表面低配位原子对声子的散射机制

由于纳米结构具有极高的表体比,声子-表面散射机制对声子的热输运性质起到关键作用.提出了表面低配位原子对声子的散射机制,并且结合量子微扰理论与键序理论推导出该机制的散射率.由于散射率正比于材料的表体比,这种散射机制对声子输运的重要性随着纳米结构尺寸的减小而增大.散射率正比于声子频率的4次方,所以这种散射机制对高频声子的作用远远强于对低频声子的作用.基于声子玻尔兹曼输运方程,计算了硅纳米薄膜和硅纳米线的热导率,发现本文模型比传统的声子-边界散射模型更接近实验值.此发现不仅有助于理解声子-表面散射的物理机制,也有助于应用声子表面工程调控纳米结构的热输运性质.     

Positron scattering from hydrocarbons

The total cross sections for positron impact on hydrocarbons have been calculated using the additivity rule in which the total cross section for a molecule is the sum of the total cross section for the constituent atoms. The energy range considered is from a few eV to several thousand eV. The total cross sections for positron impact on an atom are calculated by employing a complex spherical potential which comprises of a static, polarization and an absorption potential. We have good agreement with the experimental results for hydrocarbons for positron energy ⩾100 eV. Our results also agree with the available calculations for CH₄ and C₂H₂ which employed full molecular wavefunctions beyond 100 eV. Our absorption cross sections also agree with molecular wave-function calculations for C₂H₂ and CH₄ beyond 100 eV. We have shown the Bethe plots fore ⁺−C ande ⁺−H scattering systems and Bethe parameters have been extracted. We have fitted the cross section for positron impact on hydrocarbons in the formσ _t(C _n H _m )=naE ⁻ b+mcE ⁻ d in the energy range 300–5000 eV wherea=195.0543,b=0.7986,c=371.1757 andd=1.1379 withE in eV andσ _t in 10⁻¹⁶ cm².     

Positron scattering and ionization of neon atoms—theoretical investigations

Although positron scattering with inert gas atoms has been studied in theory as well as in experiment, there are discrepancies. The present work reports all the major total cross sections of e+-neon scattering at incident energies above ionization threshold, originating from a complex potential formalism. Elastic and cumulative inelastic scatterings are treated in the complex spherical e+-atom potential. Our total inelastic cross section includes positronium formation together with ionization and excitation channels in Ne. Because of the Ps formation channel it is difficult to separate out ionization cross sections from the total inelastic cross sections. An approximate method similar to electron-atom scattering has been applied to bifurcate ionization and cumulative excitation cross sections at energies from threshold to 2000 eV. Comparisons of present results with available data are made. An important outcome of this work is the relative contribution of different scattering processes, which we have shown by a bar-chart at the ionization peak.     

超冷温度下钾和铯原子间弹性散射特性的精确计算

本文分别用量子方法和半经典方法计算了超冷钾和铯原子之间弹性碰撞的s波散射长度,有效力程和p波散射长度等散射参数. 超冷温度下³⁹K-¹³³Cs原子间的弹性散射截面主要为s波贡献,随着碰撞能量的增加散射截面有丰富的形状共振出现, 计算发现单重态和三重态截面分别存在显著的g波和d波形状共振.另外,本文应用简并内态近似方法获得了⁴¹K-¹³³Cs 超精细态相互作用时的s波散射长度.     

Total electron scattering cross-sections for O atoms: O2 and O3 molecules

Total (elastic+inelastic) cross sections fore ⁻-O,e ⁻-O₂ ande ⁻-O₃ scattering have been calculated at sample energies between 100 and 1000eV. The basice ⁻-O atom scattering amplitudes are obtained from the partial wave analysis with a complex optical potential. Thee ⁻-O₂ ande ⁻-O₃ cross-sections are obtained through the independent atom model. Oure ⁻-O₂ cross-sections reproduce the experimental data quite well. Adequate comparisons are made in all the three cases.     

Elastic scattering of positrons by xenon atoms

Summary Partial and total cross-sections of positron scattering by Xe atoms are calculated at twenty values of incident energy ranging from 0.01 Ry to 4.00 Ry. The calculations are carried out via model potential (describing the positron target interaction) and a polarization potential introduced previously by Lane and Geltmann (1967). The comparison between our results and those obtained by other authors shows significant agreement and supports our simple model scattering process. The author of this paper has agreed to not receive the proofs for correction.     

