keV Proton Impact Research Articles

Three-Body Dynamics in Single Ionization of Atomic Hydrogen by 75 keV Proton Impact

We have measured and calculated doubly differential cross sections for ionization of atomic hydrogen using 75-keV proton impact for fixed projectile energy losses as a function of scattering angle.

Scattering Angle Dependence of Double Differential Cross Sections for Dissociative Ionization of H2 by Proton Impact

We have measured double differential cross sections for dissociative ionization of H2 by 75 keV proton impact for fixed energy losses as a function scattering angle. The data exhibit an oscillating pattern with a frequency about twice as large as found earlier for non-dissociative ionization.

Differential cross sections for single ionization of H2by 75-keV proton impact

We have calculated triply differential cross sections (TDCS) and doubly differential cross sections (DDCS) for single ionization of H{sub 2} by 75-keV proton impact using the molecular three-body distorted-wave-eikonal initial-state (M3DW-EIS) approach. Previously published measured DDCS (differential in the projectile scattering angle and integrated over the ejected electron angles) found pronounced structures at relatively large angles that were interpreted as an interference resulting from the two-centered potential of the molecule. Theory treating H{sub 2} as atomic H multiplied by a molecular interference factor only predicts the observed structure when assumptions are made about the molecular orientation. Here we apply the M3DW-EIS method, which does not rely on such an ad hoc approach, but rather treats the interference from first principles.

Classical trajectory Monte Carlo model calculations for ionization of atomic hydrogen by 75-keV proton impact

Cross sections differential with respect to the energy loss and scattering angle of the projectile have been calculated for ionization of atomic hydrogen by 75-keV proton impact using the classical trajectory Monte Carlo method. The results are compared with the experimental data measured by Laforge et al. [Phys. Rev. Lett. 103, 053201 (2009)] and Schulz et al. [Phys. Rev. A 81, 052705 (2010)], as well as with the predictions of several quantum-mechanical theoretical models. The analysis of the deviations between the classical and the quantum-mechanical results shows that the three-body fragmentation dynamics cannot be understood purely classically; for the description of the process the quantum-mechanical treatment of the interplay between the electron-projectile and the projectile--target-nucleus interaction is unavoidable.

Doubly differential spectra of scattered protons in ionization of atomic hydrogen

We have measured and calculated doubly differential cross sections for ionization of atomic hydrogen using $75$-keV proton impact for fixed projectile energy losses as a function of scattering angle. This collision system represents a pure three-body system and thus offers an accurate test of the theoretical description of the few-body dynamics without any complications presented by electron correlation in many-electron targets. Comparison between experiment and several theoretical models reveals that the projectile-target nucleus interaction is best described by the operator of a second-order term of the transition amplitude. Higher-order contributions in the projectile-electron interaction, on the other hand, are more appropriately accounted for in the final-state wave function.

Mass spectrometry (fragmentation ratios) of DNA base molecules following 80 keV proton impact with separation of direct ionization and electron capture processes

The first fragmentation ratios are presented for the ionization of gas-phase DNA bases by 80 keV (1.8 v 0 in Bohr velocity units) proton impact. Event-by-event determination of the projectile charge state post-collision enables branching ratios to be determined for electron capture (EC) by the projectile and for direct ionization (DI) of the target molecule (without projectile neutralization). Results are compared with similar experiments on uracil [11] and water [14]. In all cases, whereas both processes (EC and DI) produce the same fragment ion groups, greater fragmentation ratios are observed for EC than for DI. Moreover the fragmentation ratio is greater for thymine than for adenine, cytosine, and uracil.

Double differential projectile angular distributions for single ionization of atomic hydrogen by 75 keV proton impact

Doubly differential cross sections (DDCSp) for single ionization of atomic hydrogen by 75-keV proton impact as a function of the projectile scattering angle and energy loss have been measured.

L x-ray production cross sections from 50-300 keV proton impact on selected 4d transition elements

L x-rays were measured from the impact on thick targets of protons in the 50-300 keV energy range. The derived L sub-shell cross sections were compared with the ECPSSR theory and with measurements by others. It was found that ECPSSR over-predicts the measurements as the impact energy decreases.

