Residual Target Ion Research Articles

Ionization and Electron Capture Cross Sections for Single-Electron Removal from Biological Molecules by Swift Ion

We have studied ionization and electron capture cross sections for single-electron removal from biological molecules (adenine, cytosine, guanine, thymine, and uracil) by proton impact at energies ranging from 30 keV/amu to 10 MeV/amu. In this investigation, the three-body distorted wave method is used. The calculations are based on the independent electron model. For electron capture process, distortion in the final channel related to the Coulomb continuum states of the active electron and the projectile in the field of residual target ion is included. In case of ionization, we take all the pair-wise Coulomb interactions, which treat all interactions on equal footing in the final channel. Moreover, the asymptotic Coulomb logarithmic phase for the relative motion of two colliding nuclei is included in the initial channel. In both processes, the molecular character of the biological target is assumed to be a linear combination of their atomic orbitals (LCAO) and, for all cases, the different atomic orbitals are described with the Roothaan-Hartree-Fock (RHF) approximation. We have also calculated the total capture cross sections using simple Bragg’s additivity rule. For electron capture, the contributions to the TCS from the core orbitals of the molecule have also been explicitly analyzed. The double differential cross sections (DDCS) for electron emission as well as the total cross sections for single ionization of only uracil molecule with fast proton impact are also calculated. The results for capture and ionization cross sections are compared with other theoretical calculations and existing experimental data. We find that our calculated results are in quite better agreement with available experimental data than the other theoretical results.

Target dependence of post-collision effects in ionization by proton impact

We have measured and calculated double differential cross sections for ionization of Ne and Ar by 75 keV proton impact for a broad range of fixed projectile energy losses as a function of scattering angle. Along with data obtained previously for lighter targets this made possible a systematic analysis of post-collision effects between the scattered projectile and the continuum electron in the exit channel as a function of the target ionization potential. The data are consistent with an increasing strength of such effects with increasing ionization potential. However, second-order effects involving the projectile—residual target ion interaction also play an important role.

Fully differential cross-sections in single ionization of helium by 2 MeV/amu C6+ impact: The role of correlated wave functions

We study the effect of final-state dynamic correlation in single ionization of helium atom by 2 MeV/amu C6+ impact analyzing the fully differential cross-sections (FDCS). The transition matrix is introduced using a correlated continuum wave (CCW) and that for the initial state by an undistorted Born-approximation. The final state is modeled by the CCW function so that both the correlation and the interaction between the projectile and the residual target ion (PT interaction) are treated. The analytical expression of the transition matrix has been obtained through a series representation of the CCW function. Illustrative computations are performed in the scattering plane. A comparison of fully differential results using the present model with other theories and experimental data reveals that the dynamic correlation is responsible for the enhancement of the FDCS structure. The PT interaction is also checked in our model. We observe a satisfactory, although not perfect, agreement of the CCW calculations with experimental findings. Large discrepancies between the experimental data and theoretical predictions are found, which can at least be partly attributed to the inclusion of dynamic correlation effects.

Double differential cross sections for ion impact ionization of ammonia(NH3) and methane (CH4) molecules

SynopsisWe present a theoretical approach to calculate double-differential cross sections (DDCS) for single ionization of ammonia (NH3) and methane (CH4) molecules by the impact of proton at energies 0.25 MeV and 2 MeV respectively. The calculation are performed by the three-body formalism of three-Coulomb wave model (3C-3B). In the present formalism, the interaction of the projectile ion with the target is described by the Coulomb continuum wave functions in the initial channel. In the final channel, we have expressed the total wavefunctions as the product of pair-wise Coulomb wavefunctions among the ejected electron, projectile ion and the residual target ion, respectively.

