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Journal of Physics B Atomic Molecular and Optical Physics

Comparative studies of ion-pair formation and resonant quenching processes in collisions of Rydberg atoms with the alkaline-earth atoms

Alexander A. Narits, E. S. Mironchuk, Владимир Сергеевич Лебедев

Published Dec 17, 2013• 13 citations
Publisher

Abstract

We report the results of comparative studies of ion-pair formation and quenching processes in collisions of Rydberg Li( nl ) and Cs( nl ) atoms with Ca(4s 2 ), Sr(5s 2 ) and Ba(6s 2 ) atoms possessing small electron affinities. Our consideration includes both the cases of selectively excited Rydberg nl -states with small orbital angular momentum ( l ≪ n ) and nearly circular states with l ∼ n − 1. Calculations of the electron-transfer processes are based on the semiclassical theory of nonadiabatic transitions and exact expression for the Rydberg-covalent–ionic coupling terms. Calculations of nonresonant quenching processes are carried out within the framework of the available theory of inelastic and quasielastic transitions between Rydberg-covalent states. The ion-pair formation and resonant quenching cross sections are shown to be significantly dependent not only on the principal quantum number n but also on the orbital angular momentum l and the binding energy of the alkaline-earth anion. For each system under study we find the regions of n in which either the resonant quenching or the ion-pair formation processes are predominant. The relative role of the resonant and nonresonant mechanisms of depopulation of Rydberg states is investigated.

DOI
10.1088/0953-4075/47/1/015202

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Transfer of a weakly bound electron in collisions of Rydberg atoms with neutral particles. II. Ion-pair formation and resonant quenching of the Rb(nl) and Ne(nl) States by Ca, Sr, and Ba atoms

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Electron-transfer processes are studied in thermal collisions of Rydberg atoms with alkaline-earth Ca(4s 2), Sr(5s 2), and Ba(6s 2) atoms capable of forming negative ions with a weakly bound outermost p-electron. We consider the ion-pair formation and resonant quenching of highly excited atomic states caused by transitions between Rydberg covalent and ionic terms of a quasi-molecule produced in collisions of particles. The contributions of these reaction channels to the total depopulation cross section of Rydberg states of Rb(nl) and Ne(nl) atoms as functions of the principal quantum number n are compared for selectively excited nl-levels with l ≪ n and for states with large orbital quantum numbers l = n − 1, n − 2. It is shown that the contribution from resonant quenching dominates at small values of n, and the ion-pair formation process begins to dominate with increasing n. The values and positions of the maxima of cross sections for both processes strongly depend on the electron affinity of an alkaline-earth atom and on the orbital angular momentum l of a highly excited atom. It is shown that in the case of Rydberg atoms in states with large l ∼ n − 1, the rate constants of ion-pair formation and collisional quenching are considerably lower than those for nl-levels with l ≪ n.

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On the quenching mechanisms of Rydberg states of atoms during collisions with alkaline-earth atoms

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Collisional quenching of Rydberg states of Na(nl) atoms during the interaction with Sr (5s 2) and Ca (4s 2) atoms is theoretically studied. The effects of two quenching mechanisms are considered: the resonant one associated with transitions between ionic and covalent terms of a quasi-molecule formed during collisions and the non-resonant mechanism caused by quasi-free electron scattering by a perturbing atom. The ranges of the prevalence of these mechanisms are determined depending on the principal and orbital quantum numbers, as well as the electron affinities of the alkaline-earth atom.

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Interaction of Rydberg atoms in circular states with the alkaline-earth Ca(4s 2) and Sr(5s 2) atoms

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The resonant mechanism of interaction of alkaline-earth atoms having a low electron affinity to Rydberg atoms in circular (l = |m| = n–1) and near-circular states has been studied. To describe the dynamics of resonant processes accompanied by nonadiabatic transitions between ionic and Rydberg covalent terms of a quasimolecule, an approach based on the integration of coupled equations for the probability amplitudes has been developed taking into account the possibility of the decay of an anion in the Coulomb field of the positive ionic core of a highly excited atom. The approach involves the specific features of the problem associated with the structure of the wavefunction of a Rydberg electron in states with high orbital angular momenta l ~ n–1. This approach provides a much more accurate description of the dynamics of electronic transitions at collisions between atoms than that within the modified semiclassical Landau–Zener model. In addition, this approach makes it possible to effectively take into account many channels of the problem. The cross sections for resonant quenching of Rydberg states of the Li(nlm) atom with given principal n, orbital l = n–1, and magnetic m quantum numbers at thermal collisions with the Ca(4s 2) and Sr(5s 2) atoms have been calculated. The dependences of the results on n, m, and angle α between the relative velocity of the atoms and the normal to the plane of the orbit of the Rydberg electron have been obtained. The influence of orientational effects on the efficiency of the collisional destruction of circular and near-circular states has been studied. The results indicate a higher stability of such states to their perturbations by neutral particles as compared to usually studied nl states with low values of l (l ≪ n).

