光场横向效应导致CO2激光器混沌运转的实验研究

对横向效应导致单纵模多横模CO_2激光器失稳进行了实验研究.观测到了经各种途径到达的激光混沌态,并对其物理机制进行了探讨.     

Carbon-13 NMR spectra of solid bicyclo[3. 3. 1]nonan-9-one. Conformational studies in the solid state

Carbon-13 NMR spectra of solid polycrystalline bicyclo[3.3.1]nonan-9-one and adamantanone have been measured at 315K. The relatively narrow ¹³C linewidths observed for these solids, together with measured spin-lattice relaxation times, indicate that both these solids are orientationally disordered. Observed ¹³C chemical shifts of solid bicyclo[3.3.1]nonan-9-one indicate a twin-chair conformation for the two cyclohexanone rings.     

Bin packing: On optimizing the number of pieces packed

We consider bin-packing variations related to the well-studied problem of maximizing the total number of pieces packed into a fixed set of bins. We show that, when the objective is to minimize the total number of pieces packed subject to the constraint that no unpacked piece will fit, no polynomial-time relative approximation algorithm exists (unless, of course,P=NP). That is, we prove that it isNP-hard to guarantee packing no more than a constant multiple of the optimal number of pieces, for any constant. This appears to be the first bin-packing problem for which this property has been demonstrated. Furthermore, this result also holds for the allied packing variant which seeks to minimize the maximum number of pieces packed in any single bin. We find the situation to be markedly better for the problem of maximizing the minimum number of pieces in any bin. If all bins possess the same capacity, we prove that the familiar SPF rule is an absolute approximation algorithm with additive constant 1, and can therefore be regarded as a best possible heuristic. For the more general and difficult case in which bin capacities may differ, it turns out that SPF fails to qualify as even a relative approximation algorithm. However, we devise an implementation of the well-known FFD heuristic, which we show to be a relative approximation algorithm, yielding a worst-case performance ratio of 1/2 with additive constant 0. Moreover, we prove that (unlessP=NP) no polynomial-time algorithm can guarantee a higher ratio without sacrificing the additive constant.This author's research is supported in part by the National Science Foundation under grants ECS-8403859 and MIP-8603879.     

Af型自由基均聚反应的固化理论(Ⅲ)──高分子凝胶网络缺陷

应用高分子反应统计理论,分析了高分子网络的形成过程,讨论了A_f型自由基均聚反应凝胶网络中的缺陷悬吊环的数目及其形成几率.     

Initiation of complex-radical polymerization of methyl methacrylate in the presence of metallocenes

The initiation of radical polymerization of methyl methacrylate in the presence of benzoyl peroxide-metallocene (Cp₂Fe, Cp₂ZrCl₂, and Cp₂TiCl₂; (C₅Me₅)₂Fe, (C₅Me₅)₂ZrC_l2, and (AcC₅H₄)(C₅H₅)Fe) systems is studied. It is shown that a metallocene affects the rate of initiation and the initial rate of polymerization. On the basis of quantum-chemical calculations, a new mechanism of the initiation reaction may be advanced: Namely, the decomposition of benzoyl peroxide proceeds via the stage of complexation with a metallocene, while the nature of a metallocene determines the probability of complexation and decomposition.     

Non-Gaussian statistics of electrostatic fluctuations of hydration shells

This paper aims to understand the statistics of the electric field produced by water interfacing a non-polar solute of nanometer dimension. We study, by numerical simulations, the interface between SPC/E water and a Kihara solute, which is a hard-sphere core with a Lennard-Jones layer at its surface. The distribution of the interfacial electric field is monitored as a function of the magnitude of a point dipole placed close to the solute-water interface. The free energy surface as a function of the electric field projected on the dipole direction shows a cross-over with increasing dipole magnitude. While it is a single-well harmonic function at low dipole values, it becomes a double-well surface at intermediate dipole moment magnitudes, transforming into a single-well surface again, with a non-zero minimum position, at still higher dipoles. This transformation, reminiscent of a discontinuous phase transition in bulk materials, has a broad intermediate region where the interfacial waters fluctuate between the two minima. This region is characterized by intense field fluctuations, with non-Gaussian statistics and variance far exceeding expectations from the linear-response approximation. The excited state of the surface water is found to be lifted above the ground state by the energy required to break approximately two hydrogen bonds. This state is pulled down in energy by the external electric field of the solute dipole, making it readily accessible to thermal excitations. The excited state is a surface defect in the hydrogen-bond network, creating a stress in the nearby network, but otherwise relatively localized in the region closest to the solute dipole.     

Unconventional transport in the "hole" regime of a si double quantum dot

Studies of electronic charge transport through semiconductor double quantum dots rely on a conventional "hole" model of transport in the three-electron regime. We show that experimental measurements of charge transport through a Si double quantum dot in this regime cannot be fully explained using the conventional picture. Using a Hartree-Fock (HF) formalism and relevant HF energy parameters extracted from transport data in the multiple-electron regime, we identify a novel spin-flip cotunneling process that lifts a singlet blockade.     

Tunable spin loading and T1 of a silicon spin qubit measured by single-shot readout

We demonstrate single-shot readout of a silicon quantum dot spin qubit, and we measure the spin relaxation time T1. We show that the rate of spin loading can be tuned by an order of magnitude by changing the amplitude of a pulsed-gate voltage, and the fraction of spin-up electrons loaded can also be controlled. This tunability arises because electron spins can be loaded through an orbital excited state. Using a theory that includes excited states of the dot and energy-dependent tunneling, we find that a global fit to the loading rate and spin-up fraction is in good agreement with the data.