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61.
Analysis of crossing fibers is a challenging topic in recent diffusion-weighted imaging (DWI). Resolving crossing fibers is expected to bring major changes to present tractography results based on the standard tensor model. Model free approaches, like Q-ball or diffusion spectrum imaging, as well as multi-tensor models are used to unfold the different diffusion directions mixed in a voxel of DWI data. Due to its seeming simplicity, the two-tensor model (TTM) is applied frequently to provide two positive-definite tensors and the relative population fraction modeling two crossing fiber branches. However, problems with uniqueness and noise instability are apparent. To stabilize the fit, several of the 13 physical parameters are fixed ad hoc, before fitting the model to the data. Our analysis of the TTM aims at fitting procedures where ad hoc parameters are avoided. Revealing sources of instability, we show that the model's inherent ambiguity can be reduced to one scalar parameter which only influences the fraction and the eigenvalues of the TTM, whereas the diffusion directions are not affected. Based on this, two fitting strategies are proposed: the parsimonious strategy detects the main diffusion directions without extra parameter fixation, to determine the eigenvalues and the population fraction an empirically motivated condition must be added. The expensive strategy determines all 13 physical parameters of the TTM by a fit to DWIs alone; no additional assumption is necessary. Ill-posedness of the model in case of noisy data is cured by denoising of the data and by L-curve regularization combined with global minimization performing a least-squares fit of the full model. By model simulations and real data applications, we demonstrate the feasibility of our fitting strategies and achieve convincing results. Using clinically affordable diffusion acquisition paradigms (encoding numbers: 21, 2*15, 2*21) and b values (b = 500–1500 s/mm2), this methodology can place the TTM parameters involved in crossing fibers on a more empirical basis than fitting procedures with technical assumptions. 相似文献
62.
Pedestrians’ road-crossing behavior can often interrupt traffic flow and cause vehicle queueing. In this paper, we propose some moving rules for modeling the interaction of vehicles and pedestrians. The modified visual angle car-following model is presented for the movement of vehicles with consideration of the lateral effect of waiting pedestrians. The pedestrians’ behavior is summarized as consisting of three steps: pedestrian arrival, gap acceptance, and pedestrian crossing. Some characteristic parameters of pedestrians are introduced to characterize pedestrians’ behavior. Simulation results show that the interaction of vehicles and pedestrians lowers the traffic capacity and increases delays to both vehicles and pedestrians. 相似文献
63.
Taiki Nakatomi Shoichi Koido Yuya Watabe Toshiyuki Takayanagi 《International journal of quantum chemistry》2019,119(11):e25908
The mechanisms including spin-inversion have been systematically studied for the M+ + OCS → MS+ + CO/MO+ + CS (M denotes a transition metal from Sc to Cu) ion-molecule reactions using the automated reaction path search method. We used the lowest mixed-spin potential energy surface obtained from the diagonalization of the spin-coupled Hamiltonian matrix, whose diagonal elements are taken to be the lowest two spin states. This scheme can effectively locate approximate minimum energy crossing points between the two potential energy surfaces with different spin multiplicities. The spin-orbit couplings at spin-inversion points have been calculated to understand the efficiencies of nonadiabatic transitions. The obtained reaction pathways and the calculated spin-orbit couplings are employed to interpret previous experimental studies. 相似文献
64.
Anna S. Savchenkova Alexander S. Semenikhin Ivan V. Chechet Sergey G. Matveev Alexander A. Konnov Alexander M. Mebel 《Journal of computational chemistry》2019,40(2):387-399
Ab initio and density functional CCSD(T)-F12/cc-pVQZ-f12//B2PLYPD3/6-311G** calculations have been performed to unravel the reaction mechanism of triplet and singlet methylene CH2 with ketene CH2CO. The computed potential energy diagrams and molecular properties have been then utilized in Rice–Ramsperger–Kassel–Marcus-Master Equation (RRKM-ME) calculations of the reaction rate constants and product branching ratios combined with the use of nonadiabatic transition state theory for spin-forbidden triplet-singlet isomerization. The results indicate that the most important channels of the reaction of ketene with triplet methylene lead to the formation of the HCCO + CH3 and C2H4 + CO products, where the former channel is preferable at higher temperatures from 1000 K and above. In the C2H4 + CO product pair, the ethylene molecule can be formed either adiabatically in the triplet electronic state or via triplet-singlet intersystem crossing in the singlet electronic state occurring in the vicinity of the CH2COCH2 intermediate or along the pathway of CO elimination from the initial CH2CH2CO complex. The predominant products of the reaction of ketene with singlet methylene have been shown to be C2H4 + CO. The formation of these products mostly proceeds via a well-skipping mechanism but at high pressures may to some extent involve collisional stabilization of the CH3CHCO and cyclic CH2COCH2 intermediates followed by their thermal unimolecular decomposition. The calculated rate constants at different pressures from 0.01 to 100 atm have been fitted by the modified Arrhenius expressions in the temperature range of 300–3000 K, which are proposed for kinetic modeling of ketene reactions in combustion. © 2018 Wiley Periodicals, Inc. 相似文献
65.
