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排序方式: 共有917条查询结果,搜索用时 312 毫秒
1.
J. E. Smart 《Journal of Radioanalytical and Nuclear Chemistry》1998,235(1-2):105-108
An effective means of detecting airborne radioactive aerosol plumes has been developed and tested on aircraft platforms. The
Real-Time Airborne Radiation Analysis and Collection (RTARAC) system was mounted in the wing pod of a Navy P-3 where it sampled
20 cubic meters of air per minute on each of eleven sequentially advanced filters. A 140% intrinsic gemanium detector counted
radioactive particles collected on the 15 cm circular filters in real-time. Gamma-energy spectrum and near real-time analysis
of the sample were displayed on a laptop computer. 相似文献
2.
M. Gutowski I. Dabkowska J. Rak S. Xu J.M. Nilles D. Radisic K.H. Bowen Jr 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,20(3):431-439
The photoelectron spectra (PES) of anions of uracil-glycine and uracil-phenylalanine complexes reveal broad features with
maxima at 1.8 and 2.0 eV. The results of ab initio density functional B3LYP and second order M?ller-Plesset theory calculations indicate that the excess electron occupies a
π* orbital localized on uracil. The excess electron attachment to the complex can induce a barrier-free proton transfer (BFPT)
from the carboxylic group of glycine to the O8 atom of uracil. As a result, the four most stable structures of the anion of
uracil-glycine complex can be characterized as the neutral radical of hydrogenated uracil solvated by the anion of deprotonated
glycine. The similarity between the PES spectra for the uracil complexes with glycine and phenylalanine suggests that the
BFPT is also operative in the case of the latter anionic species. The BFPT to the O8 atom of uracil may be related to the
damage of nucleic acid bases by low energy electrons because the O8 atom is involved in a hydrogen bond with adenine in the
standard Watson-Crick pairing scheme.
Received 6 April 2002 Published online 13 September 2002 相似文献
3.
Jnos Szanyi Ja Hun Kwak Sarah Burton Jose A. Rodriguez Charles H.F. Peden 《Journal of Electron Spectroscopy and Related Phenomena》2006,150(2-3):164-170
Adsorbed ionic NOx species formed upon the interaction of NO2 with dehydrated or hydrated Na- and Ba-Y, FAU zeolites were characterized using FT-IR/TPD, solid state NMR, and XANES techniques. NO2 disproportionates on both dehydrated catalyst materials forming NO+ and NO3− species. These ionic species are stabilized by their interactions with the negatively charged zeolite framework and the charge compensating cations (Na+ and Ba2+), respectively. Although the nature of the adsorbed NOx species formed on the two catalysts is similar, their thermal stabilities are strongly dependent on the charge compensating cations. In the presence of water in the channels of these zeolite materials new paths open for reactions between NO+ and H2O, and NO2 and H2O, resulting in significant changes in the adsorbed ionic species observed. These combined spectroscopic investigations afforded the understanding of the interactions between water and NO2 on these zeolite catalysts. 相似文献
4.
High-resolution Fourier transform spectra of CH3OH have been investigated in the infrared region from 930 to 1450 cm−1 in order to map the torsion-rotation energy manifolds associated with the ν7 in-plane CH3 rock, the ν11 out-of-plane CH3 rock, and the ν6 OH bend. Upper-state term values have been determined from the assigned spectral subbands, and have been fitted to power-series expansions to obtain substate origins and effective B-values for the three modes. The substate origins have been grouped into related families according to systematic trends observed in the torsion-vibration energy map, but there are substantial differences from the traditional torsional patterns. There appears to be significant torsion-mediated spectral mixing, and a variety of “forbidden” torsional combination subbands with |Δυt|>1 have been observed, where υt denotes the torsional quantum number (equivalent to υ12). For example, coupling of the (υ6,υt)=(1,0) OH bend to nearby torsionally excited (υ7,υt)=(1,1) CH3-rock and (υ8,υt)=(1,1) CO-stretch states introduces (υ6,υt)=(1,0)←(0,1) subbands into the spectrum and makes the ν7+ν12−ν12 torsional hot band stronger than the ν7 fundamental. The results suggest a picture of strong coupling among the OH-bending, CH3-rocking, and CO-stretching modes that significantly modifies the traditional energy structure and raises interesting and provocative questions about the torsion-vibration identity of a number of the observed states. 相似文献
5.
The electron-stimulated desorption (ESD) of D− and H− ions from condensed D2O and H2O films is investigated. Three low-energy peaks are observed in the ESD anion yield, which are identified as arising from excitation of 2B1, 2A1 and 2B2 dissociative electron attachment (DEA) resonances. Additional structure is observed between 18 and 32 eV, which may be due to ion pair formation or to DEA resonances involving the 2a1 orbital. The ion yield resulting from excitation of the 2B1 resonance increases as the film is heated. We attribute the increase in the ion yield to thermally induced hydrogen bond breaking near the surface, which enhances the lifetimes of the excited states that lead to desorption. 相似文献
6.
