Influence of Integral Instability of Kramers-Kronig Transformation on Photoabsorption Cross Sections

The photoabsorption cross sections of condensed atoms and molecules have proven to be dependent not only on the imaginary parts but also on the real parts of the polarizabilities due to the strong interatomic interactions in condensed environment. The real parts of the polarizabilities calculated usually by using thefamous Kramers-Kronig transformation (KKT) from the photoabsorption cross sections of the isolated atoms are very sensitive to the accuracy of the implementation method of the infinite integral in the KKT. The influence of the integral instability of the KKT and the real part of the polarizability on the variation of the photoabsorption cross sections with the number density and the structure of the condensed matter has been studied in the present work for the first time. The conclusion is that the integration method with interpolation has given more reasonable results than the direct truncation method if some appropriate interpolation functions have been used. Some notes and conclusions have also been given for the applications of the alternative coupled expressions of photoabsorption cross sections.     

Macroscopic local-field effects on photoabsorption processes

The influence of the local-field effect on the photoabsorption cross sections of the atoms which are embedded in the macroscopic medium has been studied by a set of alternative expressions in detail. Some notes on the validity of some different local-field models used to study the photoabsorption cross sections of atoms in condensed matter have been given for the first time. Our results indicate that the local fields can have substantial and different influence on the photoabsorption cross section of atoms in condensed matter for different models. Clausius-Mossotti model and Onsager model have proved to be more reasonable to describe the local field in gas, liquid, or even some simple solid, while Glauber-Lewenstein model probably is wrong in these conditions except for the ideal gas. A procedure which can avoid the errors introduced by Kramers-Kronig transformation has been implemented in this work. This procedure can guarantee that the theoretical studies on the local field effects will not be influenced by the integral instability of the Kramers-Kronig transformation.     

Photoabsorption cross sections of the alkaline-earth-metal elements under strong interaction conditions

The strong interatomic interaction effects on photoabsorption cross sections of a private atom have been interpreted by the contribution of both the real and imaginary parts of atomic polarizability in the present work. Some unresolved differences between the photoabsorption cross sections of isolated atoms and those of condensed atoms, especially near thresholds and resonances, probably could be interpreted by the competition between the imaginary and real parts of atomic frequency-dependent polarizabilities.     

Variation of photoabsorption cross section of atomic silver from isolated to condensed conditions

The influence of the local-field on the photoabsorption cross section of the atomic silver is studied in detail by using the Clausius--Mossotti (CM) model and the Onsager model separately. The variations of the photoabsorption cross section of atomic silver with number density and radius of the environmental interaction cavity are studied systematically by using more general expressions for the photoabsorption cross sections, proposed by Sun et al recently. It has proved to be reasonable to model the optical response properties of bulky material by coupling the property of isolated atom with the environmental effects in the present work.     

Empirical photoabsorption cross sections for C,N, O,F and Cl obtained from molecular measurements between 50 and 340 Å

For many years, it has been known that many molecular photoabsorption cross sections in the hard X-ray region could be approximately obtained by summing the individual contributions of each atom to the total cross section. This article reports on an attempt to express 15 molecular cross sections as sums of empirical atomic cross sections for five constituent atoms C, N, O, F and Cl. For most wavelength ranges, the empirical atomic cross sections generated from a best fit to the molecular data were capable of describing the measured molecular cross sections to within 2% accuracy. In the 260–300 Å region where broad structures occur in the molecular spectra, the above model was slightly worse, perhaps indicating the appearance of molecular resonance effects discussed by other authors. It is suggested here that sums of empirical atomic cross sections will provide a useful means for prediction of molecular photoabsorption cross sections for molecules other than those investigated here for this limited wavelength region although it is acknowledge that the theoretical basis for this procedure is questionable.     

An alternative view of condensed-phase photoionization

This paper proposes an accurate valuable interpretation scheme to study the evolvement of the photoionization processes from the isolated to the condensed atoms by a unique ab initio method. The variations of the photoionization cross sections of the atomic sodium with the photoelectron energy and the boundary radius of the atomic configuration space are studied in this new scheme by the R-matrix method. The discrepancy in the photoionization spectra of the isolated and the condensed sodium has been explained quantitatively and understood successfully by this alternative view in detail for the first time.     

