Charge transfer betweenO+ions and helium

Emission cross sections of the first negative band system of N 2 produced by proton impact

The 26Mg(7Li,8B)25Ne reaction at 78.9 MeV was used to measure the mass-excess of 25Ne (-2.18 ± 0.10 MeV) and several low- lying excited states. Mass-excesses for 43Ar (-31.98 ± 0.07 MeV), 45Ar (-29.727 ± 0.06 MeV), and 46Ar (-29.732 ± 0.07 MeV) were determined from the 48Ca(α,9,7Be)43,45Ar reactions at 77.7 MeV and the 48Ca(6Li,8B)46Ar reaction at 80.1 MeV. Several excited states of 43Ar and 45Ar were observed. These and the excited states of 25Ne are compared with theoretical predictions. Ground state differential cross sections are given for all reactions, ranging from 0.1 to 1 μb/sr at forward angles. Upper limits of about 1 to 25 nb/sr were determined for the ground state cross sections of the reactions 48Ca(α,8,10,11B)44,42,41Cl and 48Ca(α,10-13C)42-39S at 110 MeV. Recoil coincidence techniques were used to measure the mass-excess of the particle- unbound nucleus 10Li(33.83 ± 0.25 MeV) by the 9Be(9Be,8B)10Li reaction at 121 MeV; the ground state cross section was 30 nb/sr. The 14C(9Be,8B)15B reaction at 120 MeV was also investigated using coincidence techniques. Contamination problems only permitted determining a ground state cross section upper limit of 50 nb/sr. A simple mass relation is derived, similar in approach to the Garvey-Kelson method but taking more explicit account of shell effects. Comparison is made with the Garvey- Kelson relation, and predictions of masses and of the stability of neutron-excess light nuclei are given for both methods. The modified mass relation is shown to offer better account for highly neutron-excess nuclei, including the values reported here for 25Ne and 43,45,46Ar.

The rotational transitions in low energy collisions between two polar molecules are studied by two semiclassical methods. One method is the impact-parameter close-coupling (IPCC) method which is applied to the relatively high energy collisions. For the lower energy collisions, the perturbed rotational state (PRS) method is adopted within the framework of the semiclassical impact-parameter method. The intermolecular potential is approximated by the long range dipole-dipole interaction. By using these two approaches, rotational cross sections are evaluated over a wide velocity range for the first time. A scaling procedure is introduced to enable us to evaluate the cross sections for a wide variety of polarmolecule pairs which are not necessarily identical. The validity of the sudden approximation is also studied.

It is shown that insertion of a deformation parameter ${\ensuremath{\beta}}_{6}$ in coupled channels calculations may be necessary for proper predictions of the neutron total cross sections (${\ensuremath{\sigma}}_{T}$) at incident energies below \ensuremath{\sim}700 keV. This is illustrated in the present work with the ${\ensuremath{\sigma}}_{T}$ measurements of Whalen et al. on the $^{182,184,186}\mathrm{W}$ isotopes. In a previous report it has been established that these data could not be well reproduced using coupled channel calculations involving only ${\ensuremath{\beta}}_{2}$ and ${\ensuremath{\beta}}_{4}$ deformations. It is shown presently that this discrepancy can be removed if, provided that self-shielding corrections to these measurements are made, a small deformation parameter ${\ensuremath{\beta}}_{6}$ is considered in coupled channel analyses. As the W isotopes are lying at the rotational edge of the $A=190\ensuremath{-}200$ transitional mass region, it is also emphasized that band mixing effects on the model predictions might be of some importance. These effects have been investigated for $^{182,184,186}\mathrm{W}$ as well as for $^{183}\mathrm{W}$ using realistic collective wave functions.NUCLEAR REACTIONS Neutron total and scattering cross sections. Tungsten isotopes. Statistical model calculations. Realistic collective models. Coupled channels analyses. Deduced ${\ensuremath{\beta}}_{6}$ deformation parameters. Calculated inelastic cross sections for ground state and vibrational band levels.

Doubly excited KLL states were populated by triple electron capture in collisions of fast $(v=4.5--6.6\mathrm{a}.\mathrm{u}.)$ ${\mathrm{C}}^{6+}$ ions with Ar atoms. Measurements of the Auger-electron emission in the direction of the ion beam were used to determine the absolute single differential cross sections for the triple electron capture to all autoionizing KLL states. The results were compared with cross sections calculated within the independent-particle model, in which the simultaneous capture of all three target electrons was assumed. Single electron capture probabilities, employed by the model, were calculated using the two-center semiclassical close-coupling method, based on an atomic orbital expansion. In order to allow comparison of the measured zero-degree differential cross sections with calculated total cross sections, the Auger-electron emission from the doubly excited KLL states was assumed isotropic. Model calculations were found to be in a good agreement with the experimental data. An adequate description of the triple electron capture by the model implies that the projectile screening and electron-electron correlation effects in multiple electron capture are significantly reduced in fast, highly charged ion-atom collisions.