Elastic scattering of electrons by argon atoms

The partial wave method with a central potential has been applied to investigate the elastic scattering of electrons by the argon atoms in the intermediate energy range (100 eV − 1 keV). The central potential includes the effects of the static field, exchange and polarization. The results are in good accord with recent experimental data. A brief account of this investigation was presented at the X Int. Conf. on the Physics of Electronic and Atomic Collisions held at Paris in July 1977.     

Differential elastic cross-sections of excited exotic hydrogen atoms

Differential cross-sections for the elastic scattering of excited exotic hydrogen atoms by hydrogenic atoms are calculated for the first time. The calculations are performed for exotic atoms (μ a,π a; a=p,d,t) in the states with the principal quantum numbers n=2-10 at relative motion energies E=0.001-15 eV and at scattering angles ϑ =0–180^00B0;. This revised version was published online in August 2006 with corrections to the Cover Date.     

Positron scattering from alkaline-earth elements

The total (elastic + inelastic) cross sections fore ⁺ impact on alkaline-earth elements from Be to Ra are calculated by employing a complex spherical optical potential. This potential has static, polarization and absorption components. The positron energy range is from a few eV to several thousand eV. We have compared our elastic cross sections for Mg and Ca with the other available results and the agreement is good for energies above 100eV. We have also compared our absorption cross sections withe ⁻ ionization cross sections at high energies where our absorption cross sections are in good accord. We have made Bethe plots fore ⁺ scattering on these elements.     

Single-photon scattering by two separated atoms in a supercavity

We study the single-photon scattering along a one-dimensional cavity array with two distant two-level atoms in a supercavity,which aims to simulate a recent x-ray experiment [Nature 482,199(2012)].Without introducing dissipation,we find that when one atom is exactly located at a node of a mode of the supercavity and the other is at the antinode of that mode,no splitting of the reflectivity peak can appear.Nevertheless,the atom at the node significantly changes the positions of the reflectivity valleys.On the other hand,when the atom is shifted a little from the exact node,then the splitting can appear.We also explain these results with an analysis based on the general formal scattering theory.Our result implies the importance of non-resonant modes of the supercavity in our problem.     

Elastic scattering of electrons and positrons by cadmium atoms

ABSTRACT

The differential, integrated elastic, total and momentum transfer cross sections along with Sherman function for the elastic scattering of electrons and positrons by cadmium atoms have been evaluated from the partial wave solution of the Dirac relativistic scattering equations for a projectile-atom complex potential at the energy range 6.4 eV < E < 1.0 keV. For various scattering quantities, a comparison of our results exhibits better agreement with the experimental data than the other available theoretical values.     

Total and transport elastic cross-sections for excited muonic and pionic hydrogen atoms

The results of an experiment on muon catalyzed dd-fussion in HD gas are presented. The experiment was performed at the muon beam of PSI using a high-pressure ionization chamber filled with pure HD-gas of low D₂ concentration on the level 1%, at temperatures 50, 150 and 300 K. The non-resonant character of ddμ-molecule formation on HD molecules was confirmed by measuring the ratio of yields of the two ddμ-fusion channels, R=Y(³He,n)/Y(³H,p), which proved to be close to unity. The ddμ formation rate was found to vary from λ_ddμ-HD=0.05· 10⁶ s^-1 at T=50 K to λ_ddμ-HD=0.12· 10⁶ s^-1 at T=300 K, in agreement with the theoretical prediction. A prominent peak at t<60 ns was observed in the time spectrum of fusion neutrons indicating a resonant contribution of ddμ formation from epithermal dμ atoms. This revised version was published online in August 2006 with corrections to the Cover Date.     

Elastic scattering of sodium and cesium atoms at ultracold temperatures

The elastic scattering properties in a mixture of sodium and cesium atoms are investigated at cold and ultracold temperatures. Based on the accurate interatomic potential for the NaCs mixture, the interspecies s-wave scattering lengths, the effective ranges and the p-wave scattering lengths are calculated by the quantal method and the semiclassical method, respectively. The s-wave scattering lengths are 512.7a₀ for the singlet state and 33.4a₀ for the triplet state. In addition, the spin-change and elastic cross sections are also calculated, and the g-wave shape resonance is found in the total elastic cross sections.     

