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Electron removal (target ionization and/or projectile electron loss) in neutral–atom collisions is theoretically studied for the impact of H 0 , He 0 and He + beams on noble gases (He, Ne and Ar). These reactions are investigated theoretically within the Continuum Distorted Wave-Eikonal Initial State model. New features have been included in the theoretical model: (i) a scaled projectile charge depending on its velocity and charge, (ii) a dynamic projectile-effective-charge depending on the momentum transfer, and (iii) a dynamic target-effective-charge depending on the kinematics of the emitted electron. The energy and angular spectra of emitted electrons from the target and from the projectile are calculated and compared with the available experimental data. Also, the influence of each one of the corrections on the resulting spectra will be studied. [ABSTRACT FROM AUTHOR]

We measure double differential cross sections (DDCS) of electrons emitted from CH4 molecules in collisions with 250 keV protons. The projectile ions are obtained from a 400 kV electron cyclotron resonance-based ion accelerator (ECRIA). We study the energy and angular distributions of the electron DDCS. The observed double and single differential and the total cross section are compared with the state-of-the-art continuum distorted wave eikonal initial state (CDW-EIS) model predictions. Two different approaches are used considering the different target descriptions: complete neglect of differential overlap (CNDO) and molecular orbital (MO) approximations. The MO model uses two different scaling parameters (d = 0.7 and 1.0). In the energy distribution of the DDCS, the carbon KLL Auger line is also observed at 240 eV. The single differential cross section (SDCS) and total cross section (TCS) are derived. Both the MO-based CDW-EIS models are in good agreement with the experimental results; however, the CNDO approach overestimates the data. [ABSTRACT FROM AUTHOR]

Electron removal (target ionization and/or projectile electron loss) in neutral–atom collisions is theoretically studied for the impact of H0, He0 and He+ beams on noble gases (He, Ne and Ar). These reactions are investigated theoretically within the Continuum Distorted Wave-Eikonal Initial State model. New features have been included in the theoretical model: (i) a scaled projectile charge depending on its velocity and charge, (ii) a dynamic projectile-effective-charge depending on the momentum transfer, and (iii) a dynamic target-effective-charge depending on the kinematics of the emitted electron. The energy and angular spectra of emitted electrons from the target and from the projectile are calculated and compared with the available experimental data. Also, the influence of each one of the corrections on the resulting spectra will be studied.

We measure double differential cross sections (DDCS) of electrons emitted from CH4 molecules in collisions with 250 keV protons. The projectile ions are obtained from a 400 kV electron cyclotron resonance-based ion accelerator (ECRIA). We study the energy and angular distributions of the electron DDCS. The observed double and single differential and the total cross section are compared with the state-of-the-art continuum distorted wave eikonal initial state (CDW-EIS) model predictions. Two different approaches are used considering the different target descriptions: complete neglect of differential overlap (CNDO) and molecular orbital (MO) approximations. The MO model uses two different scaling parameters (d = 0.7 and 1.0). In the energy distribution of the DDCS, the carbon KLL Auger line is also observed at 240 eV. The single differential cross section (SDCS) and total cross section (TCS) are derived. Both the MO-based CDW-EIS models are in good agreement with the experimental results; however, the CNDO approach overestimates the data.

Absolute double differential cross sections (DDCS) of electrons emitted from uracil and 5-bromouracil (BrU) in collisions with protons of energy 200 keV have been measured for various forward and backward emission angles over wide range of electron energies. The measured DDCS are compared with the continuum distorted wave-eikonal initial state (CDW-EIS) calculations. The optimized structure of the BrU was estimated along with the population analysis of all the occupied orbitals using a self-consistent field density. A comparison between the measured DDCS data for the two molecules show that the cross section of low energy electrons emitted from BrU is substantially larger than that for uracil. The BrU-to-uracil DDCS ratios obtained from the present measurements indicate an enhancement of the electron emission by a factor which is as large as 2.0 to 2.5. These electrons being the major agent for damaging the DNA/RNA of the malignant tissues, the present results are expected to provide an important input for the radiosensitization effect in hadron therapy. It is noteworthy to mention that the CDW-EIS calculations for Coulomb ionization cannot predict such enhancement. A large angular asymmetry is observed for uracil with a broad structure, which is absent in case of BrU.