We have recently published a new semiclassical method, generalized Gaussian wave packet dynamics, which extends Gaussian wave packet dynamics into complex phase space. Although we were able to give an accurate formulation of the method, we had at the time of writing that paper only an intuitive, heuristic understanding of the deeper causes which make the method work. A more mathematical understanding was needed. To close this gap we show in this paper the equivalence of the new method with a first order expansion of ℏ of the Schrödinger equation. We further prove that the new method is equivalent to the stationary phase approximation, using the usual WKB formula for the propagator. The latter equivalence enables us to show that all the symmetry properties of time-dependent quantum mechanics also hold in the new semiclassical theory. Finally, we provide some elaboration of the method, and clarify several issues that were not discussed before. With this new insight we are able to formulate a simple rule for the calculation of semiclassical wave functions that contain contributions from more than one branch. This corrects for the divergence of semiclassical wave functions near caustics, a problem that we encountered in the preceding paper.

Electron collisional excitation of the low-lying 1${s}^{2}$2${s}^{2}$2${p}^{5}$ and 1${s}^{2}$2s2${p}^{6}$ states of F-like selenium to the singly excited M-shell states is studied using a relativistic distorted-wave model and multiconfigurational relativistic Hartree-Fock bound states. Results are presented for all 2-3 transitions from the low-lying 2${s}^{2}$2${p}^{5}$ $^{2}\mathrm{P}_{3/2}$ and $^{2}\mathrm{P}_{1/2}$ levels, and also the 2s2${p}^{6}$ $^{2}\mathrm{S}_{1/2}$ level. We find a number of strong dipole-allowed 2p-3d cross sections with peak values near threshold in excess of ${10}^{\mathrm{\ensuremath{-}}20}$ ${\mathrm{cm}}^{2}$, and derive Gaunt factors which are in good agreement with values used in the literature (0.15--0.20) for most strong transitions.Very strong monopole 2p-3p excitation cross sections have been important in soft-x-ray laser theory, and are found to be as strong as the largest dipole-allowed cross sections for F-like selenium. Theoretical output powers for the strong lines of the 2p-3s, 2p-3d, and 2s-3p transition arrays are computed and presented for plasma conditions of ${N}_{e}$=3\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ and ${T}_{e}$=1.0 keV. These results are compared in detail for proportionality against gf values for each array separately, as a test of how well line intensities might be judged from gf values in the absence of detailed theoretical intensity results. We find that for the 2p-3s and 2p-3d arrays, the intensities are in fair agreement with gf values within the array, while the agreement is much poorer in the case of the 2s-3p array. The two weaker arrays 2p-3s and 2s-3p are found to radiate more per unit gf than the 2p-3d transition array, in agreement with earlier observations in the Ne-like sequence. Theoretical line positions are tabulated for all strong 2-3 lines, and found to be in good agreement with experimental results for most strong transitions. Gains on the 3-3 transitions in between 2 and 4 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ are predicted for four lines under the plasma conditions quoted. Such conditions are similar to those of the recent extreme-uv laser experiments at the Lawrence Livermore National Laboratory, yet no F-like 3-3 amplification has yet been observed for F-like transitions. The discrepancy is currently a mystery.

This work is concerned with the development of approximate methods to describe relatively large chemical systems. This effort has been divided into two primary directions: First, we have extended and applied a semiclassical transition state theory (SCTST) originally proposed by Miller to obtain microcanonical and canonical (thermal) rates for chemical reactions described by a nonseparable Hamiltonian, i.e. most reactions. Second, we have developed a method to describe the fluctuations of decay rates of individual energy states from the average RRKM rate in systems where the direct calculation of individual rates would be impossible. Combined with the semiclassical theory this latter effort has provided a direct comparison to the experimental results of Moore and coworkers. In SCTST, the Hamiltonian is expanded about the barrier and the ``good`` action-angle variables are obtained perturbatively; a WKB analysis of the effectively one-dimensional reactive direction then provides the transmission probabilities. The advantages of this local approximate treatment are that it includes tunneling effects and anharmonicity, and it systematically provides a multi-dimensional dividing surface in phase space. The SCTST thermal rate expression has been reformulated providing increased numerical efficiency (as compared to a naive Boltzmann average), an appealing link to conventional transition state theory (involving a ``prereactive`` partition function depending on the action of the reactive mode), and the ability to go beyond the perturbative approximation.

