J. Hanni

Spatial and temporal correlation in dynamic, multi-electron quantum systems

Cross sections for ionization with excitation and for double excitation in helium are evaluated in a full second Born calculation. These full second Born calculations are compared to calculations in the independent electron approximation, where spatial correlation between the electrons is removed. Comparison is also made to calculations in the independent time approximation, where time correlation between the electrons is removed. The two-electron transitions considered here are caused by interactions with incident protons and electrons with velocities ranging between 2 and 10 au. Good agreement is found between our full calculations and experiment, except for the lowest velocities, where higher Born terms are expected to be significant. Spatial electron correlation, arising from internal electron-electron interactions, and time correlation, arising from time ordering of the external interactions, can both give rise to observable effects. Our method may be used for photon impact.

Time ordering in multi-electron dynamics

Time ordering of interactions in dynamic quantum multi-electron systems provides a constraint that interconnects the time evolution of different electrons. In energy space, time ordering appears as the principal value contribution from the Green function, which corresponds to the asymptotic condition that specifies whether the system has outgoing (or possibly incoming) scattered waves. We report evidence of effects of time correlation found by comparing calculations to recent spectropolarimetric data.

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.

Investigation of excitation and ionization excitation of He following electron and proton impact using EUV polarimetry

A detailed investigation of excitation of He (1s2) 1S to He (1snp) 1P degree(s) (n equals 2 - 5) states and ionization-excitation of He (1s2) 1S to He (2p) 2P degree(s) and He (3p) 2P degree(s) states in e- + He and H+ + He collision systems is presented for a wide range of projectile velocities (2.2 a.u. < v < 6.9 a.u.). Specifically new experimental data are presented on measurements of the degree of linear polarization for excitation and excitation-ionization of He following proton impact in the extreme ultraviolet (EUV) wavelengths. These measurements have been performed using a characterized molybdenum/silicon multilayer mirror polarimeter (MLM) whose polarization characteristics have been optimized for EUV emission of He and He+. Furthermore, the proton experimental results are compared with theoretical polarization data using the first Born approximation and recent atomic orbital close coupling (AOCC) calculations for the excitation process. A comprehensive comparison of experimental data for negatively and positively charged projectiles at equal impact velocities is also given in order to elucidate differences in the collision mechanisms of two electron targets. It is important to note that these results are relevant for astrophysical diagnostics such as solar flares.

Scattering angle-integrated (total) and magnetic sublevel cross-sections and degree of linear polarization for electron and proton induced excitation [HeI (n=2–5)] of helium

Experimental scattering-angle-integrated (total) cross-sections �, (scattering) angle-integrated magnetic sublevel cross-sections �ML , and degree of linear polarization data have been measured in the extreme ultraviolet (EUV) wavelength region following decay of HeI ð1snpÞ 1 P 0 ðn ¼ 2–5Þ states induced by electron and proton impact on a neutral helium target. These measurements are compared with a first Born approach as well as more sophisticated theoretical calculations. Specifically, theoretical values for electron impact include convergent close-coupling (CCC) and R-matrix with pseudo states (RMPS) methods in addition to first Born (Born 1) approximation while proton induced excitation cross-sections are compared with atomic-orbital close-coupling (AOCC) and first Born predictions.

EUV photon emission spectroscopy for diagnostics of single and multiple-electron capture processes in 80 keV Ar8++N2 collisions

We present the first EUV emission spectra following single and multiple-electron capture for the 80 keV Ar8++N2 multi charged ion-molecular collision system. Our analysis provides evidence of single and double capture processes leading to radiative deexcitation of Ar7+*(nl) n=3–6 and Ar6+*(3lnl′) n=3–5 projectile states. Furthermore we have identified numerous Nq+ (q=2,3) ionic target lines in the EUV spectral range (10–18 nm), owing to multiple ionization of the N2 molecule and consecutive dissociation plus excitation of the fragments. The subsequent photon emission arises from high lying states of the target such as N2+*(1s22s2lnl′) 2,4L,n=2–10 and N3+*(1s22lnl′) 1,3L,n=2–5 with excitation energies up to 52 eV and 78 eV, respectively. The N2+ and N3+ excited dissociated fragments are due to multiply electron capture processes involving up to six electrons.

Non-statistical magnetic substate populations following excitation of helium by electron and proton impact

The first experimental magnetic substrate scattering angle-integrated cross sections following excitation of He (1s2)1S to He (1snp)1Po (n=2–5) in e−+He and H++He collision systems have been determined using our differential cross sections and polarization fraction data in the extreme ultraviolet (EUV) range. The derived magnetic sublevel integrated cross sections, σ0 and σ1, for ML=0,±1 have been studied over a wide range of projectile velocities: 1.4 to 8.5 a.u. and 1.4 to 7.5 a.u. for electron and proton impact on helium, respectively. In addition, the electron and proton collision data are compared with theoretical predictions using our improved first Born approximation (IFB), convergent close coupling (CCC) calculations for electron impact, and our recent atomic orbital close coupling (AOCC) calculations for protons. We have found that the electron results match very well the CCC predictions at all energies. However, the experimental proton data slightly deviate from the AOCC approach at intermedia...

Auger decay of triply excited states of Li and Be+ projectiles following ion-molecule interactions

In this study, we present high resolution electron spectra and new identifications of numerous triply excited states of the Li-isoelectronic sequence produced by intermediate velocity Li+ and Be+ projectiles interacting with CH4 molecules under single collision conditions. In particular, we show here as typical examples high resolution lithium and beryllium projectile Auger electron spectra following double K-shell excitation in 200 keV Li++CH4 and 300 keV Be++CH4 collisions. For the beryllium case, the impact velocity is 1.15 a.u. Many new lines appearing in these spectra have been assigned to triply excited Li and Be+ and doubly excited Li+ and Be2+ states. Only recently due to the pioneering theoretical predictions by Chung and coworkers (1), such collisional line structures can now be unambiguously identified. We have not only identified the lowest lying, most prominent 2s22p, 2s2p2 and 2p3 triply excited states (2), but also new excitation and deexcitation channels of higher lying 2l2l′nl′ hollow ato...

Polarization study of the extreme ultraviolet (EUV) emission from helium following electron and proton impact

A detailed investigation of excitation of He (1s2)1S to HeI (1 snp)1Po (n=2–5) states and ionization-excitation of He (1s2)1S to HeII (2p)2Po and HeII (3p)2Po states following electron and proton impact on He is presented for a wide range of projectile velocities (2.2 a.u.<v<6.9 a.u.). Specifically, new experimental data are presented on measurements of the degree of linear polarization for excitation and ionization-excitation of He following proton impact in the extreme ultraviolet (EUV) wavelength region. Furthermore, the proton experimental results are compared with theoretical polarization data using the first Born approximation and our recent atomic orbital close coupling (AOCC) calculations for the excitation process. A comprehensive comparison of experimental data for negatively and positively charged projectiles at equal impact velocities is given in order to elucidate differences in the collision mechanisms of two electron targets.

Magnetic sublevel population in 1s-2p excitation of helium by fast electrons and protons

We report experimental and theoretical results for the magnetic sublevel population of the helium atom in collisions with fast (vi = 3-9 au) electrons and protons. Cross sections for excitation of magnetic sublevels with M = 0 and ±1 have been obtained using polarization measurements of emitted radiation in combination with differential cross sections. Calculations have been carried out using the expansion of the transition amplitude in the Born series over the projectile-target interaction through the second order. Results of calculations are in agreement with experimental data. We find that the particle-antiparticle Z± difference exceeds the statistical error of measurement up to collision velocities vi≈6 au for excitation of sublevels with M = 0.