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21.
22.
J.F. Wager 《Journal of luminescence》2008,128(11):1851-1855
A simple assessment procedure is described which facilitates optimal selection of an electroluminescent phosphor based on the measurement of two optically deduced energies—absorption and emission. These two energies are used as input for an appraisal analysis based on a classical configuration-coordinate diagram model of the phosphor. A phosphor performance figure-of-merit is proposed as a consequence of the formulation of this assessment procedure. This procedure is then used to rationalize the relative performance of five electroluminescent phosphors (ZnS:Mn, BaAl2S4:Eu, SrS:Ce, SrGa2S4:Ce, CaGa2S4:Ce). Additionally, two cathodoluminescent phosphors (ZnS:Ag and ZnS:Cu) are appraised using this methodology, suggesting that this procedure may be of some utility in the evaluation of other types of phosphors. 相似文献
23.
The de-excitation process of FA(type I) centers in KCl:Na has been investigated by measuring the hot luminescence spectrum from optically excited FA centers with time-resolved spectroscopy. The experimental results are analyzed by using a model that describes a time evolution of the phonon wave packet during the vibronic relaxation process from the Franck-Condon state to a relaxed excited state. From the analysis of the experimental data, information on the vibronic mixing between 2p and 2s states, whose magnitude varies during the relaxation process, and the adiabatic potential energy curves of 2s and 2p states are extracted. The present results are compared with the already known ones of the FA(type II) centers. 相似文献
24.
Temperature Dependence of Photoluminescence from Single and Ensemble InAs/GaAs Quantum Dots
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We investigate the temperature dependence of photoluminescence from single and ensemble InAs/GaAs quantum dots systematically. As temperature increases, the exciton emission peak for single quantum dot shows broadening and redshift. For ensemble quantum dots, however, the exciton emission peak shows narrowing and fast redshift. We use a simple steady-state rate equation model to simulate the experimental data of photoluminescence spectra. It is confirmed that carrier-phonon scattering gives the broadening of the exciton emission peak in single quantum dots while the effects of carrier thermal escape and retrapping play an important role in the narrowing and fast redshift of the exciton emission peak in ensemble quantum dots. 相似文献
25.
Theoretical progress in the research of photoinduced phase transitions is reviewed with closely related experiments. After a brief introduction of stochastic evolution in statistical systems and domino effects in localized electron systems, we treat photoinduced dynamics in itinerant-electron systems. Relevant interactions are required in the models to describe the fast and ultrafast charge-lattice-coupled dynamics after photoexcitations. First, we discuss neutral-ionic transitions in the mixed-stack charge-transfer complex, TTF-CA. When induced by intrachain charge-transfer photoexcitations, the dynamics of the ionic-to-neutral transition are characterized by a threshold behavior, while those of the neutral-to-ionic transition by an almost linear behavior. The difference originates from the different electron correlations in the neutral and ionic phases. Second, we deal with halogen-bridged metal complexes, which show metal, Mott insulator, charge-density-wave, and charge–polarization phases. The latter two phases have different broken symmetries. The charge-density-wave to charge–polarization transition is much more easily achieved than the reverse transition. This is clarified by considering microscopic charge-transfer processes. The transition from the charge-density-wave to Mott insulator phases and that from the Mott insulator to metal phases proceed much faster than those between the low-symmetry phases. Next, we discuss ultrafast, inverse spin-Peierls transitions in an organic radical crystal and alkali-TCNQ from the viewpoint of intradimer and interdimer charge-transfer excitations. Then, we study photogenerated electrons in the quantum paraelectric perovskite, SrTiO3, which are assumed to couple differently with soft-anharmonic phonons and breathing-type high-energy phonons. The different electron–phonon couplings result in two types of polarons, a “super-paraelectric large polaron” with a quasi-global parity violation, and an “off-center-type self-trapped polaron” with only a local parity violation. The former is equivalent to a charged and conductive ferroelectric domain, which greatly enhances both the quasi-static electric susceptibility and the electric conductivity. Finally, we outline the development of time-resolved X-ray diffraction experiments, which directly accesses the dynamics of electronic, atomic and molecular motions in photoexcited materials. They are extremely useful when a three-dimensional structural long-range order is established and changes the symmetry. 相似文献
26.
