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排序方式: 共有161条查询结果,搜索用时 15 毫秒
1.
A nonadiabatic quantum dynamic model has been developed to study the process of electron emission from a low-work-function metal surface. The process is initiated by scattering a highly vibrationally excited NO molecule from a surface composed of a Cs layer covering a Ru crystal. The model addresses the increasing quantum yield of the electron emission as a function of the molecular vibrational excitation and incident kinetic energy. The reaction mechanism is identified as a long-range harpooning electron transfer to a molecular ion which is then accelerated toward the surface. Upon impact, the molecular ion emits its excess electron. 相似文献
2.
Rahimipour S Litichever-Coslovsky N Alaluf M Freeman D Ehrenberg B Weiner L Mazur Y Fridkin M Koch Y 《Photochemistry and photobiology》2005,81(2):250-258
A combination of light, oxygen and a photosensitizer is used to induce death of cancer cells by photodynamic therapy. In this study, we have synthesized several new methyl helianthrone derivatives and compared their phototoxicity with that of hypericin. In contrast to hypericin, methyl helianthrones are soluble in aqueous solutions and have a broad range of light absorbance, which allows the use of polychromatic light. Structural modifications of methyl helianthrone demonstrated that substitution of hydrogen atoms of methyl helianthrone at Positions 2 and 5 with Br atoms or methylation of its phenolic hydroxyls, significantly increases the corresponding singlet oxygen quantum yield and their phototoxicity toward alphaT3-1, M2R and LNCaP cells. The phototoxicity of some of these compounds was similar to that of hypericin. Methyl helianthrones, like hypericin, accumulated mainly in the perinuclear region as evident by confocal microscopy. Irradiation of cells pretreated with methyl helianthrone derivatives generates intracellular reactive oxygen species and lipid free radicals, as shown by a fluorescentic probe and electron paramagnetic resonance methods, respectively. The phototoxicity of these methyl helianthrones as well as their ability to oxidize membrane lipids were significantly decreased on addition of specific Type-II inhibitors, suggesting the involvement of singlet oxygen as the main oxidant. 相似文献
3.
Time resolved chemiluminescence spectra of pure tetramethyldioxetane are obtained for the first time using pulsed infrared laser excitation. The only major product, acetone, is shown to be formed by at least two pathways: a unimolecular, collision free, decomposition and a collision induced mechanism. Spectral and temporal behaviour indicate that neither singlet nor triplet acetone are the primary products. Other possibilities are discussed. 相似文献
4.
We consider Albrecht's theory for Raman scattering of fundamentals in the far and pre-resonance regions. Destructive interferences inherent to the A and B terms augment the conventionally suppressed C term dramatically. Raman excitation profiles for the ν1(a1g) and ν6(e2g fundamentals in benzene can be well fitted with theoretical C-term profiles involving the 1E1u(π-π*) state at 1800 A. 相似文献
5.
Atomistic-scale simulations of the initial chemical events in the thermal initiation of triacetonetriperoxide 总被引:1,自引:0,他引:1
van Duin AC Zeiri Y Dubnikova F Kosloff R Goddard WA 《Journal of the American Chemical Society》2005,127(31):11053-11062
To study the initial chemical events related to the detonation of triacetonetriperoxide (TATP), we have performed a series of molecular dynamics (MD) simulations. In these simulations we used the ReaxFF reactive force field, which we have extended to reproduce the quantum mechanics (QM)-derived relative energies of the reactants, products, intermediates, and transition states related to the TATP unimolecular decomposition. We find excellent agreement between the QM-predicted reaction products and those observed from 100 independent ReaxFF unimolecular MD cookoff simulations. Furthermore, the primary reaction products and average initiation temperature observed in these 100 independent unimolecular cookoff simulations match closely with those observed from a TATP condensed-phase cookoff simulation, indicating that unimolecular decomposition dominates the thermal initiation of the TATP condensed phase. Our simulations demonstrate that thermal initiation of condensed-phase TATP is entropy-driven (rather than enthalpy-driven), since the initial reaction (which mainly leads to the formation of acetone, O(2), and several unstable C(3)H(6)O(2) isomers) is almost energy-neutral. The O(2) generated in the initiation steps is subsequently utilized in exothermic secondary reactions, leading finally to formation of water and a wide range of small hydrocarbons, acids, aldehydes, ketones, ethers, and alcohols. 相似文献
6.
