使用时间切片离子速度成像技术研究CF2Cl2光解动力学

本文使用时间切片离子速度成像技术结合共振增强多光子电离技术研究了CF₂Cl₂分子在235 nm附近的光解动力学. 通过测量CF₂Cl₂分子在235 nm附近单光子解离产生的氯原子影像,直接得到了解离产物的速度分布和角分布. 激发态氯原子的速度分布包含高动能组分和低动能组分,分别对应³Q₀电子态的直接解离和由于内转换引起的基态预解离. 基态氯原子的速度分布也包含高动能组分和低动能组分,分别对应³Q₀与¹Q₁电子态的预解离和由于内转换引起的基态预解离. 自由基解离通道被确认,二次解离通道和三体解离通道被排除.     

Distribution of magnetic dipole fields in amorphous ferromagnetic and speromagnetic alloys and in crystalline and amorphous spin glasses

For binary alloys with only one sort of magnetic atoms we have calculated, by means of computer simulations, the distribution of magnetic dipole fields at regular lattice sites and suitably defined interstitial sites for the magnetic ground state of amorphous ferromagnetic and speromagnetic alloys and crystalline and amorphous spin glasses. Thereby we have considered spatially random arrangements of atoms and have neglected all magnetic correlations in the case of speromagnets and spin glasses. We have derived symmetric and isotropic distribution functions, which are fairly well described by Gaussian distributions for large concentrations of magnetic atoms only. The distribution functions are characterized by their second moments, which are important for the discussion of NMR and μ⁺ SR relaxation rates in these materials.     

Fragment mass and kinetic energy distribution in isomeric fission of 240Pu

Fragment mass and kinetic energy distributions have been measured for isomeric fission of ²⁴⁰Pu. The mass distribution is asymmetric with the average heavy fragment mass nearly equal to that found for ground state spontaneous fission of ²⁴⁰Pu, but slightly lower than for n_th + ²³⁹Pu-fission. The average total fragment kinetic energy appears to be higher in isomeric fission (179.5_?0.7^+1.5 MeV) than in spontaneous fission from the ground state (176.8 ± 1.8 MeV).     

Effect of lattice relaxation on spin density of nitrogen-vacancy centers in diamond and oscillator strength calculations

Using a generalized Hubbard Hamiltonian, many-electron wavefunctions of negatively charged (NV⁻) and neutral nitrogen-vacancy (NV⁰) centers in diamond were calculated. We report the effect of symmetric relaxation of surrounding atoms on the spin density, calculated from the many electron wavefunctions in the ground and excited states. We evaluated the error, that, arises in estimation of spin density when lattice relaxation effect is neglected in Electron Paramagnetic Resonance experiment and showed that the ground state spin density distribution is accessible in outward relaxations. The computed oscillator strengths give a higher efficiency for the 1.945 eV photoluminescence (PL) line of NV⁻ with respect to 2.156 eV PL line of NV⁰ which agrees well with experiment. This result is explained based on the largest the ground state spin among available values for the NV⁻ with respect to NV⁰. The transition probability between degenerate ground and excited states slightly depends on the S _z value. Finally, we report on the electronic configurations which contribute to the ground and excited states and discuss the population variation of electronic configurations with relaxation.     

基于拖尾分布的高分辨率合成孔径雷达图像建模

基于中心极限定理的合成孔径雷达(SAR)图像统计分布不能反映高分辨率SAR图像尖峰和厚尾的统计特征.文中使用广义中心极限定理,由雷达回波的实部和虚部的对称稳定分布,得到SAR图像的拖尾分布(幅值图像的拖尾Rayleigh分布以及强度图像的拖尾指数分布),并以拖尾Rayleigh分布为例,讨论了拖尾分布的代数拖尾特征以及尖峰厚尾的统计特性.为了实现拖尾分布对高分辨率SAR图像的精确建模,基于第二类统计量,提出了对数累积量的参数估计方法,从而高效估计出拖尾分布的参数.真实SAR图像的建模实例表明,基于广义中心极限定理的拖尾分布可以精确描述高分辨率SAR图像的尖峰和厚尾的统计特征.     

