future. 相似文献
53.
From the de Broglie matter wave hypothesis and Planck’s energy quantization law, and assuming conservation of energy in the
absorption of a photon and its consequent conversion to kinetic energy of motion by a material particle initially at rest,
one can deduce a simple mathematical relationship between the wavelength λ (or frequency ν), of the photon absorbed by the
particle at rest, and the resulting de Broglie matter wave length, λD, of the particle with kinetic energy of motion of mv2/2. The relationship so deduced, λD∝√λ, suggests that visible wavelengths of light, from about 4000 ?, in the violet, to beyond about 7000 ?, in the red, on
absorption by an electron at rest, lead to material electron wavelengths, λD, of the order of the size of the electron transfer proteins seen in the photosynthetic reaction centers of photosynthesizing
organisms, at about a size of 50–100 ?. In addition to understanding the mechanism of photosynthesis as a material wave mediated
phenomenon, further areas of importance of the relations pointed out in this paper are in the design of experiments to gain
a deeper understanding of the basic tenets of wave mechanics, and in the use of tunable lasers to probe various properties
of material waves, and to precisely control their properties for applications including lithography. 相似文献
54.
A novel approach to the fabrication of metal ring-disk (RD) microelectrodes is presented that employs flexible chemical vapor deposition (CVD) and electrode modification techniques. Specifically, the development of a copper ring-disk microelectrode is described utilizing a combination of CVD coating, electroetching, and electroplating. Initially, a 25 μm diameter tungsten wire is concentrically coated by CVD with an insulating layer of silica, a layer of tungsten metal, and finally, a second outer layer of silica. The copper surface was prepared by first creating micrometer cavities by electrochemical etching the tungsten in hydroxide solutions followed by electrodeposition of copper from aqueous solutions of Cu(II). Each step of the process was characterized by scanning electron microscopy, optical microscopy, and cyclic voltammetry, demonstrating the preparation of a viable metal-based dual ring-disk microelectrode system. For the purpose of demonstrating the concept of introducing specific selectivity into the device, amperometric detection of galactose in 0.1 M NaOH was performed at +0.60 V in bulk solution and after flow injection analysis in a capillary column. 相似文献
55.
Chitosan samples were irradiated by 60Co γ-rays in the presence of hydrogen peroxide with radiation dose from 10 kGy to 100 kGy. The degradation was monitored by gel permeation chromatography (GPC), revealing the existence of a synergetic effect on the degradation. Structures of the degraded products were characterized with Fourier-transform infrared spectra (FT-IR), ultraviolet-visible spectral (UV-vis) analysis, and X-ray diffraction (XRD). Results showed that the crystallinity of chitosan decreases with degradation, and the crystalline state of water-soluble chitosan is entirely different from that of water-insoluble chitosan. An elemental analysis method was employed to investigate changes in the element content of chitosan after degradation. Mechanism of chitosan radiation degradation with and without hydrogen peroxide was also discussed. 相似文献
56.
Mechanical properties decay and morphological behaviour of biodegradable films for agricultural mulching in real scale experiment 总被引:1,自引:0,他引:1
Giacomo Scarascia-Mugnozza Giuliano Vox Barbara Immirzi 《Polymer Degradation and Stability》2006,91(11):2801-2808
The use of plastic materials in agriculture causes the serious drawback of huge quantities of waste. The introduction of biodegradable materials, which can be disposed directly into the soil, can be one possible solution to this problem. Biodegradable materials are actually innovative materials; therefore, their physical properties must be evaluated in relation to their functionality during the use in field. In the present research results of experimental tests carried out on biodegradable films used in strawberries protected cultivation are presented. The decay of some relevant physical parameters of biodegradable films during the cultivation period was monitored by laboratory tests (SEM analysis, mechanical tensile tests and infrared reflectance spectroscopy). Infrared spectroscopy clearly indicated that the mechanical degradation starts from the starch component of the material. Tensile tests showed that the value of elongation at break of biodegradable materials decreased in some cases by 300% after 10 days of field application. 相似文献
57.
G. K. Vlasov D. N. Vylegzhanin N. A. Dolgikh E. I. Chizhikoya 《International Journal of Infrared and Millimeter Waves》1995,16(1):33-73
The possibility of the IR-radiation detecting in crystals of direct-gap semiconductors, caused by effects of IR-quenching of probe visible-range radiation within the region of a crystal relative transparency, is studied both theoretically and experimentally. The comparison of some mechanisms investigated allows to conclude that the most probable explanation of the IR-quenching effect, experimentally observed in the CdS crystal, is the mechanism of probe radiation absorption with photon energy deficit with respect to exciton resonance, which is eliminated due to exchange interaction of a free exciton in the intermediate state with spherical excitons localized on manyelectron atoms of impurity. 相似文献
58.
G. K. Vlasov E. I. Chizhikova D. N. Vylegzhanin 《International Journal of Infrared and Millimeter Waves》1994,15(1):121-135
The theory of new type detectors based on the quenching of secondary emission in direct-gap semiconductors (lines of Raman light scattering due to interaction between free and bound excitons in the crystal, and also bands of edge radiation) caused by IR or submillimeter radiation is proposed. The results obtained are confirmed by the experiment performed for CdS crystal excited by ultraviolet radiation of mercury lamp, at liquid helium temperature. 相似文献
59.
钾原子中多种受激辐射参与的混频过程 总被引:2,自引:1,他引:1
当双原子共振激发钾原子于6S能级时,由光泵受激辐射和电离复合受激辐射参与的六波或四波混频,产生了位于紫外和紫区的诸多相干辐射、文中对多种混频机制进行了分析和讨论,并对所测得的相干辐射线进行了标识. 相似文献
60.
It is shown that new parametersX can be defined such that the heat capacity CxT(S/T)x is negative, even when the canonical ensemble [i.e., at fixed T=(U/S)Y and YX] is stable. This implies an extension of the classical theory of polytropes from ideal gases to general fluids. As examples of negative heat capacity systems we treat blackbody radiation and general gas systems with nonsingular T. For the case of a simple ideal gas we even exhibit an apparatus which enforces a constraint X(p, V)=const that makes Cx<0. We then show that it is possible to infer the statistical mechanics of canonicallyunstable systems-for which even the traditional heat capacities are negative-by imposing constraints that stabilize the associated noncanonical ensembles. Two explicit models are discussed. 相似文献