The 4f-4f emissions from lanthanide trication (Ln3+) complexes are widely used in bioimaging probes. The emission intensity from Ln3+ depends on the surroundings, and thus, the design of appropriate photo-antenna ligands is indispensable. In this study, we focus on two probes for detecting hydrogen peroxide, for which emission intensities from Tb3+ are enhanced chemo-selectively by the H2O2-mediated oxidation of ligands. To understand the mechanism, the Gibbs free energy profiles of the ground and excited states related to emission and quenching are computed by combining our approximation—called the energy shift method—and density functional theory. The different emission intensities are mainly attributed to different activation barriers for excitation energy transfer from the ligand-centered triplet (T1) to the Tb3+-centered excited state. Additionally, quenching from T1 to the ground state via intersystem crossing was inhibited by intramolecular hydrogen bonds only in the highly emissive Tb3+ complexes. © 2018 Wiley Periodicals, Inc. 相似文献
66.
67.
Energy-Levels Crossing and Radial Dirac Equation: Supersymmetry and Quasi-Parity Spectral Signatures
Omar Mustafa 《International Journal of Theoretical Physics》2008,47(5):1300-1311
The (3+1)-dimensional Dirac equation with position dependent mass in 4-vector electromagnetic fields is considered. Using
two over-simplified examples (the Dirac-Coulomb and Dirac-oscillator fields), we report energy-levels crossing as a spectral
property or as an effect of the hidden supersymmetric quantum mechanical language and/or quasi-parity signatures. Under different
settings of the related interactions’ way-of-coupling into Dirac equation, it is observed that the two ultimate/effective
descendents, Dirac-Coulomb and Dirac-oscillator, exhibit different conditions on the energy-levels crossings. 相似文献
68.
V. S. Pavlovich A. P. Stupak L. G. Pikulik D. T. Kozhich T. A. Pavich 《Journal of Applied Spectroscopy》2008,75(3):319-324
The molar extinction coefficient, oscillator strength, natural fluorescence lifetime, and fluorescence quantum yield have
been determined for 9,10-diiodoanthracene in ethanol at 20°C. The temperature effect on the quantum yield was studied in the
range 120–300 K. The fluorescence lifetime was measured at 77 K. During glassification of ethanol, the fluorescence intensity
of 9,10-diiodoanthracene increases by more than 50 times due to the activation nature of the intersystem crossing from the
S1(1B2u+) state. The activation energy and pre-exponential factor for the probability of the intersystem {ie319-01}-crossing are found.
The long-wavelength shift of the absorption spectrum with increasing bulk polarizability of the solvent is interpreted as
evidence of changes in the nonspecific interactions.
__________
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 304–308, May–June, 2008. 相似文献
69.
John Asplund Gregory Clark Garner Cochran va Czabarka Arran Hamm Gwen Spencer Lszl Szkely Libby Taylor Zhiyu Wang 《组合设计杂志》2019,27(10):586-597
The crossing number of a graph is the smallest number of edge crossings over all drawings of in the plane. For any , the ‐planar crossing number of , is defined as the minimum of over all graphs with . Pach et al [Comput. Geom.: Theory Appl. 68 (2018), pp. 2–6] showed that for every , we have and that this bound does not remain true if we replace the constant by any number smaller than . We improve the upper bound to as . For the class of bipartite graphs, we show that the best constant is exactly for every . The results extend to the rectilinear variant of the ‐planar crossing number. 相似文献
70.
A symmetrical host material, 2,7-di(9,9-dimethyl-9H-fluoren-1-yl)-9H-thioxanthen-9-one (DMBFTX), with TADF property was firstly developed. The red phosphorescent OLED based on this TADF host displays a lower EQEs rolloff of 38.8% at a luminance of 10 000 cd/m2 as compared to 71.2% of commercial mCP host, which is resulted from the upconversion of DMBFTX from triplet to singlet. 相似文献