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.     

Near-threshold electron-impact ionization of magnesium

The effective electron-impact ionization cross section of the magnesium atom is studied in detail in the energy range from the ionization threshold to 16 eV. A large number of particular features caused both by atomic autoionization states and by the formation and decay of a short-lived state of the negative ion of the magnesium atom are revealed. These particular features are identified using experimental and theoretical data on the photoionization (photoabsorption) and on the spectra of ejected electrons.     

Charge oscillations and photoabsorption of the Thomas-Fermi-Dirac-Weizsäcker atom

Following the work of Bloch and Ball et al. the theory of linear response of a hydrodynamic Thomas-Fermi model is extended to a general density dependent energy functional. The theory is applied to the Thomas-Fermi-Dirac-Weizsäcker atom. Normal mode solutions and the atomic photoabsorption cross section are calculated.     

Density-dependent photoabsorption cross sections of atomic Xe

The evolution of the photoabsorption cross sections of atomic xenon with number densities varying from ideal gas to condensed matter has been studied by an alternative view in the present work. The alternative expressions of the photoabsorption cross sections presented by Sun et al recently were used with the local field models that has proven to be generalized easily to multiatomic systems including molecules and condensed phase systems. The present results show that the variation of the photoabsorption cross sections of atomic xenon in the giant resonance region from the isolated to the condensed conditions is very small, which agrees well with the variation law of the solid and gas experiments.     

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).     

Photoionization of the outer electrons in noble gas endohedral atoms

We suggest a prominent modification of the outer shell photoionization cross section in noble gas (NG) endohedral atoms NG@C _n under the action of the electron shell of fullerene C _n . This shell leads to two important effects: a strong enhancement of the cross section due to fullerene shell polarization under the action of the incoming electromagnetic wave and to prominent oscillation of this cross section due to the reflection of a photoelectron from the NG by the fullerene shell. Both factors lead to powerful maxima in the outer shell ionization cross sections of NG@C _n , which we call giant endohedral resonances. The oscillator strength reaches a very large value in the atomic scale, 25. We consider atoms of all noble gases except He. The polarization of the fullerene shell is expressed in terms of the total photoabsorption cross section of the fullerene. The photoelectron reflection is taken into account in the framework of the so-called bubble potential, which is a spherical δ-type potential. It is assumed in the derivations that the NG is centrally located in the fullerene. It is also assumed, in accordance with the existing experimental data, that the fullerene radius R _C is much larger than the atomic radius r _A and the thickness Δ_C of the fullerene shell. As was demonstrated recently, these assumptions allow us to represent the NG@C _n photoionization cross section as a product of the NG cross section and two well-defined calculated factors. The text was submitted by the authors in English.     

Surface plasma oscillations in layer metal particles

Light scattering cross section and positions of two surface plasma resonances in a layer metal particle containing dielectric foreign nucleus are calculated within the random phase approximation. The results obtained provide a possible explanation of contradictions in size dependence of surface plasma resonance position [1, 2] and are able to explain two peak structure of photoabsorption cross section of small silver particle observed in [1].     

Giant dipole resonance in 4d photo absorption of atomic barium

We calculate the photoabsorption cross section for the 4d¹⁰ and 5p⁶ shells of atomic barium using the random phase approximation (RPAE) including relaxation. For both shells the oscillator strength goes mainly into a collective vibration of the entire shell.     

Low multipole states in 18O from the open-shell linear response model

The photonuclear cross section for the isovector electric dipole in ¹⁸O is calculated using the open-shell linear response method. The model accounts for the pygmy resonance and provides a simple interpretation for the main peak at 23.7 MeV observed in photoabsorption cross sections. The model reproduces qualitatively the main differences and common features of the ¹⁶O and ¹⁸O photoabsorption cross sections. The strength distributions of the isoscalar quadrupole and octupole states in ¹⁸O are also presented.     

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 by the electron subsystem of a semiconductor nanoparticle

The IR photoabsorption cross section of a semiconductor nanoparticle has been calculated. Light is absorbed by conduction electrons and trapped electrons in the volume and surface of the nanoparticle. Electron concentrations have been obtained by minimizing the total free energy of charges in the system. The photoabsorption cross section has two characteristic maxima corresponding to the absorption by conduction electrons and by trapped electrons in the nanoparticle volume. The number of trapped electrons on the surface is relatively small, so that they do not contribute to the total cross section.     

