State-selective Cross Sections Research Articles

Theoretical investigations of collisions between highly charged ions and atomic H and He

In the present work, the quantum-mechanical molecular-orbital close-coupling (QMOCC) method and the atomic-orbital close-coupling (AOCC) method have been applied to investigate Solar Wind ions (Neq+ Oq+ Nq+ Cq+) colliding with neutral H and He. The multi-reference single- and double-excitation configuration interaction (MRDCI) approach has been applied to compute the adiabatic potential and nonadiabatic couplings, and important configurations have been selected to obtain potentials of very high excited states. Total and state-selective cross sections have been calculated for a large energy range and compared to available experimental and theoretical data.

State-selective electron capture in collisions of C6+, N7+ with Hydrogen at intermediate energies

We present a joint study of the electron capture process in multicharged ions on atomic Hydrogen at intermediate energies. We have applied two methods, the Eikonal Impulse Approximation (EIA) and the hydrogenic-Classical Trajectory Monte Carlo (CTMC) method. Good agreement is found in both total and state-selective electron capture cross sections.

On the signal depletion induced by stretching excitation of methane in the reaction with the F atom

Exciting a stretching mode of a chemical bond should help its breaking during a chemical reaction, according to conventional wisdom. In several recent studies of the reactions of stretch-excited methane (and isotopologues) with the F atom, counterintuitively, we found that the induced reactant vibrations instead inhibit the bond rupture and slow down the overall reaction rate. This intriguing observation has been qualitatively ascribed to the vibrationally induced steric effects of the reaction in previous reports. However, quantitative determination of the reactivity suppression in terms of reaction cross sections remains lacking. In this report, we scrutinize the physical meaning of this (product) signal depletion phenomenon and fill the gap. Through a systematic investigation we further elucidate the additional reaction dynamics information that can be retrieved from the depletion measurements. The resultant rotationally state-selected reaction cross sections for both the vibrational ground and excited states are presented, and the stereodynamical implications are delineated.

State selective photo-recombination cross sections in Be-like C and Al ions

We report results from detailed state selective photo-recombination study along with the doubly excited autoionizing resonances in Be-like C2+ and Al9+ ions. In the present investigation, the primary focus is on detailed energy profiles of the individual photo-recombination cross sections. The calculation was carried out for the excited Rydberg states of type 1s22sns(1Se) which interact with the odd-parity continua up to the C3+ and Al10+ 2p threshold limit. The numerical evaluation has been performed at a fine energy mesh across all the autoionizing Rydberg series of resonances 1s22pns(1P0) converging to Li-like ion 2p threshold. The method of calculation keeps the essential ingredients of the Feshbach projection operator approximation. The photo-ionization cross sections have been evaluated with and without relativistic effects included into the R-matrix numerical procedures, while the allowance for both quantum interference between dielectronic and radiative recombination, and overlapping resonances has been done utilizing results from the earlier R-matrix Floquet calculation. We discuss all these results with respect to the effect of quantum interference term on the energy dependence profile of photo-recombination cross section for studied transitions.

Six-dimensional and seven-dimensional quantum dynamics study of the OH + CH4 → H2O + CH3 reaction

The reaction dynamics of hydroxyl radical with methane has been investigated using time-dependent wave packet approach within reduced six- and seven-dimensional models. Initial state-selected total reaction probabilities and integral cross sections for the hydrogen abstraction reaction have been computed on the empirical potential energy surface developed by Espinosa-García et al. [J. Chem. Phys. 112, 5731 (2000)]. Excitations of the CH stretching mode and/or the CH3 umbrella mode enhance the reaction. They are, however, both less efficient than translational energy in promoting the reaction, at least at low collision energies. Also, we studied the accuracy of two approximations: centrifugal sudden (CS) and J-shifting (JS), in the calculations of the integral cross sections by a comparison to coupled-channel (CC) calculations. The integral cross sections obtained indicated that the CS approximation works well over the whole energy range studied, and the JS approximation gives accurate cross sections at low collision energies, while noticeably overestimates them at relatively high collision energies. In addition, the OH radical acts as a good spectator as it has a negligible effect on the reaction.