温度对NH_3气体紫外吸收截面影响研究

为了研究NH3气体在紫外203～220 nm内吸收截面随温度的变化规律,采用高分辨率光栅单色仪、氘灯光源、闭式气样室和配气装置,测量NH3气体温度由308 K升高至397 K的吸收截面.NH3气体吸收截面由离散吸收和连续吸收两部分组成.结果表明,随着温度的升高,基态剩余量子旋转、振动迁移到激发态的概率减少,最终导致离散吸收截面峰值的降低.随着温度由308 K升高至397 K,在特征波长212.5 nm处,离散吸收截面峰值的最大相对减幅为46%.NH3气体在这个波段的吸收截面存在明显的等波长间隔分布特征,约为4 nm.随着温度升高,峰值位置未见变化.连续吸收截面整体上随温度升高而减小,且这种减小趋势随波长红移逐渐减弱.由于NH3气体吸收截面随温度的变化呈现较大的变化,实测时应对气体浓度在线测量结果进行温度补偿计算.     

高密度体系光电离截面新表达式的应用

利用实验光谱学的Beer Lambert定律和介质中的麦克斯韦方程组，建立了精确的光吸收（光电离）截面表达式，并通过一个随体系粒子数密度和宏观复介电常数而变化的变换函数，将严格截面公式与Fano和Cooper 1968年建立的理想气体的光电离截面公式直接联系起来.建议：1)当知道某密度下正确的体系宏观复介电常数β,γ时，可直接由严格表达式求得高密度下非理想状态的正确光电离截面；2)或当知道该体系的粒子微观极化率η,ζ和其理想气体的精确截面时，用上述变换公式间接求得其他密度时的光电离截面.对氩原子和氙原子的应用表明：当缺乏β,γ时，可由某一合理的宏观电极化率物理（例如克劳修斯-莫索缔）模型来计算β,γ，从而获得高密度体系的截面.这样获得的结果符合被散射物质的光电离（光吸收）截面随体系粒子数密度增大而增加的客观散射现象.而且，考虑了高密度真实体系中粒子间相互作用的宏观电极化率模型越正确，如此求得的散射截面的误差就越小. 关键词： 光电离截面 光吸收截面 介电常数 极化率     

Total photoabsorption cross sections for high-Z ELEMENTS in the energy range 7–20 MeV

Total photoabsorption cross sections for 13 isotopes between Sm and Bi were measured in the energy range 7–20 MeV with uncertainties generally better than 0.3 % using the narrow beam attenuation method. The measured cross sections after subtracting the photonuclear contribution are generally in good agreement with the latest NBS calculations of atomic cross sections.     

Total cross sections for photoabsorption on light nuclei in the energy range 600–1500 MeV

Experimental data of the GRAAL Collaboration on the total cross sections for photoabsorption on deuterium and carbon targets at gamma-ray energies in the range between 600 and 1500 MeV are presented. The experiment was performed in a beam of photons obtained by the method of the Compton backscattering of laser photons at the electron storage ring of the European Synchrotron Radiation Facility (ESRF, Grenoble, France) by using a wide-aperture detector covering a solid angle close to 4π. The total photoabsorption cross sections were determined by two independent methods: by subtracting the emptytarget background and by summing partial cross sections for meson photoproduction. The total cross sections for photoabsorption on quasifree protons and neutrons are shown to agree both in magnitude and in shape within a 5% precision of the measurements. In contrast to data previously available in the literature, both cross sections show distinctly the F ₁₅(1680) resonance at a photon energy of about 1 GeV. Data obtained in the present experiment for the cross sections for photoabsorption on a free and a bound nucleon are compared. This comparison reveals a substantial role of two-nucleon correlations in estimating the nuclear-medium effect on meson photoproduction.     

Free electron in compressed inert gases

The behavior of excess and intrinsic free electrons inside compressed inert gases is described as a function of pressure by using a pairwise approximation for the electron interaction with atomic surroundings. The change of sign from negative to positive for the xenon atom electric potential inside condensed xenon is predicted to occur at a pressure around 3 GPa, preventing slow electron embedding into solid xenon from the gas phase at higher pressure. To overcome this difficulty, the electrons should be injected into a solid sample just before its pulsed shock loading. The ionization of xenon by pressure and its further metallization are described by decreasing the forbidden gap at the expense of increasing the xenon ground electronic term and simultaneous splitting of the upper ionized electronic state. A good coincidence between the calculated and measured pressure of the dielectric-metal transition in xenon is demonstrated. The text was submitted by the authors in English.     