Enhanced superconductivity by correlations between two Kondo impurities

We study the interplay between magnetic correlations of two Kondo impurities and superconducting singlet pairing. Performing a Schrieffer-Wolff transformation in the zero-bandwidth limit of the two-impurity Anderson model we obtain the Hamiltonian of two magnetic impurities and we add a superconducting term to the conduction electrons. The model allows us to study the effect of the magnetic correlation between the impurities on the superconducting ground state. At zero temperature, different superconducting ground states can be obtained depending on the magnitude of magnetic coupling between S₁ and S₂. For increasing coupling, the superconducting region is enlarged showing an interesting result: in the strong coupling limit, where the impurities are in a very strong ferromagnetic correlation state, half of the conduction electrons are decoupled from the local moments of the impurities and take advantage of the superconducting pairing lowering the ground state energy. On the contrary, when the coupling between S₁and S₂ decreases, the scenario of the two independent Kondo impurities in presence of superconductivity emerges and all the conduction electrons are involved in the pair breaking physics. At finite temperature, we obtain the phase diagram and we observe a region of parameters where the re-entrance phenomenon occurs.     

Energy loss and nucleon exchange in the reaction of 86Kr with 166Er

Projectile-like products from the reaction ⁸⁶Kr on ¹⁶⁶Er at 8.18 MeV/amu have been measured using a large-area position-sensitive ionization chamber. The observed element distributions for a given energy loss are found to be asymmetric. An analysis of the data in terms of the residual kinetic energy above the Coulomb barrier, however, yields symmetric and Gaussian-shaped element distributions. At large energy losses, the centroids of these Gaussian distributions exhibit a drift towards symmetric mass splits. The correlation between the variance of the element distribution σ_z² and the number of exchanged nucleons N_ex is discussed. The exchange mechanism, as described by a simple one-body dissipation model, can account for a large portion of the measured energy loss.     

Analysis of ν2 of H2Se

The infrared absorption spectrum of ν₂ of H₂Se in the region from 885 to 1347 cm^?1 was obtained with a resolution limit of 0.025 cm^?1 on the University of Denver 50-cm FTIR spectrometer system. We have assigned 1604 lines for the six isotopomers of H₂Se, including 25 lines for the H₂⁷⁴Se isotopomer, and have analyzed them using Watson's A-reduced Hamiltonian in the I^r rotational representation. Ground state constants for each of the five most abundant isotopomers were obtained from fits of microwave transitions combined with weighted averaged ground state combination differences formed from the infrared bands (010), (020), (100), and (001). Upper state constants for each of the five most abundant isotopomers were obtained from least-squares fits of the spectral lines of ν₂, keeping the ground state constants fixed to the values determined from our ground state fits. An alternate set of ground state constants together with isotopic mass adjustment constants for all six isotopomers was determined by simultaneously fitting all available microwave transitions and infrared ground state combination differences. Keeping this set of ground state constants fixed, a single set of upper state constants and isotopic mass adjustment constants for the ν₂ band was determined by a simultaneous fit of infrared spectral lines from all six isotopomers.     

Simultaneous analysis of the microwave and infrared spectra of 14ND3 and 15ND3 for the ν2 excited state

The data on the inversion spectrum in the ν₂ state of ¹⁴ND₃ [F. Scappini, A. Guarnieri, and G. DiLonardo, J. Mol. Spectrosc.95, 20–29 (1982)] have been extended by measuring frequencies of 25 new transitions. A simultaneous least-squares analysis of these data with the ground state microwave transition frequencies and the diode laser measurements of the ν₂ band has been carried out. Improved sets of molecular parameters have been obtained for ¹⁴ND₃ and ¹⁵ND₃, including the ground and ν₂ state inversion splittings, ν₂ band origins, rotational and centrifugal distortion constants, and the parameters of the Δk = ±3n vibrational-rotational interactions.     

