Differential cross section surfaces for low energy scattering of electrons and positrons from rare gas atoms

The differential cross sections for scattering of electrons and positrons from He, Ne, Ar, Kr, and Xe at projectile energies below the inelastic thresholds are calculated using a model potential approach in which the interaction between the projectile and the target atom is partitioned into static, exchange (for electrons), and correlation-polarization parts. Two different forms of the parameter-free correlation-polarization potential are suggested; in both cases the correlation-polarization potential is determined by smoothly matching the asymptotic form of the polarization potential (1/r ⁴) to the correlation potential at the outermost orbital radius of the target atom. The results of angular distributions are presented in the form of contours of constant differential cross sections as well as in the form of differential cross section surfaces in three-dimensional plots. Both of these presentations display the locations of the principal maxima and minima of the differential cross sections as well as the critical points in a very useful manner.     

Elastic scattering slope and total cross section relations

The empirical equality of $\text{B}{}^2\text{σ}{}_{\mathrm{<mtext>t</mtext>}}$ is noted, for the pomeron terms in NN, πN, KN, ?N, ωN, and φN scattering, where B is the elastic slope parameter and σ_t is the total cross section. This ratio increases slowly with energy, but remains the same in all channels. This is equivalent to a relation between the diffractive interaction radii and opacities; the opacity scales with the square of the radius. We conjecture that this systematics extends to ψN scattering. We contrast $\text{B}{}^2\text{σ}{}_{\mathrm{<mtext>t</mtext>}}$ universaility with the slope predictions of an f-coupled pomeron model. Some other predictions of the f-dominance hypothesis are tested against data.     

The total cross section for the production of heavy quarks in hadronic collisions

We present the results of a full calculation of the QCD O(α_S³) radiative corrections to the total cross section for the production of a heavy quark pair. We find large contributions for parton subenergies near threshold and well above threshold. The implications for the production of top and bottom quarks at collider energies are discussed.     

The neutron zero-energy total cross section and scattering lengths for tritium

New results for the neutron total cross section for tritium extrapolate to 1.70 ± 0.03 b at zero energy. With a theoretical constraint A₁/A₀=0.92 ± 0.04, the scattering lengths A₁=3.60 ± 0.10, A₀ = 3.91 ± 0.12, and A_c=3.68 ± 0.05 fm can be inferred independently of, but consistently with, the previous experimental value for the coherent length A_c=3.82 ± 0.24 fm.     

Absolute differential bremsstrahlung cross section for the scattering of 0.6-keV electrons by xenon atoms

The spectral distribution of the absolute differential cross section for total bremsstrahlung, including ordinary and polarization, for the scattering of 0.6-keV electrons from free xenon atoms has been measured. The bremsstrahlung photons have been detected at an angle of 97° with respect to the electron motion direction. The investigations have been carried out in the ultrasoft x-ray spectral region 60–240 eV. The results are compared with the total cross sections for bremsstrahlung calculated in several approximations for the scattering of 5-and 25-keV electrons by Xe atoms. Both the value and the spectral distribution of the experimental absolute cross sections are most satisfactorily reproduced by the calculations in the distorted partial wave approximation.     

Logarithmically accurate total cross sections of fast electron scattering by atoms

A simple analytical formula for the total integrated cross section of fast electron scattering by atoms is obtained. This formula, being logarithmically accurate, is derived from the approximate rule of sum that does not take into account exchange effects in the atomic wave function (Hartree approximation). It is shown that exchange effects can be neglected in that scattered electron energy range where the first Born approximation is valid. The results of calculation for He, Ne, and Ar atoms agree well with experimental data.     

Elastic scattering and total reaction cross section for the He + Al system

The elastic scattering of the radioactive halo nucleus ⁶He on ²⁷Al target was measured at four energies close to the Coulomb barrier using the RIBRAS (Radioactive Ion Beams in Brazil) facility. The São Paulo Potential (SPP) was used and its diffuseness and imaginary strength were adjusted to fit the elastic scattering angular distributions. Reaction cross-sections were extracted from the optical model fits. The reduced reaction cross-sections of ⁶He on ²⁷Al are similar to those for stable, weakly bound projectiles as ^6,7Li, ⁹Be and larger than stable, tightly bound projectile as ¹⁶O on ²⁷Al.     

Measurements of total and positronium-formation cross sections for the scattering of positrons by alkali atoms

We present a review of our recent measurements of total cross sections (Q _T's) for the scattering of positrons by Na, K, and Rb, and positronium-formation cross sections (Q _Ps's) for Na and K. For our total cross section measurements, a beam transmission technique has been used. For ourQ _Ps measurements, our approach involves setting upper and lower limits onQ _Ps using a combination of (1) measuring the transmission of the positron beam with the angular discrimination of the apparatus made as poor as possible, and (2) measuring the 511 keV annihilation gamma rays in coincidence produced by the decay of para-Ps formed in the scattering cell. Comparison with theoretical calculations shows that our measuredQ _T's andQ _Ps's for Na and K agree reasonably well with a close coupling approximation (CCA) calculation which takes into account the formation of Ps in then=1 andn=2 states. In the 3–10 eV energy range, this calculation predicts a peak in theQ _T's andQ _Ps's for K which also appears in our measurements. The absence of such a peak in our measuredQ _T's andQ _Ps's (preliminary) for Na in this energy range is also consistent with the same theory. Comparisons with five-state CCA calculations ofQ _T which do not take Ps-formation into account also show good agreement with our positron-Na, K, and RbQ _T measurements for energies above 20 eV, but show dramatic departures from our measurements below 10 eV for K and Rb.     