Single and double electron capture in p-He and α-He collisions

The differential and total cross sections for both single and double electron capture in collisions of and He2+ with ground state helium atom have been studied by means of the four-body model of target continuum distorted wave (TCDW-4B) approximation in the energy range from 30 to 1000 keV amu–1. In this model, distortion in the final channel related to the Coulomb continuum states of the active electron(s) in the field of residual target ion are included. The calculations are based on the independent electron model. The present computed results are compared with the available experimental and other theoretical results. Total cross sections are found to be in good agreement with the measurements. We have also analysed differential cross sections (DCS) for both single and double electron capture in the collision of proton and α-particles with helium atoms at different projectile energies. The present DCS data exhibits the typical steeply decreasing dependence on the projectile scattering angles, but neither oscillating structures characteristic of interference effects nor peaks reminiscent of the Thomas peak are observed at different projectile energies. The obtained results for the DCS into the ground state are compared with the experimental data and overall a satisfactory agreement has been found. Finally we have also studied the variation of double to single capture differential cross-section ratios with projectile scattering angles at different impact energies.

Single ionization of water molecules in collisions with bare ions

We present the double differential cross sections (DDCSs) for the direct ionization of water molecules by impact of fully stripped helium, carbon and oxygen atoms, respectively. In the present formalism, we have represented the wavefunction in the entrance channel as the product of a plane wave for the projectile and an accurate one-center-molecular wavefunction of the water molecule by Moccia (1964 J. Chem. Phys. 40 2186). In the exit channel, we have expressed the total wavefunction as the product of pair-wise Coulomb wavefunctions among the ejected electron, projectile ion and the residual target ion, respectively. The contributions of DDCSs for five different molecular orbitals of water to the spectrum of angular distributions have been analyzed for several electron emission energies. The present results for DDCSs are compared with existing experimental and theoretical findings. We find an overall good agreement between our calculated results and the experimental findings for electron emission cross sections. In addition, DDCS results for ionization from different orbitals at a few electron emission energies are given in tabular form.

Four-body charge transfer processes in collisions of bare projectile ions with helium atoms

Single-electron capture by a bare ion from a helium atom at intermediate and high energies in the framework of four-body distorted wave (DW-4B) approximation in both prior and post form has been considered. In the entrance channel, the initial bound state wave function is distorted by the incoming projectile ion, and the corresponding distortion is related to the Coulomb continuum states of the active electron and the residual target ion in the field of the projectile ion respectively. Continuum states of the active electron and the projectile ion in the field of the residual target ion are also included in the exit channel. It may be mentioned that the effect of dynamic electron correlation is explicitly taken into account through the complete perturbation potential. The total single-electron capture cross sections are obtained by summing over all contributions up to n = 3 shells and sub-shells respectively. In addition, the differential cross sections for alpha particle–helium collision are calculated at impact energies of 60, 150, 300, 450, and 630 keV amu−1, respectively. The cross sections exhibit a monotonically decreasing angular dependence, with clear peak structures around 0.1 to 0.2 mrad being found at low impact energies. The current theoretical results, both in prior and post forms of the transition amplitude for symmetric and asymmetric collision, are compared with the available theoretical and experimental results. Current computed results have been found to be satisfactory in comparison with other theoretical and experimental findings.

Double ionization of the hydrogen chloride molecule: Influence of the target orientation on multiple differential cross sections

We report in this work multiple differential cross sections for the double ionization of isolated hydrogen chloride molecules impacted by electrons. The calculations are performed within the first Born approximation by considering a target molecular state described by means of a single-center molecular wave function. In the initial state, the incident electron is described by a plane wave, while in the final state Coulomb wave functions are used for modeling the interaction between the two ejected electrons and the residual target ion, the scattered electron being described by a plane wave. Additionally, a Gamow factor is introduced to take into account the Coulomb repulsion between the two outgoing particles. The contributions to the various differential cross sections of the four outermost orbitals of the hydrogen chloride molecule, namely, 4σ, 5σ, 2πx, and 2πy, are then reported for specific target orientations and by considering only the case of two electrons ejected from the same molecular orbital. The obtained results point out the effect of the target orientation on both the (e, 3e) and the (e, 3−1e) collisions and clearly evidence the signature of the well-known double ionization mechanisms, namely, the shake-off and the two-step 1 processes for all the cases investigated.

