Cross Sections For Double Excitation Research Articles

Double excitation cross sections of molecular hydrogen induced by electron impact

In this work we present our results for calculations of doubly excited states of potential curves of H2 and the corresponding double excitation cross sections by electron impact using the First Born Approximation. The wave functions and potential energy curves were calculated using the Configuration Interaction method.

Electron- and positron-impact atomic scattering calculations using propagating exterior complex scaling

Calculations are reported for four-body electron-helium collisions and positron-hydrogen collisions, in the S-wave model, using the time-independent propagating exterior complex scaling (PECS) method. The PECS S-wave calculations for three-body processes in electron-helium collisions compare favourably with previous convergent close-coupling (CCC) and time-dependent exterior complex scaling (ECS) calculations, and exhibit smooth cross section profiles. The PECS four-body double-excitation cross sections are significantly different from CCC calculations and highlight the need for an accurate representation of the resonant helium final-state wave functions when undertaking these calculations. Results are also presented for positron-hydrogen collisions in an S-wave model using an electron-positron potential of V12 = − (8 + (r1 − r2)2)−1/2. This model is representative of the full problem, and the results demonstrate that ECS-based methods can accurately calculate scattering, ionization and positronium formation cross sections in this three-body rearrangement collision.

Differential cross sections for double excitation of theautoionizing states of helium by 100-150 keV proton impact

Autoionizing (2s2) 1S, (2p2) 1D and (2s2p) 1P resonances in projectile energy loss spectra arestudied theoretically for protons scattered on atomic helium.The differential resonance yield, as a function of the scatteringangle, is the quantity of primary attention. The effects of theCoulomb interaction in the final state and the projectile-coreinteraction on the differential resonance yield are studied.Comparison with the experimental data of Schulz et al (1995)(Schulz M, Htwe W T, Gauss A D, Peacher J L and Vajnai T 1995 Phys. Rev. A 51 2140-50) is presented. Comparison is also made withcalculations for differential cross sections of doubleexcitation.

First Double Excitation Cross Sections of Helium Measured for 100-keV Proton Impact

Excitation cross sections of the (2s{sup 2}){sup 1}S, (2p{sup 2}){sup 1}D , and (2s2p){sup 1}P autoionizing states of helium, produced in collisions with 100-keV protons, have been measured for the first time. Using a high resolution electron spectroscopy together with a recently proposed parametrization of autoionizing resonances distorted by Coulomb interaction in the final state makes it possible to extract from electron spectra {ital total cross sections} as well as {ital magnetic sublevel populations.} These new experimental data are briefly compared with out theoretical calculations. {copyright} {ital 1997} {ital The American Physical Society}

Multielectron Processes in Heavy Ion–Atom Collisions at Intermediate Velocity

Using high resolution x-ray spectroscopy, we have measured projectile electron single and multiple cross sections when a two-electron Ar{sup 16+} ion collides with neutral target atoms. For a fixed impact velocity (v{sub p}=23 a.u. ) , but using various targets from He to Xe, a range from the perturbative regime to the strong interaction regime has been investigated. Double excitation cross sections are found to be well reproduced by an independent electron model. First measurements of capture-ionization cross sections are also reported and show the importance of this often-neglected process. {copyright} {ital 1997} {ital The American Physical Society}

Electron correlation in the double excitation of the helium atom by fast charged particles

Double excitation cross sections to the 2 s2 p state of the helium atom by fast proton and antiproton impact are calculated by perturbation expansion through second order. The initial and the final states are described by correlated wavefunctions. The importance of different mechanisms and of the electron correlation is analyzed. Cross sections for negative projectiles are found to be higher than those for positive projectiles. The present results are compared with other theoretical calculations and with the experimental data.

Differential double-excitation cross sections in 50-150-keV proton-helium collisions.

We have measured projectile-energy-loss spectra for 50-, 100-, and 150-keV [ital p]+He collisions. From the data we obtained differential double-excitation cross sections as a function of projectile scattering angle. At 150 keV a pronounced peak structure was observed at about 0.7 mrad for double excitation to the (2[ital p][sup 2]) [sup 1][ital D] and (2[ital s]2[ital p]) [sup 1][ital P] states. Our data provide indications for the dominance of a first-order mechanism involving the electron-electron interaction in double excitation for 150 keV at small scattering angles. At lower projectile energies and larger scattering angles a second-order mechanism appears to be of the same order of magnitude as the first-order mechanism. In these regimes, interference effects between the first- and second-order mechanisms could be important.

Differential double excitation cross sections in proton-helium collisions studied by energy-loss spectroscopy.