原子分子真空紫外光吸收和光电离绝对截面的精确测量

介绍了一种精确测量原子分子真空紫外光吸收和光电离绝对截面的实验方法。该方法的主要特点是使用圆筒形多级电极，有效地收集光电离产生的电子和离子，并利用高通量的同步辐射作光源，精确地测量分子（原子）的绝对光吸收、光电离和光离解截面，以及光电离量子产率和光学振子强度分布等。利用该方法获得了Ｘｅ原子和Ｎ２分子的一些实验结果。     

Glory undulations in the total cross section for scattering of heavy rare gas atoms

In a supersonic molecular beam apparatus the velocity dependence of the total cross section for scattering of argon and krypton beams by argon, krypton and xenon has been measured for relative velocities ranging from 0.55 to 4.30 km · s^-1. For all these systems a number of glory extrema were well resolved. The measured cross sections are compared with cross sections calculated on the basis of recently proposed pair potentials. For the systems Ar-Ar, Kr-Kr and Ar-Kr the glory structure is fairly well described by the potentials, however, the energy dependence of the cross sections is not. The available potentials for the systems Ar-Xe and Kr-Xe describe the glory structure less accurately. Rather severe changes of the potentials will be required to obtain agreement with the total cross section data. A qualitative analysis of the required changes is presented.     

Giant resonance in the total photoabsorption cross section of Z ≈ 90 nuclei

Total photoabsorption cross sections of ²³²Th, ²³⁵U, ²³⁸U and ²³⁹Pu have been measured in the giant resonance region by the absorption method. Measured cross sections were approximated by two Lorentz lines. Lorentz line parameters, integrated cross sections, deformation parameters and quadrupole moments are given. The analysis of the nuclear optical anisotropy evolution with the increase of Z shows that Z ≈ 90 nuclei seem to be transitional, similar to N ≈ 90 nuclei. A comparison of experimental cross sections with dynamic collective model calculations has been performed. The evidence that quadrupole photoabsorption occurs in the 20–25 MeV energy region is also presented.     

Near-threshold electron-impact ionization of calcium atoms

The electron-impact ionization of calcium atoms is studied in the near-threshold energy range (from 6.11 to 16 eV). Experiments were performed by the method of intersecting electron and atomic beams with the recording of formed positive calcium ions. The electron beam (ΔE _1/2 = 0.15 eV) was formed using a hypocycloidal electron monochromator. An analysis of the specific features of ionization cross sections revealed a contribution from the excitation and decay of low-lying autoionization atomic states, which converge to the excitation thresholds of the 3d, 4p, and 5s ionic levels, and resonances (long-lived states of negative ions). The specific features of cross sections are identified using the experimental and theoretical data on photoionization (photoabsorption).     

An analysis of available absolute total cross section data for noble gas systems

The most important literature data on absolute total cross sections for noble gas systems have been surveyed. A comparison has been made with total cross sections calculated from various interatomic potentials proposed in the literature and with the total cross sections deduced from the long range part of the potential theoretically derived by Tang et al. for the noble gas systems.     

Photoabsorption on nuclei

We calculate the total photoabsorption cross section on nuclei in the energy range from 300 MeV to 1 GeV within the framework of a semi-classical phase space model. Besides medium modifications like Fermi motion and Pauli blocking we focus on the collision broadening of the involved resonances. The resonance contributions to the elementary cross section are fixed by fits to partial wave amplitudes of pion photoproduction. The cross sections for NR → NN, needed for the calculation of collision broadening, are obtained by detailed balance from a fit to NN → NNπ cross sections. We show that a reasonable collision broadening is not able to explain the experimentally observed disappearance of the D₁₃(1520)-resonance in the photoabsorption cross section on nuclei.     

L x‐ray transitions in F‐like to Na‐like xenon ions determined at a room temperature electron beam ion trap

Fluorine‐like to sodium‐like xenon ions were investigated by wavelength‐dispersive x‐ray spectrometry at the Dresden Electron Beam Ion Trap (EBIT) working at room temperature. In addition to the precise measurement of the L x‐ray transition energies following ionization, excitation and recombination processes in the highly charged xenon ions, the spectra were analyzed for different gas pressures in the trap. Multiconfiguration Dirac‐Fock calculations are introduced that were applied to the energies, transition probabilities and excitation cross‐sections. The production of neon‐like xenon ions was demonstrated even at very high gas pressures. Copyright © 2005 John Wiley & Sons, Ltd.