Single- and double-electron-capture processes in the collisions of C4+ions with He

The nonradiative single-electron-capture (SEC) and double-electron-capture (DEC) processes for C${}^{4+}$(1${s}^{2}$) colliding with He atoms are investigated by using the quantum-mechanical molecular-orbital close-coupling method. Total and state-selective electron-capture cross sections are calculated in the energy range of 10${}^{\ensuremath{-}6}$--6 keV/amu. For energies less than 2 keV/amu, the DEC dominates the electron-capture processes. As the energy increases, the SEC cross sections increase while the DEC cross sections decrease; when $E$ g 2 keV/amu, the SEC process becomes the dominant mechanism. It is found that the SEC processes mainly result from the electron capture to $n$ = 3 channels of C${}^{3+}$ ion for energy below 0.7 keV/amu and from $n$ = 2 channels for $Eg0.7$ keV/amu. Compared with the available theoretical calculations, better agreement is achieved between the present results and the available measurements for total DEC and SEC cross sections. For state-resolved cross sections, excellent agreement is obtained for two-electron capture to C${}^{2+}$(2${s}^{2}$), C${}^{2+}$(2$s$2$p$), and C${}^{2+}$(2${p}^{2}$) and one-electron capture to C${}^{3+}$(2$s$) and C${}^{3+}$(2$p$) between the present calculations and the available theoretical and experimental results.

Development and evaluation of a many-electron–multicenter classical-trajectory Monte Carlo model in charge-exchange processes involving collisions of multiply charged ions and CO

A many-electron--multicenter classical-trajectory Monte Carlo model is developed and tested in charge-exchange studies involving projectiles with charges from $+$3 to $+$10 and the CO molecule. The present model is contrasted against simpler single-center and independent-atom approaches. Total and state-selective charge-exchange cross sections are presented and are compared to recent data from the Jet Propulsion Laboratory where available.

Cross Sections of the Processes Induced by Electron Collisions with H2+, HeH+, and Their Isotopes

The accuracy of cross sections given by theoretical calculations is evaluated on the collision processes of molecular ions and electrons. The processes focused on are dissociative recombination, dissociative excitation, and rotational and vibrational transitions of the molecular ions of H2+, HeH+, and their isotopes, which are relevant to divertor plasmas. Adopting the multichannel quantum defect theory, we calculated the state-selective cross sections for various states and energies. The validity of those calculations is investigated by comparing with experimental data under some limited conditions, and the calculations are verified from physical viewpoints.

Excitation cross sections for Li3 +, Ne10 + and Ar18 ++H(1s) collisions of interest in fusion plasma diagnostics

We have calculated state-selective excitation cross sections in fully stripped Li3 +, Ne10 + and Ar18 ++H(1s) collisions from low (1 keV/amu) to high (1000 keV/amu) impact energies, relevant in fusion plasma diagnostics. In order to cover this broad impact energy range, three different theoretical methods have been employed: the semi-classical molecular and one-centre atomic-orbital close-coupling approaches, and the classical trajectory Monte Carlo method. Recommended partial excitation cross sections are provided by merging the results obtained with each method in the energy range where they are the most accurate.

Relaxation of rotational-vibrational energy and volume viscosity in H–H2 mixtures

We investigate a kinetic model for H-H2 mixtures in a regime where translational/rotational and vibrational-resonant energy exchanges are fast whereas vibrational energy variations are slow. In a relaxation regime, the effective volume viscosity is found to involve contributions from the rotational volume viscosity, the vibrational volume viscosity, the relaxation pressure, and the perturbed source term. In the thermodynamic equilibrium limit, the sum of these four terms converges toward the one-temperature two-mode volume viscosity. The theoretical results are applied to the calculation of the volume viscosities of molecular hydrogen in the trace limit on the basis of a complete set of state-selected cross sections for the H + H2(v, j) system.

State-Selective and Total Single-Capture Cross Sections for Fast Collisions of Multiply Charged Ions with Helium Atoms

The four-body boundary corrected first Born approximation (CB1-4B) is used to calculate the single electron capture cross sections for collisions between fully stripped ions (He2+, Be4+, B5+ and C6+) and helium target at intermediate and high impact energies. The main goal of this study is to assess the usefulness of the CB1-4B method at intermediate and high impact energies for these collisions. Detailed comparisons with the measurements are carried out and the obtained theoretical cross sections are in reasonable agreement with the available experimental data.

Investigation of state-selective cross-sections for excitation processes of the collisions of He2++ H(1s) in strong magnetic fields

Based on the excitation cross-sections in collisions of H(1s) atoms with He2+ obtained by using the classical trajectory Monte Carlo method, the state-selective cross-sections of excitation processes for different n and m, where n and m are the principal and magnetic quantum numbers respectively, are studied with the application of strong longitudinal and transverse magnetic fields. Meanwhile, the precise energy levels for atom H in strong magnetic fields are obtained by non-perturbative quantum method. It is found that there is some strong separation of the state-selective cross-sections among different magnetic quantum states. Such behaviors are related to the variation of the energy levels and the diamagnetic terms induced by the applied magnetic fields. The diamagnetic terms in transverse magnetic fields result in the rapid increase of the cross-sections for the state of negative m at 25keV/u, which is further indicated by the trajectory in this case. In some cases the decrease of the total excitation cross-sections is found to be due to the rise of the energy levels caused by the magnetic fields. The orbital angular momentum along the direction of the magnetic field is not conserved absolutely; this phenomenon is found also in the trajectories and agrees with our analysis.