We have measured ion energy-loss spectra for 150 keV proton-helium collisions as a function of the projectile scattering angle. From the data we obtained double excitation cross sections differential in the proton scattering angle as well as the ratios of both double excitation and single ionization to single excitation. In these ratios pronounced peak structures are observed at about 0.7 mrad. Two alternative interpretations of these peak structures are offered: They may be due to binary collisions between the projectile and the target electrons, or they could be a manifestation of an interference between different transition amplitudes leading to the same final state of the collision.

Electron-electron interactions in ion-atom collisions studied by energy-loss spectroscopy

We have performed a series of experiments on the University of Missouri-Rolla ion energy-loss spectrometer to study the role of electron-electron interactions in ion-atom collisions. In one experiment we have taken energy-loss spectra for 50 keV protons colliding with He. From the data we obtained cross sections for double excitation of the target atom. In a second experiment we measured coincidences between neutralized projectiles and target recoil ions in p + He collisions at projectile energies of 50 and 100 keV. From these data we obtained the ratio of transfer ionization to single capture as a function of the projectile scattering angle. In a preliminary experiment we also used this coincidence technique for projectiles which did not undergo charge exchange and which were energy analyzed to study single and double ionization.

Technique for the evaluation of double excitation of atoms by fast charged particles.

A technique for evaluating cross sections for two-electron excitation in collisions of atoms with fast particles of charge ${\mathit{Z}}_{\mathit{p}}$ is presented. The atomic wave function is approximated by a sum of pair products of one-electron wave functions, with the coefficients chosen by diagonalizing the fully correlated two-electron Hamiltonian. Thus spatial correlation is included in both the asymptotic and scattering regions by using these configuration-interaction (CI) wave functions for initial, intermediate, and final states. Use of CI wave function also allows the first-order contributions to be expressed in closed, analytical form. Both the energy-conserving and energy-nonconserving parts of the second-order amplitude are evaluated. The former (a correlated generalization of the independent-electron approximation) is analytical and the latter is a one-dimensional integral. In helium it is found that the double-excitation cross sections are sensitive to the sign of the projectile charge, but that the energy region where this sensitivity is of the same order as for double ionization is 0.1 to 0.5 MeV/amu, whereas the latter has peak charge sensitivity at 1.5 MeV/amu. Comparison is made with some experimental results.

Double excitation of helium by fast protons and antiprotons

The charge sensitivity of this two-electron process is investigated as a tool for understanding correlation. The helium wave function is approximated by a sum of pair-products of one-electron wave functions, with the coefficients chosen by diagonalizing the fully correlated two-electron Hamiltonian. Thus, spatial correlation is included in both the asymptotic and scattering regions by using these configuration interaction (CI) wave functions for initial, intermediate and final states. Use of CI wave functions also allows the first-order contributions to be expressed in closed, analytical form. Both the energy-conserving and energy-nonconserving parts of the second-order amplitude are found. The former (a correlated generalization of the independent electron approximation) is analytical and the latter is a one-dimensional integral. It is found that the double excitation cross sections are sensitive to the sign of the projectile charge, but that the energy region where this sensitivity is of the same order as for double ionization is 0.1 to 0.5 MeV/amu, whereas the latter has peak charge sensitivity at 1.5 MeV/amu. Comparison is made with the experimental results of Pedersen and Hvelplund [1] and Giese et al. [2].

Mechanisms of double excitation in ion-atom collisions at high velocities.

Double-excitation cross sections of helium atoms by bare heavy-ion projectiles at high energies are calculated within the one-center atomic-orbital expansion method including the electron-electron interaction between correlated helium wave functions. The resulting excitation cross sections to 2${\mathit{s}}^{2}$ $^{1}$${\mathit{S}}^{\mathit{e}}$, 2${\mathit{p}}^{2}$ $^{1}$${\mathit{D}}^{\mathit{e}}$, 2s2p $^{1}$${\mathit{P}}^{\mathit{o}}$, and 2${\mathit{p}}^{2}$ $^{1}$${\mathit{S}}^{\mathit{e}}$ states are analyzed. We conclude that double excitations at high energies by low-${\mathit{Z}}_{\mathit{p}}$ projectiles are dominated by a first-order process in which the two-electron transition proceeds through one electron-projectile interaction in conjunction with shakeup or rearrangement due to electron-electron interactions. For high-${\mathit{Z}}_{\mathit{p}}$ projectiles at a given velocity, on the other hand, the excitation is dominated by a second-order process in which the transition proceeds through two successive electron-projectile interactions.