Three-Body Dynamics in Single Ionization of Atomic Hydrogen by 75 keV Proton Impact

Doubly differential cross sections for single ionization of atomic hydrogen by 75 keV proton impact have been measured and calculated as a function of the projectile scattering angle and energy loss. This pure three-body collision system represents a fundamental test case for the study of the reaction dynamics in few-body systems. A comparison between theory and experiment reveals that three-body dynamics is important at all scattering angles and that an accurate description of the role of the projectile-target-nucleus interaction remains a major challenge to theory.

Orientation and impact-parameter dependence of dissociative ionization of H2 by slow ion impact

We have used the cold target recoil ion momentum spectroscopy imaging technique to investigate dissociative ionization of H2 by 25 keV proton impact. A kinematically complete picture of the dissociative ionization dynamics for slow proton collision has been obtained. The results show a strong impact-parameter dependence of the fragmentation process of H2. This clearly emerged in the energy distributions of the H+ ions generated for different impact parameters. At large impact parameters the H+ ions equally share the energy liberated in the collision whereas at small impact parameters, the energy sharing is quite asymmetric. We also observed a strong dependence of the electron emission on molecular alignment. The momentum distribution of the emitted electron generated for a fixed-in-space H2 molecule displays that the electrons are more likely to be emitted perpendicular to the molecular axis.

Relative intensities of L X-rays excited by 75–300 keV proton impact on elements with Z = 39–50

L X-rays excited by 75–300keV protons impacting thick targets of Y, Zr, Nb, Mo, Rh, Pd, Ag and Sn were measured using an ultra-thin window Si(Li) detector. The intensities of the Lℓ and Lβ2,15 X-ray transitions relative to the Lα(L3M4,5) X-rays were derived from the data and compared with theoretical predictions. Discrepancies in the results are attributed to multiple vacancy production due to the collisions. Vacancy fractions for the M and N shells of the target atoms are deduced from the results.

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.

Ejected-electron spectrum in low-energy proton-hydrogen collisions

The behavior of the ejected electron spectra resulting from low-energy ion-atom collisions relevant to momentum imaging experiments is explored and the origin of the oscillatory structures these spectra display as a function of collision energy is discussed. This is aided by consideration of the time-dependent, electronic Sch\odinger equation that is solved at a fixed impact parameter for 1--25 keV proton impact of atomic hydrogen utilizing the lattice, Fourier collocation technique and split-operator time propagation. At a large internuclear separation after the collision the bound states of the target and projectile are projected out for $nl4$ and the resulting continuum wave function is examined. Techniques, such as multigridding, are investigated to extend the propagation of the wave function to significantly larger final distances.

Total L-shell X-ray production cross sections by 400–700 keV proton impact for elements with 34 ≤ Z ≤ 53

Total L-shell X-ray production cross sections induced by protons with energies between 400 and 700 keV were measured for elements with atomic number Z between 34 and 53. The ECPSSR theory describes appropriately the results. This model modifies the plane wave born approximation by considering projectile energy loss (E), Coulomb deflection of the incoming ion (C), polarization and change in electron binding energies through a perturbed stationary states method (PSS) and relativistic values of target electron mass (R). A comparison is given with previously published data for proton energies below 1 MeV and 26 ≤ Z ≤ 53, based on a scaling obtained from a reduced velocity parameter ξ L R . The results show that the scaling for these atomic numbers and energy ranges is adequate and a semi-empirical expression to calculate those cross sections is proposed.

Quantum-mechanical description of ionization, capture, and excitation in proton collisions with atomic oxygen

The basis generator method is used to propagate effective single-particle equations for ion-atom collisions to provide a mean-field description for the proton-oxygen collision system. The pure capture and ionization as well as transfer ionization cross sections are calculated using two alternative prescriptions to extract the physical information from the propagated orbitals. The sensitivity of the resonant single capture cross section to the effective single-particle potential for atomic oxygen is tested. Cross sections are calculated for 1--2000 keV proton impact and good overall agreement is found with available experiments, while some of the results are at variance with a recent model calculation. The single-particle excitation amplitudes are combined in the LS coupling scheme to obtain excitation cross sections for particular atomic oxygen configurations. The latter are compared with recent measurements for proton and electron impact. Reasonable agreement is found for many one-electron excitations and some two-electron processes.

