Surface excitation probabilities in surface electron spectroscopies

The phenomenon of surface excitation is competitive in nature for elastic and other inelastic scattering processes in surface electron spectroscopies; the knowledge of influence of surface excitations in electron energy loss spectra is then essential for quantitative surface analysis with these spectroscopies. The inelastic scattering of an electron moving in the vicinity of a surface is considered in a self-energy formalism to estimate the contribution of surface excitation in electron-solid interactions via the total surface excitation probability. The formulation uses the optical bulk dielectric function and provides the spatial and angular dependence of the differential and total inelastic cross-sections. The kinetic energy range of probing electrons considered is 100-5000 eV and the numerical evaluation of total surface excitation probabilities are performed for several metals, Au, Ag, Cu, Ni, Fe and Ti; empirical formulae for the surface excitation probability are given for each of these materials and compared with experimental results for the surface excitation parameter. The total surface excitation probability is higher in Ag as compared to other metals under consideration, for identical conditions of electron-solid interactions.     

Extending the excitation sculpting concept for selective excitation

Nowadays, excitation sculpting is probably the most efficient way to achieve selectivity in an NMR experiment, since it associates very clean frequency selection with "user-friendliness." In the present report, it is shown that the excitation sculpting concept, originally based on a double pulse field gradient echo acting on a selected transverse magnetization, can be extended through new experiments designed to act on longitudinal magnetization. This leads to outstanding performances, especially when the transverse relaxation rate is a limiting factor as, for example, in the case of biological macromolecules. Several new sequences are proposed, aiming at the selection of magnetization aligned either/both on a transverse axis or/and on the z-axis. Their potentialities are illustrated in light of different applications including multiplet-selective excitation, band-selective excitation, and water suppression.     

Chiral approach to nuclear matter: role of two-pion exchange with virtual delta-isobar excitation

We extend a recent three-loop calculation of nuclear matter by including the effects from two-pion exchange with single and double virtual Δ(1232)-isobar excitation. Regularization dependent short-range contributions from pion-loops are encoded in a few NN-contact coupling constants. The empirical saturation point of isospin-symmetric nuclear matter, , ρ₀=0.16 fm⁻³, can be well reproduced by adjusting the strength of a two-body term linear in density (and tuning an emerging three-body term quadratic in density). The nuclear matter compressibility comes out as K=304 MeV. The real single-particle potential U(p,k_f0) is substantially improved by the inclusion of the chiral πNΔ-dynamics: it grows now monotonically with the nucleon momentum p. The effective nucleon mass at the Fermi surface takes on a realistic value of M^*(k_f0)=0.88M. As a consequence of these features, the critical temperature of the liquid-gas phase transition gets lowered to the value T_c15 MeV. In this work we continue the complex-valued single-particle potential U(p,k_f)+iW(p,k_f) into the region above the Fermi surface p>k_f. The effects of 2π-exchange with virtual Δ-excitation on the nuclear energy density functional are also investigated. The effective nucleon mass associated with the kinetic energy density is . Furthermore, we find that the isospin properties of nuclear matter get significantly improved by including the chiral πNΔ-dynamics. Instead of bending downward above ρ₀ as in previous calculations, the energy per particle of pure neutron matter and the asymmetry energy A(k_f) now grow monotonically with density. In the density regime ρ=2ρ_n<0.2 fm⁻³ relevant for conventional nuclear physics our results agree well with sophisticated many-body calculations and (semi)-empirical values.     

原子数据库在研究同核异能态诱发辐射中的应用

如何人为地诱发同核异能态辐射,从而利用储存在其中的巨大能量,是个备受关注的交叉学科课题。利用原子物理过程进行触发是目前较为可行的方案,原子物理参数将在里面起到关键作用。介绍了一套高效率的原子数据库,目前该数据库可对原子序数小于96的元素所有电离度离子的能级结构信息进行快速扫描,从而筛选出一些可能通过原子过程触发的同核异能态体系。通过在197Au元素电子跃迁诱导核激发过程和178Hf元素电子俘获诱导核激发过程中的应用,展示了该数据库对寻找合适同核异能态体系的作用。     

原子数据库在研究同核异能态诱发辐射中的应用

 如何人为地诱发同核异能态辐射,从而利用储存在其中的巨大能量,是个备受关注的交叉学科课题。利用原子物理过程进行触发是目前较为可行的方案,原子物理参数将在里面起到关键作用。介绍了一套高效率的原子数据库,目前该数据库可对原子序数小于96的元素所有电离度离子的能级结构信息进行快速扫描,从而筛选出一些可能通过原子过程触发的同核异能态体系。通过在¹⁹⁷Au元素电子跃迁诱导核激发过程和¹⁷⁸Hf元素电子俘获诱导核激发过程中的应用,展示了该数据库对寻找合适同核异能态体系的作用。     

Surveying the role of excitation energy in probing nuclear dissipation

A dynamical Langevin model is employed to calculate the excess of the evaporation residue cross sections of the 194pb nucleus over that predicted by the standard statistical model as a function of nuclear dissipation strength. It is shown that large excitation energy can increase the effects of nuclear dissipation on the excess of the evaporation residues and the sensitivity of this excess to the dissipation strength, and that more higher excitation energies have little contribution to further raising this sensitivity. These results suggest that on the experimental side, producing those compound systems with moderate excitation energy is sufficient for a good determination of the pre-saddle nuclear dissipation strength by measuring the evaporation residue cross section, and that forming an extremely highly excited system does not considerably improve the sensitivity of evaporation residues to the dissipation strength.     

