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48
result(s) for
"Ion-atom collisions."
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Capture dynamics of ultracold atoms in the presence of an impurity ion
We explore the quantum dynamics of a one-dimensional trapped ultracold ensemble of bosonic atoms triggered by the sudden creation of a single ion. The numerical simulations are performed by means of the ab initio multiconfiguration time-dependent Hartree method for bosons which takes into account all correlations. The dynamics is analyzed via a cluster expansion approach, adapted to bosonic systems of fixed particle number, which provides a comprehensive understanding of the occurring many-body processes. After a transient during which the atomic ensemble separates into fractions which are unbound and bound with respect to the ion, we observe an oscillation in the atomic density which we attribute to the additional length and energy scale induced by the attractive long-range atom-ion interaction. This oscillation is shown to be the main source of spatial coherence and population transfer between the bound and the unbound atomic fraction. Moreover, the dynamics exhibits collapse and revival behavior caused by the dynamical build-up of two-particle correlations demonstrating that a beyond mean-field description is indispensable.
Journal Article
Net Electron Capture in Collisions of Multiply Charged Projectiles with Biologically Relevant Molecules
A model for the description of proton collisions from molecules composed of atoms such as hydrogen, carbon, nitrogen, oxygen and phosphorus (H, C, N, O, P) was recently extended to treat collisions with multiply charged ions with a focus on net ionization. Here we complement the work by focusing on net capture. The ion–atom collisions are computed using the two-center basis generator method. The atomic net capture cross sections are then used to assemble two models for ion–molecule collisions: An independent atom model (IAM) based on the Bragg additivity rule (labeled IAM-AR), and also the so-called pixel-counting method (IAM-PCM) which introduces dependence on the orientation of the molecule during impact. The IAM-PCM leads to significantly reduced capture cross sections relative to IAM-AR at low energies, since it takes into account the overlap of effective atomic cross sectional areas. We compare our results with available experimental and other theoretical data focusing on water vapor (H2O), methane (CH4) and uracil (C4H4N2O2). For the water molecule target we also provide results from a classical-trajectory Monte Carlo approach that includes dynamical screening effects on projectile and target. For small molecules dominated by a many-electron atom, such as carbon in methane or oxygen in water, we find a saturation phenomenon for higher projectile charges (q=3) and low energies, where the net capture cross section for the molecule is dominated by the net cross section for the many-electron atom, and the net capture cross section is not proportional to the total number of valence electrons.
Journal Article
Study of single electron capture in O6+ + He collisions
Research on electron capture (EC) process are undoubtedly helpful for maturing theoretical models on ion-induced collision especially for low-energy region. In this work, a two-active-electron semiclassical asymptotic-state close-coupling method was used to calculate the total and l-solved state-selective single EC cross sections of O6+ + He collisions in the energy range of 0.3–100 keV u−1, accompanied with experimental measurements in the energy range of 2.63–37.5 keV u−1 with an uncertainty of 11% in good agreement. Above 4.5 keV u−1, the state-selective cross section of n = 5 was reported experimentally for the first time. Calculations with multiple theoretical methods were gathered and compared with present calculations. The importance of two-active-electrons correlation and large basis sets in theoretical calculations was found, and discrepancies between previous theoretical and experimental results can be explained by the present results.
Journal Article
Fragmentation Processes
Revolutionary advances in experimental techniques and spectacular increases in computer power over recent years have enabled researchers to develop a much more profound understanding of the atomic few-body problem. One area of intense focus has been the study of fragmentation processes. Covering the latest research in the field, this edited text is the first to provide a focussed and systematic treatment of fragmentation processes, bringing together contributions from a range of leading experts. As well as tackling the more established electron-impact ionization processes, (e,2e), this book also guides the reader through topics such as molecular fragmentation, ion-atom collisions and multi-photon processes. Combining a broad range of topics with an equal mix of theoretical and experimental discussion, this is an invaluable text for graduate students and researchers in atomic collisions, laser physics and chemistry.