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Quenching of Rydberg states by atoms with small electron affinities

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We discuss the theory of quenching of Rydberg states by atoms with small electron affinities and perform calculations for quenching of s and d states of Ne by Ca. There are two contributions to the quenching cross section: the ion-covalent coupling contribution, which is due to the formation of an intermediate ionic state, and the impulse contribution. The first is very sensitive to the electron affinity of the projectile and may be used for studies of weakly bound anions. The ion-covalent coupling contribution is dominant for the process of quenching of Rydberg states with large quantum defects (e.g. s states) at relatively low principal quantum number n , whereas the impulse contribution dominates the process of quenching of states with small quantum defects (e.g. d states of Ne). We present calculations of quenching of s states by Yb atoms with a hypothetical electron affinity of 2 meV and show that experimental studies of quenching of Rydberg states by Yb might help in determining the electron affinity of Yb even if it is negative. In the latter case the quenching cross section as a function of n should exhibit oscillations (Borodin V M and Kazansky A K 1992 J. Phys. B: At. Mol. Opt. Phys. 25 971).

Journal of Physics B Atomic Molecular and Optical Physics•Apr 7, 2000•22 citations
We report the results of comparative studies of ion-pair formation and quenching processes in collisions of Rydberg Li( nl ) and Cs( nl ) atoms with Ca(4s 2 ), Sr(5s 2 ) and Ba(6s 2 ) atoms possessing small electron affinities. Our consideration includes both the cases of selectively excited Rydberg nl -states with small orbital angular momentum ( l ≪ n ) and nearly circular states with l ∼ n − 1. Calculations of the electron-transfer processes are based on the semiclassical theory of nonadiabatic transitions and exact expression for the Rydberg-covalent–ionic coupling terms. Calculations of nonresonant quenching processes are carried out within the framework of the available theory of inelastic and quasielastic transitions between Rydberg-covalent states. The ion-pair formation and resonant quenching cross sections are shown to be significantly dependent not only on the principal quantum number n but also on the orbital angular momentum l and the binding energy of the alkaline-earth anion. For each system under study we find the regions of n in which either the resonant quenching or the ion-pair formation processes are predominant. The relative role of the resonant and nonresonant mechanisms of depopulation of Rydberg states is investigated.
Electron-transfer processes are studied in thermal collisions of Rydberg atoms with alkaline-earth Ca(4s 2), Sr(5s 2), and Ba(6s 2) atoms capable of forming negative ions with a weakly bound outermost p-electron. We consider the ion-pair formation and resonant quenching of highly excited atomic states caused by transitions between Rydberg covalent and ionic terms of a quasi-molecule produced in collisions of particles. The contributions of these reaction channels to the total depopulation cross section of Rydberg states of Rb(nl) and Ne(nl) atoms as functions of the principal quantum number n are compared for selectively excited nl-levels with l ≪ n and for states with large orbital quantum numbers l = n − 1, n − 2. It is shown that the contribution from resonant quenching dominates at small values of n, and the ion-pair formation process begins to dominate with increasing n. The values and positions of the maxima of cross sections for both processes strongly depend on the electron affinity of an alkaline-earth atom and on the orbital angular momentum l of a highly excited atom. It is shown that in the case of Rydberg atoms in states with large l ∼ n − 1, the rate constants of ion-pair formation and collisional quenching are considerably lower than those for nl-levels with l ≪ n.
Collisional quenching of Rydberg states of Na(nl) atoms during the interaction with Sr (5s 2) and Ca (4s 2) atoms is theoretically studied. The effects of two quenching mechanisms are considered: the resonant one associated with transitions between ionic and covalent terms of a quasi-molecule formed during collisions and the non-resonant mechanism caused by quasi-free electron scattering by a perturbing atom. The ranges of the prevalence of these mechanisms are determined depending on the principal and orbital quantum numbers, as well as the electron affinities of the alkaline-earth atom.