Cleveland ZI Pavlovskaya GE Stupic KF Wooten JB Repine JE Meersmann T 《Magnetic resonance imaging》2008,26(2):270-278
Despite the importance of the tobacco smoke particulate matter in the lungs to the etiology of pulmonary disease in cigarette smokers, little is currently known about the spatial distribution of particle deposition or the persistence of the resulting deposits in humans, and no satisfactory technique currently exists to directly observe tobacco smoke condensate in airways. In this proof-of-principle work, hyperpolarized (hp) 83Kr MRI and NMR spectroscopy are introduced as probes for tobacco smoke deposition in porous media. A reduction in the hp-83Kr longitudinal (T1) relaxation of up to 95% under near-ambient humidity, pressure and temperature conditions was observed when the krypton gas was brought into contact with surfaces that had been exposed to cigarette smoke. This smoke-induced acceleration of the 83Kr self-relaxation was observed for model glass surfaces that, in some experiments, were coated with bovine lung surfactant extract. However, a similar effect was not observed with hp-(129)Xe indicating that the 83Kr sensitivity to smoke deposition was not caused by paramagnetic species but rather by quadrupolar relaxation due to high adsorption affinity for the smoke deposits. The 83Kr T1 differences between smoke-treated and untreated surfaces were sufficient to produce a strong contrast in variable flip angle FLASH hp-83Kr MRI, suggesting that hp-83Kr may be a promising contrast agent for in vivo pulmonary MRI. 相似文献
27.
Ping Xia Zhang Yong Fang Zhao Feng You Hao Xin Ying Li 《International journal of quantum chemistry》2008,108(5):937-944
Noble‐gas‐noble‐metal hydroxides NgAuOH (Ng = Kr, Xe) are investigated at the MP2 theoretical level. All species are found to be in Cs symmetry with an approximate linear Ng? Au? O moiety. The noble‐gas‐noble‐metal bond lengths are comparable with covalent limits, and the corresponding binding energies have been computed to be 59.6 and 83.4 kJ/mol for KrAuOH and XeAuOH, respectively. Except the charge‐induced dipole contribution to the binding energies, the remainder could be ascribed to the higher‐order charge‐induction energies, dispersion energies, the contributions of multipole moments on AuOH and covalent effects. The title species are sufficiently chemical bound and are expected to be stable species theoretically. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 相似文献
28.
运用Monte-Carlo轨迹法研究了Kr(1Sg)+HF(X1Σ+;ν=1,2,3;J=4)→Kr(1Sg)+HF(X1Σ+,ν',J')的碰撞过程,从而分析Kr原子对HF基频和泛频的影响。研究表明:在初始相对平动能Et≤251.0kJ/mol时,Kr原子能够减少产生基频的粒子数反转,增加产生泛频特别是ν'=2能级的粒子数反转,并且,Kr原子有较强的弛豫高转动态(J'≥4)的能力,因此,Kr原子对HF泛频激光在提高光强和增强大气传输能力方面均有 较大的改善。 相似文献
29.
Y.H. Ren Q.H. Chen Y.B. Yu Z.A. Xu W.B. Shao Z.K. Jiao 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(4):651-656
Within the framework of the second-order Rayleigh-Schr?dinger perturbation theory, we investigate the effects of the interaction
of the electron and longitudinal-optical phonons in two-dimensional semiconductive quantum dots with respect to a general
potential. We propose a simple expression for the ground state energy, and compare it with those obtained by Landau-Pekar
strong coupling theory. It is shown both analytically and numerically that the results obtained from the second-order Rayleigh-Schr?dinger
perturbation theory could be better than those from Landau-Pekar strong coupling theory when the coupling constant is sufficiently
small. Moreover, some interesting problems, such as polarons in quasi-one-dimensional quantum wires, and quasi-zero-dimensional
asymmetric or symmetric quantum dots can be easily discussed only by taking different limits. After the numerical calculations,
we find that there exists a simple dimensional scaling and symmetry relation for the ground state polaron energy. Furthermore,
we apply our results to some weak-coupling polar semiconductors such as GaAs, CdS. It is shown that the polaronic effects
are found to be quiet appreciable if the confinement lengths and smaller than a few nanometers.
Received: 3 December 1997 / Revised: 6 July 1998 / Accepted: 17 September 1998 相似文献
30.
E. Benichou A.R. Allouche R. Antoine M. Aubert-Frecon M. Bourgoin M. Broyer Ph. Dugourd G. Hadinger D. Rayane 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2000,10(2):233-242
A non perturbative approach is used to solve the problem of a rigid linear molecule with both a permanent dipole moment and
a static dipole polarizability, in a static electric field. Eigenenergies are obtained and compared to perturbative low field
and high field approximations. Analytical expressions for the orientation parameters and for the gradient of the energy are
given. This non perturbative approach is applied to the simulation of beam deviation experiments in strong electric field.
Results of simulations are given for inhomogeneous alkali dimers. For LiNa, the simulations are compared to experimental data.
For LiK, deviation profiles have been simulated in order to prepare future experiments on this molecule.
Received 21 July 1999 and Received in final form 22 September 1999 相似文献