The ionization energies of bent and twisted double bonds. Part II. Tricyclo[4.2.2.22,5] Dodecadiene-1 (2),5 (6)

The electronic structure of the title compound (1) has been investigated using He(Iα) PE spectroscopy and assuming the validity of Koopmans' theorem. It is found that: (a) in analogy to the situation prevailing in the cyclophanes, hyperconjugation of the out-of-phase linear combination of the π-orbitals with the ethano-bridge σ orbitals is symmetry-forbidden; (b) in agreement with the ideas developed in Part I of this series [1], the strong local B_1u-type bending of the ethylene moieties does not affect their basis energy significantly; and (c) the “through-space” coupling of the two double-bond π orbitals is of the same order as the benzene π-orbital coupling in [2ⁿ] cyclophanes, even if the net contribution to the repulsion of the two double bonds is probably small. Because of the near degeneracy of the HOMO and HOMO — 1 of 1, the ground state of the radical cation 1⁺ cannot be determined uniquely: it is either²A_g or²B_1u.     

The effect of oxygen on the sputtering of metastable atoms and ions from Ba metal

By means of the Doppler-Shift-Laser-Fluorescence technique we have measured the effect of oxygen coverage on the velocity distributions and relative yields of sputtered Ba atoms and ions in both the ground and metastable excited states. Our measurements show no evidence that the excitation probabilities are correlated with the magnitude of the electric-dipole-moment matrix element to the ground state. We also find that the excitation probability of the Ba(¹ D) state from a clean metal surface depends strongly on the emission velocity υ and approaches the functional form exp$\text{(}\text{?A}\text{av}\text{)}$ at higher velocities.     

Theoretical study of structure and analytic potential energy function for the ground state of PO2 molecule

In this paper, the energy, the equilibrium geometry, and the harmonic frequency of the ground electronic state of PO₂ are computed using B3LYP, B3P86, CCSD(T), and QCISD(T) methods in conjunction with 6-311++G(3df, 3pd) and cc-pVTZ basis sets. A comparison between the computational results and the experimental values indicates that the B3P86/6-311++G(3df, 3pd) method can give better energy calculation results for the PO₂ molecule. It is shown that the ground state of the PO₂ molecule has C_2v symmetry and its ground electronic state is X²A₁. The equilibrium parameters of the structure are R_P-O=0.1465 nm, d=19.218 eV. The bent vibrational frequency ν₁=386 cm^-1, the symmetric stretching frequency ν₂=1095 cm^-1, and the asymmetric stretching frequency ν₃=1333 cm^-1 are obtained. On the basis of atomic and molecular reaction statics, the reasonable dissociation limit for the ground state of the PO₂ molecule is determined. Then the analytic potential energy function of the PO₂ molecule is first derived by using the many-body expansion theory. The potential curves correctly reproduce the configurations and the dissociation energy for the PO₂ molecule.     

Analysis of ν2 of D2S

The infrared spectrum of ν₂ of D₂S was recorded from 740 to 1100 cm^?1 on the University of Denver 50-cm FTIR spectrometer system. We have assigned 655 transitions from D₂³²S and 129 from D₂³⁴S, and have analyzed them using Watson's A-reduced Hamiltonian evaluated in the I^r representation. We used the recently published D₂³²S and D₂³⁴S ground state Hamiltonian constants [C. Camy-Peyret, J. M. Flaud, L. Lechuga-Fossat and J. W. C. Johns, J. Mol. Spectrosc.109, 300–333 (1985)]. Upper state Hamiltonian constants were obtained from a fit of the ν₂ transitions, keeping the ground state constants fixed while varying the upper state constants. The standard deviation of the D₂³²S ν₂ fit is 0.0025 cm^?1. The standard deviation of the D₂³⁴S ν₂ fit is 0.0041 cm^?1.     

A note on non-equilibrium work fluctuations and equilibrium free energies

We consider in this paper, a few important issues in non-equilibrium work fluctuations and their relations to equilibrium free energies. First we show that the Jarzynski identity can be viewed as a cumulant expansion of work. For a switching process which is nearly quasistatic the work distribution is sharply peaked and Gaussian. We show analytically that dissipation given by average work minus reversible work W_R, decreases when the process becomes more and more quasistatic. Eventually, in the quasistatic reversible limit, the dissipation vanishes. However the estimate of p, the probability of violation of the second law given by the integral of the tail of the work distribution from −∞ to W_R, increases and takes a value of 0.5 in the quasistatic limit. We show this analytically employing Gaussian integrals given by error functions and the Callen-Welton theorem that relates fluctuations to dissipation in process that is nearly quasistatic. Then we carry out Monte Carlo simulation of non-equilibrium processes in a liquid crystal system in the presence of an electric field and present results on reversible work, dissipation, probability of violation of the second law and distribution of work.     