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.     

Nuclear photoabsorption and Compton scattering at intermediate energy

Intermediate energy nuclear photoabsorption and Compton scattering within the Δ-hole approach are studied. The same Δ-nucleus dynamics used to describe a variety of pion-induced reactions are employed. A dynamical model for the photon-nucleon amplitude is constructed, including both resonant-channel and nonresonant backgrounds in addition to Δ-excitation. Medium corrections to the full amplitude are included for nuclear seattering. A doorway state expansion allows one to discuss quantitatively the role of various contributions to the Δ-hole Hamiltonian, thereby clarifying the level of sensitivity to the structure of the Δ spreading potential. The reactive content of the total cross section is diseussed. The results are compared with available photoabsorption data for ⁴He, ¹²C and ¹⁶O. We present results for the Δ contribution to the electron seattering transverse response function, for coherent π⁰ photoproduction, and for nuclear elastic Compton seattering. The photoabsorption strength is spread over a larger energy range by the Δ dynamics, in agreement with the data, but is centered at too high an energy. The helicity flip Compton cross section is especially sensitive to the Δ-nucleus interaction, such as the spin-orbit potential strength.     

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.     

The absorption cross sections of H2 and D2 from 180 to 780 Å

The absolute photoabsorption cross sections of H₂ and D₂ have been measured photoelectrically in the wavelength region from 180 to 780 Å using synchrotron radiation. At photon wavelengths shorter than 650 Å, the absorption cross section was found to be structureless and to decrease montonically with decreasing wavelength. In this spectral region, both gases have nearly equal absorption cross sections, varying only within the experimental error. At photon wavelengths greater than λ 650 Å, structure is evident in the cross section of both gases and presumably corresponds to the Rydberg States reported by various authors.     

E1 resonances in neutron-rich nuclei within the phonon damping model

The quasiparticle representation of the phonon darnping model (PDM) is developed to include the superfluid pairing correlations microscopically. The formalism is applied to calculate the photoabsorption and the electromagnetic (EM) differential cross sections of E1 excitations in neutron-rich oxygen and calcium isotopes. The calculated photoabsorption cross sections agree reasonably well with the available data for ^16,18O and ^40,48Ca. The results of calculations show that the change of the fraction of the E1 integrated strength in the region of pygmy dipole resonance (PDR) as a function of mass number A with increasing neutron number N is in agreement with the recent experimental data, and does not follow the prediction by the simple cluster model. The EM differential cross sections obtained within PDM in this work show prominent PDR peaks below 15 MeV for ^20,22O in agreement with the recent experimental observation. It is also shown that, using low-energy RI beams at around 50–60 MeV/nucleon, one can observe clean and even enhanced PDR peaks without the admixture with the GDR in the EM differential cross sections of neutron-rich nuclei.     

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.     

Photoionization cross section of iridium in the soft X-ray region

The photoionization cross section of iridium was measured experimentally for the first time in the soft X-ray region (35 eV?400 eV) by direct photoabsorption. Measurements obtained in using synchrotron radiation emitted by the LURE-ACO storage ring are compared with theoretical predictions.     

Measurement of the total photoabsorption cross section on a proton in the energy range 600–1500 MeV at the GRAAL

The total photoabsorption cross section on a free proton was measured at the GRAAL facility in the energy range E _γ = 600−1500 MeV. The large-aperture LAGRANγE detector and a liquid hydrogen target were used in the experiment performed with a back-scattered Compton gamma beam. To improve the accuracy, two alternative methods were employed. First, a subtraction method of using empty-target measurements allowed the cross section σ _tot to be evaluated directly because of a low level of the electromagnetic background. Second, an algorithm for evaluating σ _tot on the basis of summing the dominating partial cross sections was developed. Experimental results obtained for σ _tot by the two methods are compared with existing data. The article was translated by the authors. Published in Russian in Yadernaya Fizika, 2008, Vol. 71, No. 1, pp. 76–83.     

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.