Classical simulation of single-electron capture and ionization in ion-atom collisions

Classical trajectory Monte Carlo (CTMC) simulation method and the four-body formalism of boundary corrected continuum intermediate state (BCCIS-4B) approximations have been employed to determine total single electron capture cross section and total single ionization cross section in collisions of H+, He2+ and Li3+ with helium atom and energy in range of 30–200 keV/amu. Present theoretical study using CTMC method is based on three-body formalism, where interaction of the active electron in target ion has been approximated by a model potential containing both long and short range part. As a consequence, initialization of the problem has been made on a multiperiodic orbit in sharp contrast to singly periodic orbit occurring in hydrogenic model. In BCCIS-4B formalism, distortion in the final channel related to Coulomb continuum states of projectile ion and active electron in the field of residual target ion are included. Results so obtained from both methods are in very good agreement with available experimental findings.

Electron capture by fast protons from helium-like ions

Four-body formalism of boundary corrected continuum intermediate state (BCCIS-4B) approximation have been applied to calculate the single-electron capture cross sections by fast protons through some helium-like ions in a large energy range from 30–1000 keV. In this model, distortion has been taken into account in the entrance channel. In the final channel, the passive electron plays the role of screening of the target ion. However, continuum states of the projectile and the electron in the field of the residual target ion are included. The comparison of the results is made with those of other theoretical investigations and experimental findings. The present calculated results are found to be in good agreement with the available experimental findings.

Single-electron-capture processes in collisions of He2+, Liq+ (q=1,2,3),C6+, andO8+ions with helium

Cross sections for single-electron capture in collisions of He${}^{2+}$, Li${}^{q+}$ ($q$ = 1,2,3), ${\mathrm{C}}^{6+}$, and ${\mathrm{O}}^{8+}$ ions with helium atoms at incident energy ranging from 50 to 5000 keV/amu have been calculated in the framework of four-body boundary-corrected continuum intermediate state (BCCIS-4B) approximation in both prior and post forms. In this formalism, distortion in the final channel related to the Coulomb continuum states of the projectile ion and the active electron in the field of residual target ion are included. In all cases, total single-electron-capture cross sections have been calculated by summing over all contributions up to $n$ = 3 shells and subshells, respectively. It has been observed that the contribution of the capture cross section into the excited states is significant for asymmetric collision (${Z}_{P}>{Z}_{T}$) and is insignificant for symmetric collision. Numerical results for the total cross sections show good agreement with the available experimental findings, particularly in the post form. Post-prior discrepancy has been found to be within 30% except for Li${}^{+}$ + He interactions below 150 keV/amu.

Electron transfer to individual magnetic substates of 3p 2P in collisions of helium-like ions with two-electron targets

We measured the degree of polarization of the 3s 2S1/2–3p 2P3/2 transition of C3+ ions produced by one-electron transfer in the collisions of C4+ ions with H2 at energies of 30–80 keV by visible spectroscopy. The results in this work were compared with the previous works for the alignment of 3p states of lithium-like carbon ions by means of VUV spectroscopy. We also compared these with the degree of polarization of the isoelectronic transitions of other lithium-like ions, N4+ and O5+, and found the correlation between the polarization degree and the potential crossing distance in which the one-electron transfer reaction takes place. This collision system dependence in the polarization of the 3s 2S1/2–3p 2P3/2 transition was discussed with respect of the linear Stark effect by the residual target ion.

A comprehensive theory of electron emission in ion-atom collisions

The channel-distorted wave method is combined with a two-centre description of the ejected electron state to embrace target ionization as well as capture-to-continuum aspects of the ionization process in energetic ion-atom collisions. The target-ionization peak, the electron-capture-to-continuum cusp and the binary ridge appear in the appropriate regions of momentum space of the ejected electron but each one of these features is perturbed by the presence of the other collision partner. In the cusp and in the binary-ridge regions, the essential effect of the residual target ion is represented by a modified Compton profile in which the momentum distribution of the initial electron state has been replaced by a corresponding form factor. Similarly, in the region of low electron energies with respect to the target nucleus, the standard expression for target ionization is modulated by a suitable off-energy-shell factor. The present theory agrees well with experimental data.

Theoretical description of the ejected-electron spectrum in collisions of 1.5-MeV/u F9+ with helium.