Inelastic processes in the scattering of metastable rare gas atoms at metal surfaces

Electron spectra of various metastable rare gas atoms systematically measured on a Pt(111) surface with Rb coverages ranging from submonolayers (3%) to multilayers are presented. The different decay channels of the excited particles are discussed in terms of resonant electron exchange processes between the substrate and the projectile in relation to the work function. It is shown that below a certain value of the work function a highly excited negative rare gas atom is formed which can undergo different de-excitation processes. A careful discussion of the branching ratios into the decay channels offers a natural explanation of the variations in the electron spectra induced by alkali metal adsorption. Additionally, an attempt is made to extract information about the alkali metal chemisorption state from the observed electron spectra.     

Relationship between the cross section for deep-inelastic scattering and the cross section for inclusive annihilation

Arguments have been presented that suggest that available experimental data on the structure functions for the processes ep → eX and e ⁺ e ^? → p( $\bar p$ )X do not confirm the so-called reciprocity relation, which was obtained in the leading-logarithm approximation of perturbation theory. It has also been shown that the asymptotic relationship between the above processes that was obtained on the basis of more general considerations is compatible with existing data.     

New measurement of the elastic scattering cross section of μd muonic atoms off deuterons

We have measured the elastic scattering cross section for the process μd + d → μd + d in ultrapure gaseous deuterium at 14 atm and at room temperature. The results is σ_dd = (8 ± 2) × 10^?20 cm².     

On the interaction of excited alkali atoms with rare gas targets in scattering processes

A model potential calculation has been applied to evaluate scattering experiments for Na and K in the groundstate and the resonance state interacting with Ar. The model potential has only two free parameters which are determined by a best fit of the interatomic potentials to experimental results. Satisfactory agreement between calculated and experimental results is found for the differential cross sections in the groundstate and the excited state, for satellites in theK(4P),K(5P) and Na(3P) line profile, for the van der Waals constantsC ⁽⁶⁾ andC ⁽⁸⁾, the alkali ion-rare gas interaction and the vibrational energy levels of the Na-Ar molecule. As maior advantages we point out that for a given pair of atoms all these calculated data are given with one single pair of values for the free parameters and that with this set also the interatomic potentials for the higher alkali states (specifically up to 5f for Na and K) are obtained.     

中高能区电子与分子散射总截面的计算

实验表明,电子与分子碰撞总截面(TCS)与分子本身固有参数(极化率、键长等)有一定关系,所以可从理论上定量探索这种关系,找出包含这些参数的准经验公式 .理论上可根据散射截面的"低能极限为硬球势散射、高能极限为玻恩近似"的思想,参考实验上测得的一些分子的总截面与分子的极化率的定量关系,我们用分子的几何和物理参数模拟了一种准经验公式.?     

Scaling laws for the photoionization cross section of two-electron atoms

The cross sections for single-electron photoionization in two-electron atoms show fluctuations which decrease in amplitude when approaching the double-ionization threshold. Based on semiclassical closed orbit theory, we show that the algebraic decay of the fluctuations can be characterized in terms of a threshold law sigma proportional to |E|(mu) as E --> 0(-) with exponent mu obtained as a combination of stability exponents of the triple-collision singularity. It differs from Wannier's exponent dominating double-ionization processes. The details of the fluctuations are linked to a set of infinitely unstable classical orbits starting and ending in the nonregularizable triple collision. The findings are compared with quantum calculations for a model system, namely, collinear helium.     

Steady-state differential cross section for elastic scattering

Lower limits are found on the differential cross section for elastic scattering and on its derivatives with respect to the momentum transfer t at t = 0 at high energies on the basis of completely general considerations. The results are expressed in terms of the measurable total cross sections.Translated from Izvestiya VUZ. Fizika, No. 4, pp. 103–108, April, 1970.     

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.     

Predicting the total charm cross section

We discuss the energy dependence of the total charm cross section and some of its theoretical uncertainties including the quark mass, scale choice and the parton densities.     

Upper limit of the elastic scattering cross section and the relationship between the total cross sections and the s-amplitude

Proceeding from general principles, an upper limit is obtained for the differential elastic scattering cross section of zero-spin particles having small momentum transfer and arbitrary energies, in terms of the experimentally observable total cross sections and of the ratio of the real to the imaginary parts of the forward-scattering amplitude. An important relationship is also obtained between the total cross sections and the partial s-amplitude.     

Excitation cross section for iridium atoms by slow electrons

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 5, pp. 711–716, May, 1991.     

Radial Sternheimer shielding-antishielding for rare gas atoms

The radial dependence of Sternheimer quadrupole shielding-antishielding factor is reported for He, Ne, Ar and Kr atoms at the coupled Hartree-Fock level of accuracy. The results would be useful in the interpretation of the nuclear quadrupole interactions measured at the rare gas nuclei in different chemical environment such as the various van der Waals molecules.