L. Allen Kindman Charles E. Cohen Michael H. Zeldin Yehuda Ben-Shaul Jerome A. Schiff 《Photochemistry and photobiology》1978,27(6):787-794
Abstract— Absorption measurements in the 600–720 nm region of dark-grown cells of Euglena gracilis Klebs var. bacillaris Cori were made in vivo at room temperature using computer-assisted spectrophotometry. Dark-grown wild-type cells have a prominent absorption maximum at 634 nm due to protochlorophyll(ide) absorption. Upon illumination, the absorption at 634nm decreases and a peak appears at 674nm, representing the phototransformation of protochlorophyll(ide) to chlorophyll(ide). Using difference spectroscopy, the resynthesis in the dark of protochlorophyll(ide) by previously-illuminated wild-type dark-grown cells was found to begin at about 10min after illumination and reached completion by about 25 min, the amount of protochlorophyll(ide) resynthesized being equivalent to that of dark-grown cells. Resynthesis of protochlorophyll(ide) following a second illumination follows the same kinetics, indicating that protochlorophyll(ide) resynthesis is under tight regulation, possibly via feedback control. Cells of dark-grown wild-type and W3BUL, a mutant lacking protochlorophyll(ide) contain a component absorbing at 658 nm which does not undergo phototransformation when examined by difference spectroscopy at room and liquid N2 temperatures. Following the phototransformation of protochlorophyll(ide) 634 to chlorophyll(ide) 674, the chlorophyll(ide) 674 shifts to shorter wavelengths, ultimately to 671 nm. Possible relationships among the various spectroscopic forms of protochlorophyll(ide) and chlorophyll(ide) at room temperature and liquid N2 temperature in Euglena and higher plants are presented. It is concluded that Euglena, unlike older, etiolated higher plants, contains only protochlorophyll(ide) 634, making it an excellent system in which to examine the phototransformation of this pigment species in the absence of other forms. 相似文献
7.
Different methods for the interpretation of linear dichroic spectra of molecules incorporated in uniaxial matrices are discussed. A method based on the combination of both polarized absorption and emission measurements is described for the resolution of absorption and emission spectra into their different polarized components, and for the investigation of molecular distributions in the oriented matrix. The distributions of some planar molecules of different shapes incorporated in stretched polyethylene films are presented. 相似文献
8.
Decomposition of triacetone triperoxide is an entropic explosion 总被引:1,自引:0,他引:1
Dubnikova F Kosloff R Almog J Zeiri Y Boese R Itzhaky H Alt A Keinan E 《Journal of the American Chemical Society》2005,127(4):1146-1159
Both X-ray crystallography and electronic structure calculations using the cc-pVDZ basis set at the DFT B3LYP level were employed to study the explosive properties of triacetone triperoxide (TATP) and diacetone diperoxide (DADP). The thermal decomposition pathway of TATP was investigated by a series of calculations that identified transition states, intermediates, and the final products. Counterintuitively, these calculations predict that the explosion of TATP is not a thermochemically highly favored event. It rather involves entropy burst, which is the result of formation of one ozone and three acetone molecules from every molecule of TATP in the solid state. 相似文献
9.
Yehuda Shalom 《Transactions of the American Mathematical Society》1999,351(8):3387-3412
Let be any locally compact non-discrete field. We show that finite invariant measures for -algebraic actions are obtained only via actions of compact groups. This extends both Borel's density and fixed point theorems over local fields (for semisimple/solvable groups, resp.). We then prove that for -algebraic actions, finitely additive finite invariant measures are obtained only via actions of amenable groups. This gives a new criterion for Zariski density of subgroups and is shown to have representation theoretic applications. The main one is to Kazhdan's property for algebraic groups, which we investigate and strengthen.
10.
A model is presented which demonstrates that the process of flooding and flow reversal can be explained on the basis of a film mechanism. The model predicts well the gas flow rate at which flooding and flow reversal begins and ends for a given liquid flow rate and the presence of a hysteresis loop between flooding and flow reversal. The predictions of the theory are in satisfactory agreement with experimental flooding data. 相似文献