Theory of core photoemission spectra in CeO2

By using the Anderson model with a filled valence band, we calculate the core photoemission spectra of CeO₂, and compare them with experimental results. It is shown that in the ground state of CeO₂ the 4?⁰ and 4?¹ configurations are mixed strongly due to the large hybridization between the 4? states and the valence band. In the final state of the photoemission, the 4?¹ and 4?² configurations are also mixed strongly due to the final state interaction coming from a core hole potential and the large hybridization. The fractional intensity of the 4?⁰ photoemission peak is considerably different from the weight of the 4?⁰ configuration in the ground state because of the strong final state interaction.     

The unique spherically symmetric solution of the U4-theory of gravity in the teleparallelism limit

Within the U₄-theory of gravity in the teleparallelism limit we determine the metric and torsion tensor of a spherically symmetric fluid ball. We uniquely derive a Schwarzschild solution thereby establishing the validity of the Birkhoff theorem in this framework.     

Identification of a metastable state of the VZnH2 defect in ZnO

An infrared absorption study of the Zn vacancy passivated by two hydrogen atoms (V_ZnH₂) is reported. The ground state of the defect V_ZnH₂ consists of the inequivalent O–H bonds, which are aligned parallel and perpendicular to the c-axis, respectively. A metastable state of the defect was detected with two equivalent O–H bonds oriented perpendicular to the c-axis (V_ZnH₂^?). V_ZnH₂^? has two local vibration modes at 3329.0 and 3348.4 cm⁻¹ which are the antisymmetric and symmetric combinations of the two O–H stretch modes. The metastable state of the defect is 75±9 meV above the ground state of V_ZnH₂. An activation energy of 0.96±0.12 eV for the transition from metastable to the ground state was determined.     

Ground state rotational energies of C3v quasi-spherical top molecules: Applications to 16OPF3 and PH3

Rotational spectra of the quasi-spherical symmetric molecules, ¹⁶OPF₃ and PH₃, are analyzed with all the data currently available. In both cases the current formulation that uses rotational contact transformations to completely diagonalize the ground state energy matrix appears to be inadequate, due to the smallness of the quantity |;A₀ ? B₀|. This quantity represents only 5% of A₀ or B₀ in the case of OPF₃ and 11% in the case of PH₃. Thus, both analyses require the resonance terms in ΔK = ±3 and/or ΔK = ±6 to be taken into account through diagonalization of the energy matrix. The ground state parameters as well as the ground state energies are given for both molecules.     

(e,2e) Spectroscopy of ethane

The 400-eV and 1200-eV non-coplanar symmetric (e, 2e) reaction has been used to measure the momentum distributions of electrons in the individual valence orbitals of ethane as well as to measure the complete separation energy spectra in the valence region. The shapes and relative magnitudes of the momentum distributions agree well with those calculated using the plane wave off-shell impulse approximation and double zeta basis molecular orbital wave functions. The ground state of C₂H₆⁺ is shown to be 1e_g^?1, with the vertical ionization potential being 12.25 ± 0.1 eV. Considerable structure due to configuration interaction is observed in the separation energy region 29–55 eV. Much of this structure can be assigned to the 2a_1g orbital.     

Fission of highly excited fragments from collisions of 750 A.MeV 238U-ions on 208Pb

Fragments of relativistic 750 A.MeV U-projectiles were investigated by using the fragment separator FRS for magnetic selection of reaction products including ray-tracing and ΔE-ToF techniques. For elements between Ge and Sb, measurements of isotopic yield distributions and velocities revealed three processes: fragmentation, low-energy fission, and high-energy fission. The last of these regimes is presently reported. First and second moments of distributions of mass numbers, atomic numbers and velocities of the corresponding fragments allowed us to identify ¹⁰¹ ₄₃Tc₅₆ as the most probable fragment of a high energy symmetric fission reaction. Moreover, we could deduce a hypothetical mean fissioning fragmentation product ²⁰⁸Rn and its highly excited pre-fragmentation parent ²²⁷Ra produced in a primary abrasion reaction at an excitation energy of about 290 MeV. Received: 26 January 1998 / Revised version: 16 March 1998