James H. McGuire

The ratio of cross sections for double to single ionization of helium by high energy photons and charged particles

Data and analysis for the ratio of double to single ionization in helium is reviewed for impact by photons and charged particles. In the case of photoionization there are two processes, namely, (i) photoionization where the photon is annihilated, and (ii) Compton scattering where the photon is inelastically scattered. In the case of charged particle scattering the ratio of total cross sections tends toward an asymptotic high energy value of 0.26% which is well below the value observed for photons of 1.7% at photon energies between 2 and 12 keV. Theoretical relations between various ratios have been predicted and to some extent confirmed by observations.

Spectropolarimetric measurements of the extreme-ultraviolet emission from helium following e-, H + , H2 + , andH 3 + charged particle impact

We present here comprehensive experimental data corresponding to helium extreme ultraviolet emission (EUV) for the 300-600A wavelength range in e- + He, H+ + He, and Hnn+ + He (n=2-3) collisions with impact velocities ranging from 1.4 to 6.9 a.u. The degree of linear polarization associated with HeI (1snp) 1Po -> (1s2) 1S decays for n-2-5 ((lambda) =510 to 584A) and HeII (2p) 2Po -> (1s) 2S transitions ((lambda) =304A), have been measured at intermediate and higher energies. These measurements have been conducted using a compact lightweight molybdenum/silicon multilayer mirror (MLM) spetropolarimeter. These experimental results are compared with our most advanced theoretical calculations using the close coupling method and several many body perturbation approaches. In this study, the degree of linear polarization will be discussed as a function of the projectile charge to elucidate few body excitation and ionization-excitation processes. Specifically, more sophisticated calculations including second Born approximation with full off-energy terms for the HeII (2p) 2Po sublevels have been used. The excellent agreement with our HeII Lyman-(alpha) data clearly demonstrates that off-energy terms in two-electron atomic interaction have very prominent effects. Our comprehensive experimental and theoretical database is also of great importance for the understanding of solar physics phenomena. Such results can be used as a new diagnostic tool for clarifying the role of electron and proton beams in solar flares.

Independent center, independent electron approximation for dynamics of molecules and clusters

A formalism is developed for evaluating probabilities and cross sections for multiple‐electron transitions in scattering of molecules and clusters by charged collision partners. First, the molecule is divided into subclusters each made up of identical centers (atoms). Within each subcluster coherent scattering from identical centers may lead to observable phase terms and a geometrical structure factor. Then, using a mean field approximation to describe the interactions between centers we obtain AI∼∑k∏keiδkIAIk. Second, the independent electron approximation for each center may be obtained by neglecting the correlation between electrons in each center. The probability amplitude for each center is then a product of single electron transition probability amplitudes, aIki, i.e. AIk≊∏iaiki. Finally, the independent subcluster approximation is introduced by neglecting the interactions between different subclusters in the molecule or cluster. The total probability amplitude then reduces to a simple product of am...

Polarization measurements of the extreme ultraviolet (EUV) emission following excitation and ionization-excitation of helium by fast electron, proton, and molecular hydrogen (H2+ and H3+) impact

We report intensive experimental and theoretical data corresponding to helium extreme ultraviolet (EUV) emission in the 30-60 nm wavelengths range for e- + He, H+ + He, and Hn+ + He (n=2-3) collisions with impact velocities from 1.4 to 8.5 a.u. Specifically, the degree of linear polarization associated with HeI (1snp) 1Po levels, n=2-5, (λ=51.7 to 58.4 nm for n=2-5 to n=1 transitions) and HeII (2p) 2Po states (λ=30.4 nm) have been measured at intermediate- to high-energy range using a compact lightweight molybdenum/silicon multilayer mirror polarimeter (MLM). The combination of these polarization measurements with our previous total EUV cross sections has enabled us to determine experimental magnetic substate scattering angle-integrated cross sections σ0 and σ1for ML = 0, ±1, and 1 following excitation of He (1s2) 1S to HeI (1s2p) 1Po levels and ionization-excitation to He+ (2p) 2Po for electron and proton impact at a wide range velocities. The degree of linear polarization and obtained magnetic sublevel cross sections are compared to earlier theoretical calculations together with our recent 23-state R-Matrix, 1st and 2nd Born calculations. Such a detailed comparison may shed more light on the collision processes involved in positively and negatively single charge particle impact in helium. In particular it shows that electron-electron interactions play an important role in the ionization-excitation mechanism. In addition, this comprehensive database possibly will provide a deeper understanding of electron and proton induced jets in solar flare astrophysical investigations.

Interpretation of scattering by charged particles and photons

Abstract In principle, Coulomb scattering by charged particles may be described by contributions from n -photon terms, corresponding to the n th order terms in the Born expansion for charged particles. Consequently, one may relate scattering by charged particles to scattering by photons. Thus Raman scattering, second-order Compton scattering, the Auger effect, and two electron transitions in high velocity collisions with charged particles are examples of processes that in principle are inter-related. The probability amplitude second order in the interaction with the incoming charged particle or photon may generally be separated into a real and an imaginary part. The real part contains an energy-conserving propagator and may be regarded classically, while the imaginary part does not conserve energy (in accord with the Uncertainty Principle) and disappears in the limit as h → 0 . These two terms are related by a dispersion relation. The energy non-conserving part is the same as the time ordering effect which is a measure of correlation in time. We analyze observations of the ratio of double to single ionization of helium by impact of photons and charged particles.