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Journal of Physics B Atomic Molecular and Optical Physics

Rydberg atom-ion collisions: classical overbarrier model for charge exchange

Valentin N. Ostrovsky

Published Sep 14, 1995• 34 citations
Publisher

Abstract

A novel classical overbarrier model for the charge exchange is formulated for low and medium collision velocities. The charge-exchange probability is expressed in analytical form. The results are compared with the classical Monte Carlo trajectory calculations for various partners (ground-state atom+multicharged ion, high Rydberg atom+ion with unit charge). The model reproduces well the experimental data for the collisions Na + +Na(29s).

DOI
10.1088/0953-4075/28/17/025

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Journal of Physics B Atomic Molecular and Optical Physics•Jul 11, 2000•0 citations
A novel classical overbarrier model for the charge exchange is formulated for low and medium collision velocities. The charge-exchange probability is expressed in analytical form. The results are compared with the classical Monte Carlo trajectory calculations for various partners (ground-state atom+multicharged ion, high Rydberg atom+ion with unit charge). The model reproduces well the experimental data for the collisions Na + +Na(29s).
Charge transfer collisions between slow ions and Rydberg atoms have been studied for ion charges q=1−−4q\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1--4q=1−−4, using resonant laser excitation to detect specific energy states of the collision products. The data collected show a clear resonance in the capture cross section to a particular energy state as the binding energy of the Rydberg target is varied. The resonance position differs significantly from the predictions of the classical overbarrier model, and its width becomes very small (0.025 eV) at the lowest velocities studied.
Charge-transfer collisions between slow multiply charged ions and highly excited Rydberg atoms have been studied to determine the details of the internal energy transfer accompanying charge exchange. The experiment uses lasers to define uniquely the binding energies of both initial and final states of the collision, thereby eliminating the ambiguity inherent in the selective field ionization method used for previous studies. The results clearly characterize the capture energetics over the range of ion charge q=1−−11q=1--11q=1−−11 and velocity v=0.031−−0.138a.u.v=0.031--0.138\mathrm{a}.\mathrm{u}.v=0.031−−0.138a.u. The measurements are in very good agreement with predictions of the classical trajectory Monte Carlo method, but not with predictions of other classical models.
A stochastic model of associative ionization in collisions of Rydberg atoms with ground-state atoms is presented. The conventional Duman–Shmatov–Mihajlov–Janev (DSMJ) model treats the ionization as excitation of Rydberg electron to the continuum by the electric-dipole field generated by exchange interaction within the quasi-molecular ion. The stochastic model essentially extends this treatment by taking into account redistribution of population over a range of Rydberg states prior to ionization, which is caused by non-adiabatic processes in overlapping multiple level crossings of quasi-molecular Rydberg states. The redistribution is modelled as diffusion of electrons in the Rydberg energy spectrum using a Fokker–Planck-type equation. The process of l -mixing of Rydberg states at large internuclear distances and twisting of the collision trajectories on attractive potentials are taken into account. The choice of the collision velocity distribution is also shown to be important. Associative ionization rates have been calculated for Na**( nl ) + Na collisions with n = 5–25 and l = 0, 1, 2, and compared with the available experimental data and the calculations performed using the nonlinear DSMJ model. At relatively low n the stochastic model yields a substantially better agreement with the experimental data than the DSMJ model, while the results of both models converge at large n .
Low-energy collisions of Rydberg atom-ion systems are investigated theoretically. We present the parameter space associated with suitable approaches for the dynamics of Rydberg atom-ion collisions, i.e., quantum, Langevin, and classical exchange regimes, showing that for the lowest reachable temperatures a classical treatment is appropriate. A quasiclassical trajectory method is used to study charge exchange cross sections for ${\mathrm{Li}}^{*}\text{\ensuremath{-}}{\mathrm{Li}}^{+}$ and ${\mathrm{Li}}^{*}\text{\ensuremath{-}}{\mathrm{Cs}}^{+}$ at collision energies down to 1K1\phantom{\rule{0.28em}{0ex}}\mathrm{K}1K. For cold collisions we find the charge exchange cross section deviating from the n4{n}^{4}n4 geometric scaling. Furthermore, for low-energy collisions, we find both an influence of the ionic core repulsion as well as variations for two different models used for describing the electron-core potential.
Transitions between states with the same principal quantum number n in the collision of a Rydberg atom with an ion are described within the classical approximation for the Rydberg electron. The quantum electron state is represented by an ensemble of classical trajectories. Within the dipole approximation for the interaction the final value of the orbital momentum l is calculated analytically for each initial trajectory in the ensemble. Averaging over the ensemble gives the distribution function over l. For the initial s state of the Rydberg atom the distribution functions are found also for the pairs of variables, namely: {the angular momentum; its projection on the collision velocity}; {projections of the angular momentum and the Runge - Lenz vector on the collision velocity} and {projections of the angular momentum and the Runge - Lenz vector on the plane perpendicular to the collision velocity}. The l-distributions for the orbitally polarized initial p and d Rydberg states with definite projections of the initial angular momentum on the velocity axis are also obtained. The influence of the non-Coulomb core is taken into account by an approximation. The final l-distributions (calculated for the initial s-state) are shifted towards the large-l values as the relative collision velocity vn decreases. Good agreement is achieved with the recent experimental data for Na(28d) and Na(26d) Rydberg atoms.
The classical trajectory Monte Carlo method has been used to simulate fully stripped ion-circular Rydberg atom collisions. We report that the mechanisms that govern the capture process for ions colliding with aligned Rydberg targets, namely direct and indirect velocity matching, alternate as to which is dominant. Previous experimental and theoretical results for singly charged ion-Rydberg atom collisions demonstrate that direct capture processes dominate at intermediate collision speeds. We have found that for highly charged ion-Rydberg atom collisions electron capture via indirect velocity matching is dominant at intermediate collision speeds and contributes up to 90% of the total capture cross-section.