We present the results of an extensive classical trajectory Monte Carlo (CTMC) simulation of the ejected electron spectrum in collisions of 1.5-MeV/u F[sup 9+] with helium. Excellent agreement is found with the measurements of Lee [ital et] [ital al]. [Phys. Rev. A 41, 4816 (1990)] for ejection to 0[degree]. In particular, the simulation reproduces extremely well the shape and magnitude of the electron-capture-to-the-continuum and binary peaks. We contrast this agreement with calculations utilizing various quantum-mechanical perturbation theories. Also, a continuum-distorted-wave--eikonal-initial-state approach, which describes the interaction between the outgoing electron and the residual target ion through a model potential, has been utilized. This approach is shown to be an improvement over conventional calculations based on the use of effective charges. We draw conclusions regarding the proper representation of the collision dynamics leading to electron ejection in the low-energy, electron-capture-to-the-continuum, and binary peak regimes. Calculations of the doubly differential ionization cross section for non-0[degree] ejection are displayed as well.

Ne ionization induced by the impact of 106-keV/u H+ and 3He2+: Two-center effects on the soft-electron emission.

We present an experimental study of the electron emission induced by the collisional ionization of Ne gas by 106-keV/u ${\mathrm{H}}^{+}$ and $^{3}\mathrm{He}^{2+}$ projectiles. Cross sections, doubly differential in electron energy ${\mathit{E}}_{\mathit{e}}$ and angle \ensuremath{\theta}, were measured throughout the ranges 0.5\ensuremath{\le}${\mathit{E}}_{\mathit{e}}$\ensuremath{\le}440 eV and 0\ifmmode^\circ\else\textdegree\fi{}\ensuremath{\le}\ensuremath{\theta}\ensuremath{\le}180\ifmmode^\circ\else\textdegree\fi{}. We show that measurements at electron energies down to 0.5 eV were possible after a reduction of the physical extension of the gas target to a minimum. We find a remarkable dependence of the measured distribution of soft electrons with the extension of the target. We try a model according to which this effect is attributed to the scattering of electrons into the acceptance cone defined by the detector. Our measurements enable us to perform a quantitative analysis of the shape of the soft-electron peak. Contrary to current belief, this peak is found to be strongly asymmetric. The magnitude of this asymmetry is comparable to that of the cusp resulting from electron capture into the continuum. This puts into evidence that, also for these low-energy electrons, the interaction with the two Coulomb centers, projectile and residual target ion, is essential. By analyzing the ratio of the emission obtained with $^{3}\mathrm{He}^{2+}$ and ${\mathrm{H}}^{+}$, we also study the dependence of the doubly differential cross section on the projectile charge.

Is doubly differential electron emission produced in ion-atom collisions dominant at the saddle point velocity?

The authors present evidence that doubly differential distributions of electrons produced by ion-atom ionizing collisions of intermediate energies do not show a concentration in the sense of a dominant emission centred at the velocity of the saddle point of the potential extended between the two Coulomb centres, as given by the projectile ion and the residual target ion. This evidence is sustained by measurements performed with protons of 52 and 103 keV interacting with He. Their electron distributions are represented in terms of the doubly differential cross section given by the number of electrons per unit volume in velocity space, that is not affected by transformation into different reference frames. The authors do, however, not deny the important role that the two-centre saddle potential plays in the ionization mechanism. This leads to an enhanced forward emission which, however, is distributed in the velocity interval between zero and the projectile velocity.

Electron capture into Rydberg states in collisions between multiply charged ions and hydrogen

We investigate the substate distribution σ nlm in Rydberg manifolds ( n ∼ 10) following an asymmetric charge transfer reaction, C 6+ + H(1s) → C 5+( nlm) + H +, at velocities v ⩾ 1 a.u. These calculations serve as testing ground for two novel features included in our theoretical treatment, namely, (1) the use of a multiple scattering approach, the continuum distorted wave (CDW) approximation, to describe the capture process, and (2) the explicit inclusion of the post-collision interaction (PCI) with the residual target ion to supplement the primary capture amplitude. A measure of the multiple scattering contributions is obtained by comparison with the predictions of the first Born (OBK, single scattering) approximation whereas the PCI effects are gauged by inspection of the ( l, m)−distributions obtained from the resulting CDW(OBK)-PCI model with those calculated with the standard CDW (OBK) approximation.