Electron-impact ionization of fluoromethanes – Review of experiments and binary-encounter models

Based on the semiclassical impact parameter method, a theoretical model is constructed to calculate fully differential cross sections for single ionization of helium by impact with fast C6+ ions. A good agreement with the experiment is achieved in the scattering plane, while in the perpendicular plane a structure similar to that observed experimentally is obtained. The contribution of different partial waves to the cross section is also investigated.

Spectrum-averaged cross sections of the reactions V-51(n,p), V-51(n,α), Cu-65(n,2n), Zn-64(n,p), I-127(n,2n) and Hg-199(n,n’) were measured relative to Ni-58(n,p) or In-115(n,n’). The data were compared with other experiments and with calculations. A complete uncertainty covariance matrix was derived for the data. The correlation of the present data with previous experiments is also shown. This information and the data of a few recent other experiments were used to update the evaluation of a complete set of experimental spectrum-averaged cross sections determined in the standard neutron field of Cf-252.

Based on NII spectra, some transition probabilities for 2p4f–2p3d and 2s2p23d–2s2p23p are obtained by a semi-classical method. The results are in good agreement with other measurements and the data reported by the National Institute of Standards and Technology. The transition probability for a line of 424.18nm is reported for the first time. Meanwhile, a feasible method of calculating transition parameters related to special excited configurations or highly excited states is provided.

We have investigated the recently reported discrepancy between theory and a new experiment [Palathingal et al., Phys. Rev. A 51, 2122 (1995)] for single-quantum annihilation of positrons with bound electrons. Fully relativistic calculations of total cross sections for the production and the annihilation of positrons involving an electron bound in the K or L shells of atoms have been performed in screened atomic potentials, for a number of elements ranging from germanium (Z=32) to thorium (Z=90) and for positron kinetic energies up to 4 MeV above threshold. Very good agreement with earlier calculations, that had been performed over a more restricted range of elements and energies, is obtained, also confirming that the Z and shell dependence previously seen continues at higher energies. The results presented here extend theory to energies where recent experiments have been performed, at forward angle, which were assumed to characterize the Z and shell dependence of the corresponding total cross sections. Our new theoretical predictions are not consistent with the strong dependence of the K- and L-shell bound-pair-annihilation cross sections on the nuclear charge of the target reported in those experiments. An explanation of these discrepancies is proposed, namely, that it is not correct to assume the same ratios for total cross sections as for forward distributions. This is demonstrated using existing data for the related photoeffect process, and it is seen that such forward data are indeed consistent with the pair-annihilation experiments.

Charge exchange between xenon ions and xenon atoms is the source of a detrimental low energy plasma in the vicinity of electrostatic spacecraft thrusters. Proper modeling of charge-exchange induced spacecraft interactions requires knowledge of the respective charge-exchange cross sections. Guided-ion beam measurements and semiclassical calculations are presented for xenon atom charge-exchange collisions with Xe+ and Xe2+ at energies per ion charge ranging from 1 to 300 eV. The present measurements for the symmetric Xe++Xe exchange system are in good agreement with several earlier experimental studies and semiclassical calculations based on the most recently computed Xe2+ interaction potentials. The cross sections are ∼30% higher than predictions by the Rapp and Francis model [D. Rapp and W. E. Francis, J. Chem. Phys. 37, 2631 (1962)]. The present Xe2++Xe symmetric charge exchange measurements are the first to cover the ion energy range from 40 to 600 eV. The cross sections are in good agreement with low-energy drift tube measurements and are significantly lower than previous higher energy measurements. A simple model for symmetric two-electron transfer is proposed that is in good agreement with the present measurements. The onset for the asymmetric charge-exchange process, Xe2++Xe→2Xe+, is observed to be at 10 eV. For this process, a cross section of 2.8±0.9 Å2 is measured for a Xe2+ energy of 600 eV.

L-shell ionization cross sections, evaluated as mean values of the experimental data, were compared with the first-order theoretical values obtained by the plane-wave Born and semiclassical method. Hydrogenic Dirac wave functions in the united-atom limit were employed in the calculation. The plane-wave Born approximation cross sections with Dirac-Hartree-Slater wave functions were obtained from the tables of Chen and Crasemann [At. Data Nucl. Data Tables 33, 217 (1985); 41, 257 (1989)]. The agreement between the experimental and semiclassical cross sections was generally better than 20%, except for the ${\mathit{L}}_{1}$ subshell of heavy elements and for the very adiabatic region of ${\mathit{L}}_{2,3}$ subshells. The differences between both sets of data were largely reproduced by a simplified coupled-channel calculation performed nonrelativistically for a single final state. The calculation also suggests that, due to the uncertainties of the fluorescence yields, the ionization cross sections deduced from the measured data are overestimated by 10%.