Investigation of interband optical transitions by near-resonant magneto-photoluminescence in InAs/GaAs quantum dots

Resonant photoluminescence experiments performed on self-assembled InAs/GaAs quantum dots under strong magnetic field up to 28 T give rise to an accurate determination of the interband magneto-optical transitions. As this technique minimizes the effect of the homogeneous broadening of the transitions due to the size and composition fluctuations of the dots, the experimental spectra display well-defined peaks. A good agreement is found between the experimental data and calculations using an effective mass model including the coupling between the mixed exciton-LO phonon states. Transitions involving excitonic polarons are clearly identified. Moreover, a light-hole to conduction transition is also evidenced in agreement with previous theoretical predictions.     

Surveying the role of excitation energy in probing nuclear dissipation

A dynamical Langevin model is employed to calculate the excess of the evaporation residue cross sections of the ^194Pb nucleus over that predicted by the standard statistical model as a function of nuclear dissipation strength. It is shown that large excitation energy can increase the effects of nuclear dissipation on the excess of the evaporation residues and the sensitivity of this excess to the dissipation strength,and that more higher excitation energies have little contribution to further raising this sensitivity. These results suggest that on the experimental side,producing those compound systems with moderate excitation energy is sucient for a good determination of the pre-saddle nuclear dissipation strength by measuring the evaporation residue cross section,and that forming an extremely highly excited system does not considerably improve the sensitivity of evaporation residues to the dissipation strength.     

Dependence of nuclear deorientation on nuclear spin for recoils in vacuum

Time-integral measurements have been made of the vacuum deorientation of the 2⁺ and 4⁺ states in ¹⁵⁰Sm and the 6⁺ and 8⁺ states in ¹⁵⁶Gd Coulomb-excited by 133 MeV ³⁵Cl ions. The ¹⁵⁰Sm results were deduced from γ-ray angular distributions measured in coincidence with backscattered ³⁵Cl ions. In the ¹⁵⁶Gd measurements, a fixed counter array and a sandwich target were used to measure directly the differences between the γ-ray angular distributions from nuclei recoiling in vacuum and the unperturbed γ-ray angular distributions from nuclei stopped in a thick ¹⁵⁶Gd target. The measured deorientation time constants in ps are τ₂(2⁺) = 27±4; τ₂(4⁺) = 14±4; τ₂(6⁺) = 24±3; τ₂(8⁺) = 23⁺¹⁰_?6; τ₄(2⁺) = 12±2; τ₄(4⁺) = 5±3; τ₄(6⁺) = 7.6±2.5. The 8⁺ data were analyzed assuming τ₂/τ₄ = $\text{10}\text{3}$ as predicted by the Abragam-Pound theory for $\text{I ≧ 4}$ essentially independently of the degree of quadrupole admixture. The other results are consistent with this except for the 2⁺ level which shows some quadrupole effect. The present results, which show strong deorientation of high spin levels, are in contrast to earlier work on neutron deficient Er isotopes. In light of our findings we suggest that the failure to observe deorientation in the high spin states in Er is primarily due to anomalously low g-factors associated with the backbending observed in these nuclei.     

绝对测量He原子快电子碰撞双激发的电子关联动力学

应用高分辨快电子能量损失谱仪 ,绝对测量了He原子各种光学允许和光学禁戒的双电子激发过程的动量转移依赖特性 ,报道了作为动量转移函数的以内禀关联量子数K ,T和A表征的电子态 2(0 ,1 ) 2 1 Po,2 (1 ,0 ) 2 1 Se和 2(1 ,0 ) 2 1 De的Fano参数q ,fa,ρ2 ,f和S ,同时对 2(- 1 ,0 ) 2 1 Se和 2(1 ,0 ) 2 的波函数作出定性检验 .从而在实验上通过激发动力学过程研究了电子关联效应     

Quasimolecular K X-ray excitation by bombardment of Ge atoms with Ge ions

By bombardment of Ge targets with ⁷⁴₃₂Ge⁵⁺ of 81 MeV, a continuous X-ray intensity distribution has been obtained, which ranges up to the K X-ray energies of ₆₄Gd. The shape of the spectra, corrected for the detector efficiency, confirms the assumption that this continuum is caused by K X-rays of Z = 64 quasimolecules, which are transiently formed during the adiabatic heavy-ion collision. The yield of quasimolecular radiation was determined to be about 4 x 10⁻⁵ X-rays per beam K-vacancy.     