The effects of collision energy and reagent vibrational excitation on the reactivity of the reaction H + LiH: A quasiclassical trajectory study

Quasi-classical trajectory (QCT) calculations are carried out for the reaction H+LiH (ν=0–3, j=0) using a recent analytical potential energy surface (PES) of Prudente et al. [F.V. Prudente, J.M.C. Marques, A.M. Maniero, Time-dependent wave packet calculation of the LiH+H reactive scattering on a new potential energy surface, Chem. Phys. Lett. 474 (2009) 18–22]. The reaction H+LiH proceeds through the highly exothermic LiH depletion and thermoneutral hydrogen-exchange channels. State-selected and energy-resolved reaction probability and integral reaction cross sections are reported and compared with the recent available literature data. The QCT-calculated probabilities are in good agreement with the previous quantum–mechanical (QM) results, meanwhile, the vibrational excitation of the reagent LiH molecule decreases the reactivity of the LiH depletion channel and increases the reactivity of the hydrogen-exchange channel. Moreover, this decrease and increase can be explained by examining the representative reactive trajectories for the different vibrational excitation of the reagent and the topology of the HHLi PES. Our calculated results indicate that the collision energy and the vibrational excitation of the reagent play an important role on the reactivity of this system.

Angular- and state-selective differential cross sections for single-electron capture inp-He collisions at intermediate energies

State-selective electron capture cross sections and projectile scattering-angle differential cross sections for single-electron capture in collisions of protons with He at energies ranging from 50 to 100 keV have been experimentally investigated by means of a reaction microscope. It is shown that for single-electron capture the ground-state transfer is the dominant reaction channel, and excited-state transfer has relative small contributions to the cross sections. The transfer-excitation process has a minor contribution to the total cross sections. As the projectile energy increases, ground-state transfer remains dominant, but the relative contributions of the excited-state transfers become more important. The minor contribution of transfer excitation remains almost unchanged. Furthermore, it was found that for single transfer the present results are in good agreement with independent electron model predictions in the energy regime considered. For transfer excitation, however, there are obvious discrepancies between our measurements and theoretical results. We concluded that the electron-electron-correlation effects may play a role at least in transfer excitation in collisions of protons with He in the energy range considered.

Theoretical investigation of total and state-dependent charge exchange in O6 + collisions with atomic hydrogen

The charge exchange process has been found to play a dominant role in the production of x-rays and/or extreme ultraviolet photons emitted from cometary and planetary atmospheres and from the heliosphere. Charge exchange cross sections, especially state-selective cross sections, are necessary parameters in simulations of this x-ray emission. In this study, charge exchange, or single-electron capture, due to collisions of ground state O6 +(1s2 1S) with atomic hydrogen has been investigated theoretically using the quantum-mechanical molecular-orbital close-coupling method (QMOCC). The multi-reference single- and double-excitation configuration interaction approach has been applied to compute the adiabatic potentials and nonadiabatic couplings, and the atomic basis sets used have been optimized with a method proposed previously to obtain accurate descriptions of the high-lying Rydberg states of highly charged ions. Total and final-state-selective cross sections are calculated for energies between 0.1 eV/u and 10 keV/u. The QMOCC results are compared to available experimental and theoretical data as well as to new atomic-orbital close-coupling (AOCC) and classical trajectory Monte Carlo (CTMC) calculations. A recommended set of cross sections, based on the QMOCC, AOCC and CTMC calculations, and existing data, are deduced which should aid in x-ray emission modelling studies.

Electron transfer and ionization in collisions of He-like ions with Na(3s) and Na(3p)

Single electron transfer and ionization in collisions of He-like ions (N5+, O6+, Ne8+) and Na has been investigated both experimentally and theoretically at energies around the matching velocity of the valence electron (2 to 10 keV/amu). State selective cross sections and scattering angle distributions were obtained using recoil-ion momentum spectroscopy in combination with a magneto-optically cooled Na atom target. A strong dependence of the relative cross sections on the collision energy is observed. The results are compared with Classical-Trajectory Monte Carlo (CTMC) calculations and show an overall very good agreement.