In this contribution we present ab initio results for ionization total cross sections, probabilities at zero impact parameter, and impact parameter moments of order +1 and −1 of Ne, Ar, Kr, and Xe by proton impact in an extended energy range from 100 keV up to 10 MeV. The calculations were performed by using the continuum distorted wave eikonal initial state approximation (CDW-EIS) for energies up to 1 MeV, and using the first Born approximation for larger energies. The convergence of the CDW-EIS to the first Born above 1 MeV is clear in the present results. Our inner-shell ionization cross sections are compared with the available experimental data and with the ECPSSR results. We also include in this contribution the values of the ionization probabilities at the origin, and the impact parameter dependence. These values have been employed in multiple ionization calculations showing very good description of the experimental data. Tables of the ionization probabilities are presented, disaggregated for the different initial bound states, considering all the shells for Ne and Ar, the M-N shells of Kr and the N-O shells of Xe. [ABSTRACT FROM AUTHOR]

In this contribution we present ab initio results for ionization total cross sections, probabilities at zero impact parameter, and impact parameter moments of order +1 and −1 of Ne, Ar, Kr, and Xe by proton impact in an extended energy range from 100 keV up to 10 MeV. The calculations were performed by using the continuum distorted wave eikonal initial state approximation (CDW-EIS) for energies up to 1 MeV, and using the first Born approximation for larger energies. The convergence of the CDW-EIS to the first Born above 1 MeV is clear in the present results. Our inner-shell ionization cross sections are compared with the available experimental data and with the ECPSSR results. We also include in this contribution the values of the ionization probabilities at the origin, and the impact parameter dependence. These values have been employed in multiple ionization calculations showing very good description of the experimental data. Tables of the ionization probabilities are presented, disaggregated for the different initial bound states, considering all the shells for Ne and Ar, the M-N shells of Kr and the N-O shells of Xe. [ABSTRACT FROM AUTHOR]

Abstract: Irradiation of water and other molecules of biological interest by heavy ion beams is studied. Distorted wave models are employed to investigate the corresponding inelastic collisions. Cross sections for electronic capture, ionization, and excitation processes are determined as well as equilibrium charge-states and electronic stopping power. The influence of multiple ionization in liquid water radiolysis is analyzed. [Copyright &y& Elsevier]

Abstract: Single ionization of liquid water by the impact of fast, but nonrelativistic heavy charged particles is reviewed. Special attention is focused on protons, alpha particles, and carbon ions. This phenomenon has been extensively studied by using theoretical methods during the last decades. Quantum-mechanical as well as semiclassical approaches have been developed. Nevertheless, experimental studies in this field are very scarce. Based upon both theoretical and experimental results, semiempirical formalisms have been reported. At the beginning, the first Born (B1) approximation emerged with some success in reproducing ionization cross sections corresponding to impact energies above a few hundreds of keV/u. The introduction of the distorted-wave formalism brought a remarkable improvement with respect to B1, mainly because of reproduction of the well-known two-center effects. The B1 approximation is treated here in order to be used as a reference model for all the subsequent comparisons. Also presented are the distorted-wave formalism and its variants. On the other hand, available experimental works for measuring ionization cross sections corresponding to water vapor targeted by hydrogen, helium, and carbon ions are summarized. In addition, the most relevant semiempirical approaches intended to calculate water ionization cross sections are addressed. Some of the experimental single ionization cross sections mentioned above are compared to those determined by the distorted-wave and semiempirical formalisms for liquid water. Finally, perspectives on the studies of the ionization problem are briefly commented. [Copyright &y& Elsevier]

Abstract: In this chapter we analyze the possibilities and ranges of validity of the dielectric formalism to deal with correlated bound electrons in matter by using the shellwise local plasma approximation. This model describes the response of the electrons of the same binding energy as a whole (collectively), screening the interaction with the impinging ion. It considers separately each sub-shell of target electrons, with the corresponding dielectric response. The density of electrons and the energy gap are included explicitly by employing the Levine and Louie dielectric function. The goal of this chapter is to summarize and review the capability of this model to deal with fundamental magnitudes of the atomic collisions expressed as different moments of the energy loss: ionization cross sections (single or multiple, differential, and total), stopping power (and mean excitation energy), and energy loss straggling. This review covers a wide range of the collisions of ions with gases and solids, paying special attention to multi-electronic targets. The advantages and disadvantages of the model in comparison with independent electron ones, ranges of validity and future prospect will be considered. [Copyright &y& Elsevier]

Atomic hydrogen and helium ionization by impact of 3.6 MeV/u Au53+ is investigated within classical trajectory Monte Carlo and quantum-mechanical continuum distorted wave models. For helium ionization, a description of initial and final electron states in terms of Hartree–Fock potentials is compared with hydrogenic models. Different ionization mechanisms are characterized. A strong correlation of the electron, projectile and target nucleus is observed. Good agreement is obtained between calculations performed with the different models. [Copyright &y& Elsevier]

Abstract: The total cross-sections for atomic double ionization are evaluated with a two-step approximation, by considering that the projectile ionizes successively each electron. Single ionization probabilities are calculated within the continuum distorted-wave with eikonal initial state (CDW-EIS) formalism. The initial and final atomic wave functions are obtained by solving numerically the atomic wave equations with an optimized potential model (OPM). The electron–electron correlation in the final state is investigated following three different approaches: the usual Gamow factor, a modified Gamow factor with an effective charge and a mean value of the electron–electron repulsive Coulomb wave. The calculations are compared with experimental data and good agreement is found for double ionization of He and Li atoms by H+, He2+and Li3+ impact, at intermediate energies. [Copyright &y& Elsevier]