Prospects for coherently driven nuclear radiation by Coulomb excitation

Possible experiments are discussed in which Coulomb excitation of nuclear isomers would be followed by sequential energy release. The possibility of coherent Coulomb excitation of nuclei ensconced in a crystal by channeled relativistic heavy projectiles is considered. The phase shift between neighbor-nuclei excitations may be identical to the photon phase shift for emission in the forward direction. Thus, the elementary string of atoms may radiate coherently with emission of characteristics nuclear γ rays, and the intensity of the radiation would be increased due to the summation of amplitudes. Mössbauer conditions should be important for this new type of collective radiation, which could be promising in the context of the γ-lasing problem.     

钾蒸汽中双光子激发产生2~3∏_g—α~3∑_u~+扩散带受激辐射

本文报道了在K_2-K中通过双光子共振激发原子或双光子激发分子产生2~3∏_(?)—a~3∑_u~+扩散带辐射的实验结果,并对激发机制以及扩散带辐射随激发能量和温度的变化进行了讨论.     

Phase equation with nonlinear excitation for nonlocally coupled oscillators

Some reaction-diffusion systems feature nonlocal interaction and, near the point of Hopf bifurcation, can be represented as a system of nonlocally coupled oscillators. Phase of oscillations satisfies an evolution pde which takes different forms depending on the values of parameters. In the simplest case the equation is effectively a diffusion equation which is excitation-free. However, more complex forms are possible such as the Nikolaevskii equation and the Kuramoto–Sivashinsky equation incorporating linear excitation. We analyse a situation when the phase equation is based on nonlinear excitation. We derive conditions on the values of the parameters leading to the situation and show that the values satisfying the conditions exist.     

In-phase selective excitation of overlapping multiplets

An editing experiment is presented that selects for a peak on the basis of its chemical shift and that of one of its scalar coupling partners. The selected multiplet is pure in-phase. The editing procedure can be used in conjunction with 1D TOCSY/HOHAHA and NOE measurements. The pulse sequence described is particularly suitable for small molecules; data is presented for Gramicidin S and dehydrotestosterone.     

Microscopic nuclear collective motion studied by classical equations of motion

The time-dependent variation principle is used to obtain generally non-canonical equations of motion from any class of quantum states which are parameterized by a set of continuous complex quantities. A class of states is presented whose associated classical dynamics is described by the five collective quadrupole degrees of freedom. Information about the classical dynamics of the system can be obtained from the non-canonical equations by finding physically interesting quantities which are coordinate independent and which characterize the low-energy collective motion. Approximate collective hamiltonians, of either a Bohr-Mottelson or an IBM type, can be found by insisting that the interesting physical quantities which describe the low-energy classical behavior of the many-body system are the same as those describing the classical behavior of the system given by the collective hamiltonian. The method is applied to two simple schematic models, one vibrational and one rotational, and IBM hamiltonians are obtained.     

Triplet-triplet transfer of electronic energy upon nonequilibrium vibrational excitation of triplet molecules

The specific features of the triplet-triplet (T-T) transfer of electronic excitation energy in a gas phase upon nonequilibrium vibrational excitation of the triplet molecules of a donor were studied for an anthraquinone-diacetyl donor-acceptor pair using the time-resolved slow fluoresence of anthraquinone and sensitized phosphorescence of diacetyl. It is shown that in the gas phase, which allows regular control of the number of collisions, competition between the processes of T-T transfer and intermolecular vibrational relaxation is observed for nanosecond time resolution. The T-T transfer rate for the molecular system investigated exceeded the rate of intermolecular vibrational relaxation k_V in the triplet state T₁ of the donor. The effectiveness of the T-T transfer of energy by vibrationally excited molecules turned out to be higher than the effectiveness of transfer by thermalized ones, but even the highest of them was much less than unity. An increase in the equilibrium temperature of vapors led to a decrease in the effectiveness of transfer for both vibrationally excited and thermalized triplet molecules, thus indicating the importance of the collisional complex in the intermolecular process studied. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 4, pp. 474–479, July–August, 2000.     

Measurement of compound nuclear lifetimes by the crystal blocking technique

The crystal blocking technique has been used to measure the lifetime of the 12.44 MeV 2⁺ level in ²⁸Si excited in the ²⁷Al(p, α)²⁴Mg resonance reaction at E_p = 885 keV. Both 〈110〉 axial and {111} planar blocking effects in Al single crystals have been investigated. Measurements have been made for two reaction depths, 1000 and 4000 A. Various methods of extracting the lifetime, involving both analytical calculations and computer simulation, are compared. The theoretical change in volume of an axial blocking dip due to a lifetime effect has been calculated in the continuum approximation. The results obtained from the different methods of analysis agree, the average value of the lifetime being 28 as. Although the results show no systematic dependence on reaction depth, pronounced depth effects are evident in computer simulations of the planar blocking dips.