eBook
Calculation of Energy and Angular Distributions of Electrons Produced in Intermediate-Energy p + H2 Collisions
We extend the two-centre wave-packet convergent close-coupling approach to doubly differential ionisation in proton collisions with H2 to intermediate projectile energies. The results for the doubly differential cross section at projectile energies from 48 to 200 keV are presented as a function of the energy and angle of emitted electrons. We consider a wide range of emission angles from 10 to 160∘, and compare our results to experimental data, where available. Excellent agreement between the presented results and the experimental data was found, especially for emission angles less than 130∘. For very large backward emission angles our calculations tended to slightly overestimate the experimental data when energetic electrons are ejected and the doubly differential cross section is very small. This discrepancy may be due to the large uncertainties in the experimental data in this region and the model target description. Overall, the present results show significant improvement upon currently available theoretical results and provide a consistently accurate description of this process across a wide range of incident energies.
Journal Article
Charge Transfer in He+ − He → He(1s4l, l ≥ 2) − He+ Collisions in Intermediate Energy Range
The anticrossing spectra of the helium line λ1s4l D3,F−1s2p P3=447.2 nm emitted after electron capture by He+ ions in He+−He collisions were measured for projectile energies of 10–29 keV. Furthermore, considering the excited states’ time evolution, the theoretical intensity functions were calculated. The electric field and density distributions of the target He atoms in the collision volume were taken into account, and by fitting the theoretical intensities to the measured ones, the post-collisional states of the charge-transferred He atoms were determined. The results indicate that for the intermediate projectile energy range, the electronic charge distributions were asymmetric, but the electric dipole moments did not change, as in the case of the target atoms excited directly in the collisions. This result shows that the Paul trap mechanism may play an important role in the charge transfer excitation in this energy range.
Journal Article
Differential Study of Projectile Coherence Effects on Double Capture Processes in p + Ar Collisions
We have measured differential yields for double capture and double capture accompanied by ionization in 75 keV p + Ar collisions. Data were taken for two different transverse projectile coherence lengths. A small effect of the projectile coherence properties on the yields were found for double capture, but not for double capture plus ionization. The results suggest that multiple projectile–target interactions can lead to a significant weakening of projectile coherence effects.
Journal Article
Cross section sensitivity to perturbation strengths in distorted waves for double electron capture by alpha particles from helium targets
Computer experiments are performed on total cross sections for capture of both electrons from helium targets at 100-10000 keV. Employed are four quantum-mechanical perturbative four-body distorted wave methods (one of the first and three of the second order). The goal is to determine the cross section sensitivity to the perturbation strengths in distorted waves from the second-order methods. The perturbation strength is parametrized by the Sommerfeld factor (the quotient of the nuclear charge and the relative velocity of the colliding particles). At each fixed impact energy, the sought sensitivity is monitored by gradually modifying the nuclear charges in the Sommerfeld factors. These factors reside in the Coulomb distortions of the unperturbed channels states. The focus is on the electronic distortions through the eikonal Coulomb logarithmic phases and the full Coulomb waves. The logarithmic phases are the constituents of the compound phases for the net charges of the two heavy scattering aggregates in relative motions. A striking perturbation strength sensitivity of the obtained total cross sections is recorded.
Journal Article
Preliminary Results for Observation of Radiative Double-Electron Capture by F9+,8+ on Graphene
Radiative double-electron capture, which can be considered the inverse of double photoionization, has been investigated for 2.11 MeV/u F9+ and F8+ projectiles colliding with the two-dimensional target single-layer graphene. Preliminary results for the cross sections are obtained and presented and compared with our previous measurements for the one-dimensional gas targets N2 and Ne, with the three-dimensional target thin-foil C, and with the most accurate theoretical results that currently exist. The graphene results reported here are reasonable when compared with the F9++N2, Ne results given the thicknesses of the respective targets, being larger by about a factor of four.
Journal Article