《Solid State Ionics》2006,177(26-32):2241-2244
A catalytic dense membrane reactor allows to physically separate the oxygen feed from the reactant (hydrocarbon) feed with a catalytic membrane chosen among oxide ion conducting materials. The membrane plays a double role, it provides the oxygen needed for selective oxidation and acts as a catalyst. The catalytic properties of BIMEVOX (ME = Co, Cu, Ta) membranes were examined in the mild oxidation of propene and of propane. During the complex transient state observed when the surface is rough, the nature and distribution of products are different from those obtained with mirror-polished membranes in a former work. In particular, syngas is formed with propene as well as with propane, and it precedes the production of hydrogen and coke. The complex behaviour differs according to ME and seems to be related to the different nature of electron semi-conduction induced by each dopant. 相似文献
7.
R. A. Wind P. D. Majors K. R. Minard E. J. Ackerman D. S. Daly G. R. Holtom B. D. Thrall T. J. Weber 《Applied magnetic resonance》2002,22(2):145-158
Confocal fluorescence optical microscopy and magnetic resonance microscopy are each used to study live cells in a minimally invasive way. Both techniques provide complementary information. Therefore, by examining cells simultaneously with both methodologies, more detailed information is obtained than is possible with each microscope individually. In this paper two configurations of a combined confocal and magnetic resonance microscope are described. The first configuration is capable of studying large single cells or three-dimensional cell agglomerates, whereas the second configuration is designed for the investigation of monolayers of mammalian cells. In both cases the sample compartment is part of a temperature regulated perfusion system. Images obtained with the combined system are shown forXenopus laevis oocytes, model JB6 tumor spheroids, and a single layer of Chinese hamster ovary cells. Finally, potential applications of the combined microscope are discussed. 相似文献
8.
When some parameters cannot be easily measured experimentally, mathematical models can often be used to deconvolute or interpret data collected on complex systems, such as those characteristic of many environmental problems. These models can help quantify the contributions of various physical or chemical phenomena that contribute to the overall behavior, thereby enabling the scientist to control and manipulate these phenomena, and thus to optimize the performance of the material or device. In the first case study presented here, a model is used to test the hypothesis that oxygen interactions with hydrogen on the catalyst particles of solid oxide fuel cell anodes can sometimes occur a finite distance away from the triple phase boundary (TPB), so that such reactions are not restricted to the TPB as normally assumed. The model may help explain a discrepancy between the observed structure of SOFCs and their performance. The second case study develops a simple physical model that allows engineers to design and control the sizes and shapes of mesopores in silica thin films. Such pore design can be useful for enhancing the selectivity and reactivity of environmental sensors and catalysts. This paper demonstrates the mutually beneficial interactions between experiment and modeling in the solution of a wide range of problems. 相似文献
9.
《Journal of Non》2006,352(23-25):2589-2595
Triplet electron–hole excitations were introduced into amorphous silica to study self-trapping (localization) and damage formation using density functional theory. Multiple self-trapped exciton (STE) states are found that can be differentiated based on the luminescence energy, the localization and distribution of the excess spin density of the triplet state, and relevant structural data, including the presence or absence of broken bonds. The trapping is shown to be affected by the relaxation response of the silica network, and by comparing results of quartz and amorphous silica systems the effects of the inherent disordered structures on exciton self-trapping are revealed. A key result is that the process of exciton trapping can lead directly to the formation of point defects, without thermal activation. The proposed mechanism includes a non-radiative decay from the excited to the ground state followed by structure relaxation to a defect configuration in the ground state. 相似文献
10.
Arthur Maki Robert L. Sams Jeffrey Barber Engelene t.H. Chrysostom Alfons Weber 《Journal of Molecular Spectroscopy》2004,225(2):109-122
Several new infrared absorption bands for 32S16O3 have been measured and analyzed. The principal bands observed were ν1+ν2 (at 1561 cm−1), ν1+ν4 (at 1594 cm−1), ν3+ν4 (at 1918 cm−1), and 3ν3 (at 4136 cm−1). Except for 3ν3, these bands are very complicated because of (a) the Coriolis coupling between ν2 and ν4, (b) the Fermi resonance between ν1 and 2ν4, (c) the Fermi resonance between ν1 and 2ν2, (d) ordinary l-type resonance that couples levels that differ by 2 in both the k and l quantum numbers, and (e) the vibrational l-type resonance between the A1′ and A2′ levels of ν3+ν4. The unraveling of the complex pattern of these bands was facilitated by a systematic approach to the understanding of the various interactions. Fortunately, previous work on the fundamentals permitted good estimates of many constants necessary to begin the assignments and the fit of the measurements. In addition, the use of hot band transitions accompanying the ν3 band was an essential aid in fitting the ν3+ν4 transitions since these could be directly observed for only one of four interacting states. From the hot band analysis we find that the A1′ vibrational level is 3.50 cm−1 above the A2′ level, i.e., r34=1.75236(7) cm−1. In the case of the 3ν3 band, the spectral analysis is straightforward and a weak Δk=±2, Δl3=±2 interaction between the l3=1 and l3=3 substates locates the latter A1′ and A2′ “ghost” states 22.55(4) cm−1 higher than the infrared accessible l3=1 E′ state. 相似文献