First and second Born (e, 2e) calculations are presented for excitation-ionization of ground-state helium to . Results for ionization to the ground-state ion (1s) are also given. The physical content of the approximations is discussed, in particular, the two-step mechanism which appears in the second-order term for excitation-ionization. The second Born term is calculated in the closure approximation using a new numerical method based on prolate spheroidal coordinates. Comparison is made with absolute experimental data from Paris and Rome in very asymmetric coplanar geometry - scattered electron energies of 5500, 1500 and 570 eV and ejected electron energies of 5, 10, 20, 40 and 75 eV. For excitation-ionization the second Born approximation generally gives improved agreement with the experimental data in the recoil region and second-order effects are found to be still significant at 5500 eV. The importance of the second-order term decreases with increasing ejected energy for the cases studied here.

Second-order effects in (e, 2e) excitation-ionization of helium to ( n = 2)

We present experimental and theoretical results for the electron-impact-induced ionization of ground-state helium atoms. Using a high-sensitivity toroidal electron spectrometer, we measured cross-section ratios for transitions leading to the first three excited states of the residual helium ion relative to the transition leaving the ion in the ground state. Measurements were performed for both symmetric- and asymmetric-energy-sharing kinematics. By presenting results as a ratio, a direct comparison can be made between theoretical and experimental predictions without recourse to normalization. The experimental data are compared to theoretical predictions employing various first-order models and a second-order hybrid distorted-wave $+$ convergent $R$ matrix with pseudostates (close-coupling) approach. All the first-order models fail in predicting even the approximate size of the cross-section ratios. The second-order calculations are found to describe the experimental data for asymmetric-energy-sharing with reasonable fidelity, although significant disparities are evident for the symmetric-energy-sharing cases. These comparisons demonstrate the need for further theoretical developments, in which all four charged particles are treated on an equal footing.

/pdf/ionization-and-ionization-excitation-of-helium-to-the-n-1-4-29d7pvarqs.pdf

Ionization and ionization–excitation of helium to the n = 1 – 4 states of He + by electron impact

Cross sections for the ionization-excitation of helium following electron and H+, H2+ and H3+ ion impact have been measured for He II (np) 2P0 states with n=2-5. These measurements were performed for the velocity range from 3.8-8.5 au for electrons, 1.4-7.5 au for protons and 1.4-4.0 au for H2+ and H3+ ions. The cross sections have been obtained by measuring the extreme ultraviolet (EUV) emission from the decay of the excited target atoms at 90 degrees to the projectile beam. We have compared our results to the current limited literature for He II (2p) 2P0 cross sections with good agreement observed. We have also examined the cross section ratio of ionization-excitation to excitation, sigma +*((np)2P0)/ sigma *((1snp)1P0), for electron and proton impact in comparison with the ratio of double to single ionization, sigma ++/ sigma +, as a limiting value for large n. Finally, cross section results from Hn+ impact, where n=1-3, are discussed in terms of a semiempirical scaling relation for molecular hydrogen projectiles.

https://iopscience.iop.org/article/10.1088/0953-4075/28/13/017/pdf

Cross sections for the ionization-excitation of helium by fast electrons and H+, H2+ and H3+ ions: (np) 2P0 levels, n=2-5

The cross sections for simultaneous excitation and ionisation in helium following impact of fast electrons, protons and Cq+ (q=3-6) ions have been investigated by measuring the vacuum-ultraviolet radiation from transitions in HE II (np-1s) in the wavelength region 220-315 AA. The cross sections obtained with 500-2000 eV electrons are found to be larger by a factor of 1.4-2.3 than those obtained with protons of the same velocity (0.92-3.76 MeV). The difference is discussed in terms of an interference between two excitation-ionisation mechanisms.

Simultaneous excitation and ionisation of helium by fast projectiles

We investigate the acceleration of a proton beam driven by intense tera-hertz (THz) laser field from a near critical density hydrogen plasma. Two-dimension-in-space and three-dimension-in-velocity particle-in-cell simulation results show that a relatively long wavelength and an intense THz laser can be employed for proton acceleration to high energies from near critical density plasmas. We adopt here the electromagnetic field in a long wavelength (0.33 THz) regime in contrast to the optical and/or near infrared wavelength regime, which offers distinct advantages due to their long wavelength ( λ=350 μm), such as the λ2 scaling of the electron ponderomotive energy. Simulation study delineates the evolution of THz laser field in a near critical plasma reflecting the enhancement in the electric field of laser, which can be of high relevance for staged or post ion acceleration.

/pdf/intense-tera-hertz-laser-driven-proton-acceleration-in-450pggikps.pdf

Intense tera-hertz laser driven proton acceleration in plasmas

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).

/pdf/ionization-excitation-of-helium-to-he-2p-magnetic-sublevels-2z6mm2il3z.pdf

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

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.

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

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.

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

We have calibrated a 1.5 m grazing incidence monochromator with the light of known wavelength at the advanced light source (ALS) of the Lawrence Berkeley National Laboratory (LBNL) using beamline 6.3.2. Several measurements in the 13–30 nm wavelength range with well resolved monochromatic light were examined at ALS. Such precise measurements have enabled us to find the dependence of the efficiency as function of wavelengths for the EUV monochromator in the wavelength range of 10–80 nm using previously and current determined sensitivities. We have used this calibrated monochromator for EUV diagnostics of a plasma source developed at LBNL. In particular, we have normalized previously measured EUV radiative lines arising from xenon (Xe) ions originating from charge state q = 1–10 in the wavelength range of 10–16 nm with special attention for the wavelength of 13.4 nm which might have lithographic applications.

Absolute calibration of a 1.5 m grazing incidence monochromator for extreme ultraviolet (EUV) diagnostics of a plasma source

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.

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

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