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151.
We report a novel Fourier transform method for measuring two-photon excitation spectra. We demonstrate this method using simple dye molecules and discuss its applications in two-photon fluorescence microscopy and optimal control. This method facilitates an intuitive interpretation of recent control experiments in terms of tuning the nonlinear spectrum of the exciting laser source. 相似文献
152.
Decleva P Fronzoni G Stener M de Simone M Coreno M Green JC Hazari N Plekan O 《Physical review letters》2005,95(26):263401
Photoemission from the two outermost ionizations [highest occupied molecular orbitals (HOMO and HOMO-1)] of Mg(eta(5)-C(5)H(5))(2) has been studied with synchrotron radiation in the gas phase. Strong oscillations in the HOMO-1/HOMO ratio, qualitatively similar to those well-known for fullerenes, are found. Excellent agreement with the experimental ratio is provided by accurate cross section calculations both at the density-functional theory and time-dependent density-functional theory level, indicating that a many electron response has a minor role for this effect. A comparison with the calculated values for other metal sandwich compounds indicate that the presence of oscillations is a widespread phenomenon, and a potential source of interesting information on the structural and electronic properties of the target molecule. 相似文献
153.
Choi JY Hasegawa-Johnson M Cole J 《The Journal of the Acoustical Society of America》2005,118(4):2579-2587
Acoustic cues related to the voice source, including harmonic structure and spectral tilt, were examined for relevance to prosodic boundary detection. The measurements considered here comprise five categories: duration, pitch, harmonic structure, spectral tilt, and amplitude. Distributions of the measurements and statistical analysis show that the measurements may be used to differentiate between prosodic categories. Detection experiments on the Boston University Radio Speech Corpus show equal error detection rates around 70% for accent and boundary detection, using only the acoustic measurements described, without any lexical or syntactic information. Further investigation of the detection results shows that duration and amplitude measurements, and, to a lesser degree, pitch measurements, are useful for detecting accents, while all voice source measurements except pitch measurements are useful for boundary detection. 相似文献
154.
Superlubricity of graphite 总被引:1,自引:0,他引:1
Dienwiebel M Verhoeven GS Pradeep N Frenken JW Heimberg JA Zandbergen HW 《Physical review letters》2004,92(12):126101
Using a home-built frictional force microscope that is able to detect forces in three dimensions with a lateral force resolution down to 15 pN, we have studied the energy dissipation between a tungsten tip sliding over a graphite surface in dry contact. By measuring atomic-scale friction as a function of the rotational angle between two contacting bodies, we show that the origin of the ultralow friction of graphite lies in the incommensurability between rotated graphite layers, an effect proposed under the name of "superlubricity" [Phys. Rev. B 41, 11 837 (1990)]]. 相似文献
155.
Intramolecular cation-pi interactions between a methyl pyridinium ion and a phenyl ring stabilize the folded structure of a phenyleneethynylene oligomer. 相似文献
156.
Bernstein SL Liu D Wyttenbach T Bowers MT Lee JC Gray HB Winkler JR 《Journal of the American Society for Mass Spectrometry》2004,15(10):1435-1443
The protein alpha-synuclein, implicated in Parkinson's disease, was studied by combining nano-electrospray ionization (N-ESI) mass spectrometry and ion mobility. It was found that both the charge-state distribution in the mass spectra and the average protein shape deduced from ion mobility data, depend on the pH of the spray solution. Negative-ion N-ESI of pH 7 solutions yielded a broad charge-state distribution from -6 to -16, centered at -11, and ion mobility data consistent with extended protein structures. Data obtained for pH 2.5 solutions, on the other hand, showed a narrow charge-state distribution from -6 to -11, centered at -8, and ion mobilities in agreement with compact alpha-synuclein structures. The data indicated that there are two distinct families of structures: one consisting of relatively compact proteins with eight or less negative charges and one consisting of relatively extended structures with nine or more charges. The average cross section of a-synuclein at pH 2.5 is 33% smaller than for the extended protein sprayed from pH 7 solution. Significant dimer formation was observed when sprayed from pH 7 solution but no dimers were observed from the low pH solution. A plausible mechanism for aggregate formation in solution is proposed. 相似文献
157.
Alan H. Cowley Zheng Lu Jamie N. Jones Jennifer A. Moore 《Journal of organometallic chemistry》2004,689(15):2562-2564
The salt [HC(CMe)2(NAr)2BPh][Al2Cl7] (3); Ar = 2,6-i-Pr2C6H3) has been synthesized via the in situ preparation of [HC(CMe)2(NAr)2BClPh], followed by treatment with AlCl3. X-ray analysis of 3 reveals that the BN2C3 ring of the boron cation is planar and DFT calculations indicate π-type interactions in the HOMO-6 and HOMO-7 orbitals. 相似文献
158.
Snowden M Bermudez A Kelly DR Radkiewicz-Poutsma JL 《The Journal of organic chemistry》2004,69(21):7148-7156
Explanations for stereoselectivity in the Baeyer-Villiger reaction have relied on the assumption that the antiperiplanar (app) group migrates. However, the magnitude of the preference for app-migration over gauche migration is unknown. To investigate this, the energy differences between the two were estimated from ab initio calculations. App-migration was found to be the preferred pathway since no transition structure could be located for gauche migration. Barriers for gauche migration were estimated by performing constrained optimizations. App-migration was found to be strongly favored with a barrier that is at least 3.8 kcal/mol and as much as 58.0 kcal/mol lower in energy than the gauche migration barrier. 相似文献
159.
K6Yb3(PS4)5 possesses two identical but interlocked three-dimensional frameworks. Each framework is realized by having Yb3+ atoms adopt both octahedral and trigonal prismatic sulfur coordination. 相似文献
160.
Chasse TL Smith JC Carroll RL Gorman CB 《Langmuir : the ACS journal of surfaces and colloids》2004,20(9):3501-3503
The electrochemical behavior of a film composed of a redox-active dendrimer was studied as a function of the type of counterion available during its reduction and reoxidation. The rate of permeation/migration of counterions into the film appeared to be the bottleneck to electron transfer through the film. Because the dendrimer is rather hydrophobic, increasing the hydrophobicity of the counterion increased the rate and extent of electron hopping within the films. 相似文献