Phasing of lysozyme using one－wavelength anomalous scattering of sulphur atoms through the combination of direct methods and density modification

A new method of combining one-wavelength anomalous scattering (OAS) phasing and density modification has been described, in which the improved phases from density modification axe re-introduced into OAS phasing. In this way, the phases could be improved iteratively until convergence. The OAS phasing method is based on the previously established sign-probability formula, which breaks the phase ambiguity in the OAS phasing. The implementation of this method has been available in CCP4 as OASIS. This method, although based on direct-methods, could also incorporate known phases and figures of merit into its sign-probability formula. In the implementation of OASIS, the known phases axe from the positions of the anomalous scatters. In the current method, the known phases are from the density modification. The current method was tested on phasing a lysozyme crystal using anomalous scattering of sulphur atoms with diffraction data collected on an in-house x-ray source. The resulting map was well connected for the backbone atoms and clearly traceable, with an average map correlation coefficient of 0.6622 for the backbone atoms.     

超冷钠原子弹性散射特性的精确计算

基于精确的原子之间相互作用势,系统研究了钠原子在超冷温度下的弹性散射特性,精确计算了钠原子间碰撞时的s波散射长度、有效力程、p波散射长度以及束缚态数目等散射参数.超冷温度下单重态和三重态原子间的弹性散射截面主要为s波贡献,随着碰撞能量的增加散射截面有丰富的形状共振出现,计算发现单重态和三重态散射截面分别存在显著的f波和i波形状共振.应用简并内态近似方法获得了超精细态相互作用时的s波散射长度,所得结果与精确值比较符合.     

Positron spin relaxation in solids

Summary The positron spin relaxation in a longitudinal magnetic field was investigated in some solids where Ps formation occurs. The experimental results indicate that this relaxation manifests itself in those organic compounds where the positron can excite low-lying triplet states. The values of positron slowing-down time, obtained by adopting a schematic model, range from 13 to 28 ps. This work was supported by CISM (Centro Interuniversitario di Struttura della Materia) of Ministero della Pubblica Istruzione, and by GNSM (Gruppo Nazionale di Struttura della Materia) of CNR.     

Positron implantation profile in kapton

Summary Measurements of positrons' implantation profile were made with geometry as in the majority of PAT experiments, making use of the difference in values of mean lifetimes of positrons in the absorber and in the detector. The function describing the absorption of positrons in the absorber taking into account measurement geometry was fitted to the experimental data. The correction to the exponential relation occurring in this function is the dominating factor for small thicknesses of the absorber. In this analysis various values of positrons' backscatter coefficients of the nickel and of the kapton were also taken into account.

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Riassunto Sono state effettuate misure del profilo di impianto dei positroni per mezzo della geometria come nella maggioranza degli esperimenti PAT, usando la differenza nei valori delle vite medie dei positroni nell'assorbitore e nel rilevatore. La funzione che descrive l'assorbimento di positroni nell'assorbitore, considerando la geometria di misurazione, è stata adattata ai dati sperimentali. La correzione alla relazione esponenziale che si veriffica in questa funzione è il fattore dominante per spessori ridotti dell'assorbitore. Sono stati presi in esame in quest'analisi anche vari valori dei coefficienti di backscatter dei positroni del nickel e del kapton.

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Резюме Проведены измерения профиля имплантации позитронов в случае геометрии, как в большинстве экспериментов PAT, используя различия в величинах средних времен жизни позитронов в поглотителе и в детекторе. Функция, описывающая поглощение позитронов в поглотителе с с учетом геомтрии измерения, удовлетворяет экспериментальным данным. Поправка к экспоненциальному отношению, появляющаяся в этой функции, представляет основной коэффициент для малых толщин поглотителя. В этом анализе учитываются различные величины для коэффициентов обратного рассеяния от никеля и картона.

     

Positron spin relaxation in naphthalene

Summary The remaining spin polarizationP of positrons was measured in naphthalene against longitudinal magnetic-field strength. The measuring method was based on the relative yield of Ps in triplet and singlet state. In a zero fieldP is reduced to 50% of the initial polarization and approaches 100% upon increasing the applied field to larger values. This effect, which in teflon is quite absent, appears to be determined by the specific behaviour of positrons in naphthalene. This work was supported by CISM (Centro Interuniversitario di Struttura della Materia) of Ministero Pubblica Istruzione and by GNSM (Gruppo Nazionale di Struttura della Materia) of CNR.     

Positron polarization in chiral media

Summary The positron spin depolarization, relative to the direction of a static magnetic field, was investigated in some chiral media where Ps formation occurs. The results indicate that helical positrons are able to distinguish one enantiomer from the other: the remaining polarizations—just before Ps is formed—differ by about 20% on passing from one enantiomer to the other of the same chiral species. Moreover the existence of a process—strictly connected with chirality and able to hinder any depolarizing process more efficiently than the field itself—has been made evident at low fields. This work was supported by INFM (Consorzio Nazionale di Fisica della Materia).