Charge exchange processes involving highly charged ions and targets of interest in astrophysics and fusion plasmas

Renewed interest in charge exchange processes involving highly charged ions arises because of their crucial role in the planned ITER reactor as well as to recent X-ray observations in the astrophysical context. In this work, the classical trajectory Monte Carlo method (CTMC) is used to calculate state selective single charge exchange n-level cross sections and line emission cross sections pertinent to both fields. These are contrasted to recent laboratory data from KVI for the Xe18+ + Na(3s) collision system and NIST/BERLIN-EBIT data for the Ar18+ +Ar system.

Soft X-ray emissions related to the solar wind charge exchange observed by the X-ray satellite observatories

We have observed the emission spectra in collisions of bare and hydrogen-like C and O ions with helium atoms and hydrogen molecules as target gases in the soft X-ray region using a window-less Si(Li) detector at collision energies around 100 keV. Because it is found that the 1s-2p emission is dominant in each spectrum, we indicate that the cascade from the upper states give the large population of the 2p state after the consideration using state-selective capture cross sections calculated by the TC-AOCC method.

Rovibrationally selected ion-molecule collision study using the molecular beam vacuum ultraviolet laser pulsed field ionization-photoion method: Charge transfer reaction of N2+(X 2Σg+; v+ = 0–2; N+ = 0–9) + Ar

We have developed an ion-molecule reaction apparatus for state-selected absolute total cross section measurements by implementing a high-resolution molecular beam vacuum ultraviolet (VUV) laser pulsed field ionization-photoion (PFI-PI) ion source to a double-quadrupole double-octopole ion-guide mass spectrometer. Using the total cross section measurement of the state-selected N(2)(+)(v(+), N(+)) + Ar charge transfer (CT) reaction as an example, we describe in detail the design of the VUV laser PFI-PI ion source used, which has made possible the preparation of reactant N(2)(+)(X (2)Σ(g)(+), v(+) = 0-2, N(+) = 0-9) PFI-PIs with high quantum state purity, high intensity, and high kinetic energy resolution. The PFI-PIs and prompt ions produced in the ion source are shown to have different kinetic energies, allowing the clean rejection of prompt ions from the PFI-PI beam by applying a retarding potential barrier upstream of the PFI-PI source. By optimizing the width and amplitude of the pulsed electric fields employed to the VUV-PFI-PI source, we show that the reactant N(2)(+) PFI-PI beam can be formed with a laboratory kinetic energy resolution of ΔE(lab) = ± 50 meV. As a result, the total cross section measurement can be conducted at center-of-mass kinetic energies (E(cm)'s) down to thermal energies. Absolute total rovibrationally selected cross sections σ(v(+) = 0-2, N(+) = 0-9) for the N(2)(+)(X (2)Σ(g)(+); v(+) = 0-2, N(+) = 0-9) + Ar CT reaction have been measured in the E(cm) range of 0.04-10.0 eV, revealing strong vibrational enhancements and E(cm)-dependencies of σ(v(+) = 0-2, N(+) = 0-9). The thermochemical threshold at E(cm) = 0.179 eV for the formation of Ar(+) from N(2)(+)(X; v(+) = 0, N(+)) + Ar was observed by the measured σ(v(+) = 0), confirming the narrow ΔE(cm) spread achieved in the present study. The σ(v(+) = 0-2; N(+)) values obtained here are compared with previous experimental and theoretical results. The theoretical predictions calculated based on the Landau-Zener-Stückelberg formulism are found to be in fair agreement with the present measured σ(v(+) = 1 or 2; N(+)). Taking into account of the experimental uncertainties, the measured σ(v(+) = 1 or 2, N(+)) for N(+) = 0-9 at E(cm) = 0.04-10.0 eV are found to be independent of N(+).

Four-body corrected first Born approximation for single-electron capture into arbitrary states of energetic projectiles

Single charge exchange in collisions between fast bare projectiles and heliumlike atomic systems is investigated by means of the four-body boundary-corrected first Born approximation. An extensive analytical study of the general transition amplitude for electron capture into the arbitrary ${n}^{f}{l}^{f}{m}^{f}$ final states of the projectile is carried out. The quantum-mechanical transition amplitude for both symmetric and asymmetric collisions is derived in terms of a two-dimensional numerical quadrature over real variables. An illustrative computation is performed involving state-selective and total single-capture cross sections for $p$-He and ${\mathrm{Li}}^{3+}$-He collisions at intermediate and high impact energies. Detailed comparisons with the available measurements are reported with the purpose of further assessing the validity and utility of the four-body corrected first Born method in various applications ranging from fusion research to radiotherapy.