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Journal of Physics B Atomic and Molecular Physics

Theoretical interpretation of Penning and associative ionisation in collisions between two excited rubidium atoms

L. Barbier, A. Pesnelle, M. Chéret

Published Mar 26, 1987• 11 citations
Publisher

Abstract

Associative and Penning ionisation cross sections, rho AI and rho PI are calculated for collisions of two excited atoms Rb(nl)+Rb(5p 2 P 3 2/) in the classical impact parameter formulation for nl=5d to 9d and 7s. It is shown that the very large Penning ionisation rate constants, measured by the authors and presented earlier, can be well explained by such a model in terms of direct and exchange autoionisation widths, as well as the associative ionisation contributions which have been observed to be quite different for ns and nd cases.

DOI
10.1088/0022-3700/20/6/012

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Kinetics of Rb2+ and Rb+ formation in laser-excited rubidium vapor

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Abstract We have studied the formation of the molecular ion Rb2+ and the atomic ion Rb+. These are created in laser excited rubidium vapor at the first resonance, 5s–5p and 5p-nl transitions. A theoretical model is applied to this interaction to explain the time evolution and the laser-power dependence of the population density of Rb+ and Rb2+. A set of rate equations which describe: the temporal variation of the population density of the excited states; the atomic ion density; and the electron density, were solved numerically under the experimental conditions of Barbier and Cheret. In their experiment the Rb concentration was 1×1013cm−3 and the laser power was taken to be 50–500 mW at vapor temperature = 450 K. The results showed that the main processes for producing Rb2+ are associative ionization and Hornbeck-Molnar ionization. The calculations have also showed that, the atomic ions Rb+ are formed through the Penning Ionization (PI) and photoionization processes. Moreover, a reasonable agreement between the experimental results and our calculations for the ion currents of the Rb+ and Rb2+ is obtained.

Open Physics•Apr 10, 2008•4 citations
Associative and Penning ionisation cross sections, rho AI and rho PI are calculated for collisions of two excited atoms Rb(nl)+Rb(5p 2 P 3 2/) in the classical impact parameter formulation for nl=5d to 9d and 7s. It is shown that the very large Penning ionisation rate constants, measured by the authors and presented earlier, can be well explained by such a model in terms of direct and exchange autoionisation widths, as well as the associative ionisation contributions which have been observed to be quite different for ns and nd cases.
Rb + and Rb 2 + formation in a laser-excited Rb vapour is extensively studied for s and d levels ranging from 5d to 11s levels. Collisional ionisation with a ground-state atom leads to Rb 2 + formation by Hornbeck-Molnar reaction with rate coefficients lying in the 10 -10 cm 3 s -1 range. This reaction is analysed theoretically. Penning ionisation occurs in collision with a resonant-state atom, Rb(5p), with a high efficiency (rate coefficient above 10 -8 cm 3 s -1 ). Associative ionisation between the 7s level and the 5p level is also observed (rate coefficient about 10 -10 cm 3 s -1 ). This reaction appears negligible for the neighbouring 5d level.
Penning ionization (PI) occurring during the collision of a Rydberg atom Rb(nl) with a resonant state K(4P) or Rb(5P) is analyzed theoretically. By using a semiclassical approach, we calculate the rate coefficients corresponding to both the direct and exchange decay channels. We implement the method by deriving various approximations for the exchange and direct decay widths. The radiative mechanism is found to dominate the thermal-energy quenching of alkali atoms by PI, the exchange being negligible. The calculated values are in reasonable agreement with the available experimental results.
Cette thèse est une étude des divers processus d'Ionisation collisionnelle d'atomes de rubidium très excités, à énergie thermique (450° K).Dans une cellule, les atomes alcalins excités par laser sont ionisés par collision avec un partenaire excité ou dans l'état fondamental.L'analyse des courants ioniques atomiques et moléculaires met en évidence les réactions principales d'Ionisation Penning atomique et Hornbeck Molnar associative :Rb(nl) + Rb(5p) => Rb+ + Rb + e-Rb(nl) + Rb(5s) => Rb2+ + e-L'étude théorique de ces réactions confirme les taux de réaction mesurés,La formation d'ions négatifs est aussi observée. Ces ions sont issus de la réaction :Rb(nl) + Rb(5s) => Rb+ + Rb-Les mécanismes de formation de la paire d'ions dépendent de la position en énergie du niveau excité initial.Enfin la section efficace du processus inélastique d'association d'énergie a été mesurée et interprétée :Rb(5p) + Rb(5p) => Rb(5d) + Rb(5s)
The cross section σ = (8 ± 4) × 10 −15 cm 2 was determined for the Rb(7S) + Rb(5S) → Rb(5D) + Rb(5S) excitation energy transfer process, and the quenching cross section σ q = (2 ± 1) × 10 −14 cm 2 for the Rb(5D) state in collisions with ground state Rb atoms. Applying rubidium quasimolecular asymptotic potential curves at relatively large internuclear distances, a qualitative interpretation of the experimental results is presented. It is shown that the quenching of the Rb(5D) atoms in collisions may be explained by a reversed energy pooling process Rb(5D) + Rb(5S) → Rb(5P) + Rb(5P).
The mechanisms of nonresonant excitation transfer process in thermal collisions of two rubidium atoms Rb(7S) + Rb(5S) → Rb(5D) + Rb(5S) was studied theoretically, under conditions when the initial and final states of a system of two colliding atoms are not connected with each other by electric multipole transitions. Population processes of Rb 5D states were considered using calculated adiabatic potential curves of the Rb 2 quasimolecule in asymptotic approximation. The calculated cross-sections (σ theor = 5.0 × 10 −15 cm 2 ) are in good agreement with experimental results [σ exp = (8.0 ± 4.0) × 10 −15 cm 2 ] at thermal collision energies ( T ∼ 500 K).
Abstract We have studied the formation of the molecular ion Rb2+ and the atomic ion Rb+. These are created in laser excited rubidium vapor at the first resonance, 5s–5p and 5p-nl transitions. A theoretical model is applied to this interaction to explain the time evolution and the laser-power dependence of the population density of Rb+ and Rb2+. A set of rate equations which describe: the temporal variation of the population density of the excited states; the atomic ion density; and the electron density, were solved numerically under the experimental conditions of Barbier and Cheret. In their experiment the Rb concentration was 1×1013cm−3 and the laser power was taken to be 50–500 mW at vapor temperature = 450 K. The results showed that the main processes for producing Rb2+ are associative ionization and Hornbeck-Molnar ionization. The calculations have also showed that, the atomic ions Rb+ are formed through the Penning Ionization (PI) and photoionization processes. Moreover, a reasonable agreement between the experimental results and our calculations for the ion currents of the Rb+ and Rb2+ is obtained.