The resonant mechanism of interaction of alkaline-earth atoms having a low electron affinity to Rydberg atoms in circular (l = |m| = n–1) and near-circular states has been studied. To describe the dynamics of resonant processes accompanied by nonadiabatic transitions between ionic and Rydberg covalent terms of a quasimolecule, an approach based on the integration of coupled equations for the probability amplitudes has been developed taking into account the possibility of the decay of an anion in the Coulomb field of the positive ionic core of a highly excited atom. The approach involves the specific features of the problem associated with the structure of the wavefunction of a Rydberg electron in states with high orbital angular momenta l ~ n–1. This approach provides a much more accurate description of the dynamics of electronic transitions at collisions between atoms than that within the modified semiclassical Landau–Zener model. In addition, this approach makes it possible to effectively take into account many channels of the problem. The cross sections for resonant quenching of Rydberg states of the Li(nlm) atom with given principal n, orbital l = n–1, and magnetic m quantum numbers at thermal collisions with the Ca(4s 2) and Sr(5s 2) atoms have been calculated. The dependences of the results on n, m, and angle α between the relative velocity of the atoms and the normal to the plane of the orbit of the Rydberg electron have been obtained. The influence of orientational effects on the efficiency of the collisional destruction of circular and near-circular states has been studied. The results indicate a higher stability of such states to their perturbations by neutral particles as compared to usually studied nl states with low values of l (l ≪ n).
Abstract The results of theoretical studies of the resonant quenching and ion-pair formation processes induced by collisions of Rydberg atoms with highly polar molecules possessing small electron affinities are reported. We elaborate an approach for describing collisional dynamics of both processes and demonstrate the predominant role of resonant quenching channel of reaction for the destruction of Rydberg states by electron-attaching molecules. The approach is based on the solution of the coupled differential equations for the transition amplitudes between the ionic and Rydberg covalent terms of a quasimolecule formed during a collision of particles. It takes into account the possibility of the dipole-bound anion decay in the Coulomb field of the positive ionic core and generalizes previous models of charge-transfer processes involving Rydberg atoms to the cases, when the multistate Landau–Zener approaches become inapplicable. Our calculations for Rb ( nl ) atom perturbed by C 2 H 4 SO 3 , CH 2 CHCN , CH 3 NO 2 , CH 3 CN , C 3 H 2 O 3 , and C 3 H 4 O 3 molecules show that the curves representing the dependence of the resonant quenching cross sections on the principal quantum number n are bell-shaped with the positions of maxima being shifted towards lower values of n and the peak values, σ max ( q ) , several times higher than those for the ion-pair formation, σ max
Ion-pair formation processes are studied in collisions of Rydberg atoms with neutral particles possessing small electron affinities. Nonadiabatic transitions from a Rydberg covalent term to an ionic term of a quasi-molecule are considered using the modified Landau-Zener theory supplemented with calculation of survival factors of an anion decaying in the Coulomb field of a positive ion core. Using the technique of irreducible tensor operators and the momentum representation of the wavefunction of a highly excited atom, exact expressions are obtained for transition matrix elements and the ionic-covalent coupling parameter. The approach developed in the paper provides the description beyond the scope of a conventional assumption about a small variation of the wavefunction of the Rydberg atom on the range of electron coordinates determined by the characteristic radius of the wavefunction of the anion. This allows one to correctly consider long-range effects of the interaction between a weakly bound electron and the neutral core of a negative ion in processes under study. It is shown by the example of thermal collisions of Xe( nf ) atoms with CH 3 CN molecules that this is very important for a reliable quantitative description of anion formation with a low binding energy. The results are compared with experiments and calculations performed within the framework of a number of approximate methods.
We discuss the theory of quenching of Rydberg states by atoms with small electron affinities and perform calculations for quenching of s and d states of Ne by Ca. There are two contributions to the quenching cross section: the ion-covalent coupling contribution, which is due to the formation of an intermediate ionic state, and the impulse contribution. The first is very sensitive to the electron affinity of the projectile and may be used for studies of weakly bound anions. The ion-covalent coupling contribution is dominant for the process of quenching of Rydberg states with large quantum defects (e.g. s states) at relatively low principal quantum number n , whereas the impulse contribution dominates the process of quenching of states with small quantum defects (e.g. d states of Ne). We present calculations of quenching of s states by Yb atoms with a hypothetical electron affinity of 2 meV and show that experimental studies of quenching of Rydberg states by Yb might help in determining the electron affinity of Yb even if it is negative. In the latter case the quenching cross section as a function of n should exhibit oscillations (Borodin V M and Kazansky A K 1992 J. Phys. B: At. Mol. Opt. Phys. 25 971).