S. Fülling

Ionization–excitation of helium to He+(2p) magnetic sublevels following electron, proton, and molecular hydrogen (H2+ and H3+) impact

Experimental magnetic sublevel scattering-angle-integrated cross sections following ionization–excitation of He(1s2)1S to He+(2p)2Po in e− + He and Hm+ + He (m = 1–3) collision systems have been determined using a combination of total cross sections and polarization fraction measurements in the extreme ultraviolet range. The derived magnetic sublevel cross sections, σ0 and σ1, for ML = 0 +/- 1 have been studied over a wide range of velocities (2–8.5 au) for electron impact on helium. These results are compared with previous experimental proton cross sections (2–6 au) as well as new measured data for hydrogen molecular impact (1.4–4.0 au) for equi-velocity. In addition, our electron and proton measurements are compared with earlier theoretical predictions, our recent second-Born calculations fully including off-shell energy terms, and present predictions from a hybrid first-order and second-order distorted-wave plus R-matrix (close-coupling) model (DWB1 + RMPS and DWB2 + RMPS). Finally, we have extended our polarization measurements for H+ + He collisions and found excellent agreement between theory and experiment at nearly all impact energies. However, the present second-Born and DWB1 + RMPS results deviate slightly from the experimental electron data while the DWB2 + RMPS calculations tend toward the experimental data of Forand et al (1985 J. Phys. B: At. Mol. Phys. 18 1409).

Spectroscopic studies of xenon EUV emission in the 40-80 nm wavelength range using an absolutely calibrated monochromator

We have measured and identified numerous Extreme UltraViolet (EUV) radiative line structures arising from xenon (Xe) ions in charge state q = 1 to 10 in the wavelength range 40-80 nm. To obtain reasonable intensities of different charged Xe ions, we have used a compact microwave plasma source which was designed and developed at the Lawrence Berkeley National Laboratory (LBNL). The EUV emission of the ECR plasma has been measured by a 1.5 m grazing incidence monochromator that was absolutely calibrated in the 10-80 nm wavelength range using well known and calibrated EUV light at the Advanced Light Source (ALS), LBNL. This calibration has enabled us to determine absolute intensities of previously measured EUV radiative lines in the wavelengths regions investigated for different ionization stages of Xe. In addition, emission spectra of xenon ions for corresponding measured lines have been calculated. The calculations have been carried out within the relativistic Hartree-Fock (HF) approximation. Results of calculations are found to be in good agreement with current and available experimental and theoretical data.

Higher-order contributions to ionization plus excitation cross sections in high energy e- + He and H+ + He scattering

The authors have determined absolute cross sections for ionization-excitation of He by energetic electron and proton projectiles. Using a semi-empirical analytic formula the authors have been able to decompose their He+ (np) cross section data in leading order terms which are proportional to Zpn for n = 2, 3 and 4. In particular the authors have found a strong Zp3 dependence which appears to represent quantum interference effects. The set of amplitude coefficients obtained here is universal for both their electron and proton cross section results from He+ (np) states with principal quantum numbers n = 2 to 5.

Excitation of the He+ lyman series by proton and Cq+ (q = 1–6) ion impact

Abstract The resonance line series Is-np of He + has been excited by collisions of C q + ( q = 1–6) and H + with He at MeV energies. The line intensity pattern is found to vary with the charge state of the projectile and reveals the steepest n -dependence of the n p level populations for the most pointlike projectiles, C 6 + and H + . Possible excitation mechanisms are mentioned.

Projectile‐charge dependence of ionization‐excitation of helium following collisions of MeV bare ions with Zp=1 to 6

State‐selective cross sections σ+*(nl) have been measured for ionization‐excitation of helium in collisions with fully stripped ions. The energies of the H+, Li3+, B5+, and C6+ were .83 MeV/u and 1.60 MeV/u. These relate to collision velocities of 5.8 and 8.0 a.u. Data for the Lyman series, He+(np→1s), have been measured by observation of the extreme ultra‐violet spectra in the 22 to 31 nm wavelength range. The 2p‐ 1s data is presented and analyzed in terms of a simple power law dependence on the projectile charge state, Zpn. For the two equivelocity series of cross sections the powers, n, derived from least squares fits to the data were 2.89±0.3 and 2.83±0.4.

Ionization-excitation magnetic sublevel cross sections for He+ (2p)2P0 states following fast electron and proton impact

The first experimental magnetic substate scattering-angle-integrated cross sections following ionization–excitation of He(1s 2 ) 1 St o He þ (2p) 2 P 0 in the e � þ He collision system have been determined using a combination of total cross sections and polarization fraction measurements in the extreme ultraviolet range. The derived magnetic sublevel cross sections, r0 and r1,for ML ¼ 0, � 1 have been studied over a wide range of electron velocities (2–8.5 a.u.). These results are compared with previous cross sections for equi-velocity proton impact. In addition,our experimental data are compared with earlier theoretical predictions,our recent second-Born calculations fully including off-energy shell terms, and new R-matrix results. We have extended our polarization measurements for H þ þ He collisions and found excellent agreement between theory and experiment at nearly all impact energies. However,the present second-Born and Rmatrix results deviate slightly from the experimental electron data. 2003 Elsevier Science B.V. All rights reserved.

Polarization Fractions and Magnetic Sublevel Populations Following Excitation and Ionization‐Excitation of Helium by Molecular Hydrogen (H2+ and H3+) Impact

We present recent measurements of the degree of linear polarization following the extreme ultraviolet (EUV) emission of helium in H2+ + He and in H3+ + He collision systems at impact velocities ranging from 1.4 to 4.5 a.u. and 1.4 to 3.5 a.u., respectively. The combination of these polarization fractions with our previous total EUV cross sections has enabled us to determine the first experimental magnetic substate scattering angle‐integrated cross sections, σ0 and σ1 for ML = 0, ±1, following excitation of He (1s2) 1S to HeI (1s2p) 1Po levels and ionization‐excitation to He+ (2p) 2Po for singly charged molecular hydrogen (H2+ and H3+) impact. In addition, a comprehensive comparison of these spectropolarimetric data is given for different collision processes together with our previous proton results at an extended velocity range (2.2 < v < 5.5 a.u.). Such an experimental database is of great importance for astrophysical and laboratory plasma diagnostics.

Theoretical analysis of ionization plus excitation of helium by fast electron and proton impact

Abstract We have applied theoretical many-body techniques to analyze our experimental ionization-excitation cross sections for fast e−+He and H++He collisions. By using the main first-order, second-order and interference cross terms we have derived an approximate value for the cross section ratio σ + ∗ (e − ) σ + ∗ (H + ) for He + (2p) which is in good agreement with experiment.

Excitation and ionization in fast electron and ion collisions with helium gas targets

Abstract Extensive research has been conducted in our laboratory on absolute cross section measurements of extreme ultraviolet (EUV) emission spectra following e − , H + , H + 2 and H + 3 high energy bombardment of He gas targets. In particular higher order contributions which vary as the cube of the projectile charge Z p have been studied for joint ionization-excitation of He. These terms of odd order in Z p may represent complex quantum interference effects. Using a semiempirical analytic formula we have been able to decompose our cross section data in leading order terms which are proportional to Z p n for n = 2–4 for electron and proton impact. This approach allows us to model the charge-state, velocity- and Rydberg state dependence of ionization-excitation cross sections at velocities υ / υ 0 = 3.5–8 for He + ( n p) states.