Excitation of autoionizing states of helium by 100 keV proton impact: II. Excitation cross sections and mechanisms of excitation

Mechanisms of two-electron excitation of the (2s2)1S, (2p2)1D and (2s2p)1P autoionizing states of helium are studied both experimentally and theoretically. It is shown that an explicit introduction of a kinematic factor, with a process-specific phase leads to a productive parametrization of experimental cross sections of ionization, allowing one to extract cross sections of excitation of autoionizing states. Using a new fitting procedure together with the proposed parametrization made it possible to obtain the excitation cross sections and magnetic sublevel population from electron spectra as well as, for the first time, to resolve the contribution of resonance and interference components to resonance profiles. Interference with direct ionization is shown to contribute significantly into resonance formation even for backward ejection angles. We demonstrate theoretically that the excitation cross sections thus extracted from experimental electron spectra hold information about the interaction of autoionizing states with an adjacent continuum.

Excitation of He atoms by 50-500 keV proton impact: from Paul-trap promotion to the high-energy limit

Excitation of helium atoms by 50-500 keV proton impact to singly excited states was investigated experimentally. By measuring the intensities of He I spectral lines as functions of an axial electric field applied to the collision volume, we analysed the electric charge distributions of collisionally excited states with principal quantum numbers n = 4, 5 and 7. The experimental results reflect the transition from intermediate- to high-energy processes. States with large electric dipole moments were found in the lower part of the investigated energy range indicating a quasi-molecular evolution of the collision system. States with dominant components and accordingly very small electric dipole moments were found for 400 and 500 keV proton impact, signifying that the high-energy limit is reached.

Charge distribution of He I states excited by 10-50 keV proton impact: a study of electron promotion in an atomic Paul trap

Collisional excitation of helium atoms by protons of intermediate energies is investigated experimentally. By measuring the intensities of various spectral lines as functions of an electric field applied parallel or antiparallel to the proton beam, we found that the excited He atoms are left after the collision in transient states with large electric dipole moments directed upstream. The experimental results are explained by assuming that during the collision one electron is promoted on the saddle of the two-centre Coulomb potential of the projectile and target. At intermediate energies saddle dynamics is particularly effective due to the stabilization of the electron's motion on the saddle by the Paul-trap mechanism.

Excitation of autoionizing states of helium by 100 keV proton impact: theory and experiment

A joint theoretical and experimental study of the excitation of the autoionizing and states of helium by 100 keV proton impact is presented for the first time. The role of the three-body Coulomb interaction in the final state between the ejected electron, the scattered proton and the recoil helium ion is emphasized. Calculations have been carried out with inclusion of the three-body Coulomb interaction and within an expansion of a two-electron excitation amplitude in powers of projectile - target interaction up to the second order. A new parametrization is proposed to describe resonance profiles distorted by the Coulomb interaction in the final state (CIFS). New high-resolution (up to 68 meV) measurements of electron emission spectra made it possible to resolve the near-lying and resonances and reveal an evident distortion of the resonance profiles by CIFS for forward electron ejection angles below . Processing of the experimental spectra has been done both with the new parametrization, with allowance for CIFS, and with the Shore formula. Considering the complexity of the problem, reasonable agreement is achieved between experiment and theory.

Comprehensive convergence study of TCAO close-coupling method for the excitation and ionization in keV collisions

Convergence of the two-centre atomic orbital (TCAO) close-coupling calculations for the excitation and ionization of atomic hydrogen by keV proton impact is carefully examined using various large basis sets on each centre. It is shown that reliable cross sections for the weak channels can only be obtained with a large number of basis states on the target and a limited number of bound states on the projectile. Cross sections for excitation to , , and states and for ionization are presented and compared to available experimental data and to other theoretical calculations.