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Journal of Physics B Atomic and Molecular Physics

Experimental study of Penning and Hornbeck-Molnar ionisation of rubidium atoms excited in a high s or d level (5d⩽nl⩽11s)

L. Barbier, M. Chéret

Published Mar 26, 1987• 61 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3700/20/6/011

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Electron energy distribution function in laser-excited rubidium atoms

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Journal of Physics B Atomic Molecular and Optical Physics•May 9, 2005•11 citations
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.
Highly excited and resonant states are created in a rubidium vapour in order to study the collisional ionisation with the ground state and between these different excited species. A mass analysis allows one to separate the atomic and molecular channels. Penning ionisation with a resonant state is shown to have a great efficiency (rate coefficient greater than 10 -8 cm 3 s -1 ) for all the highly excited levels under study (6d, 8s, 7d). The associative ionisation rate coefficient between these excited atoms and a ground-state atom lies in the expected range 10 -10 to 5*10 -10 cm 3 s -1 and seems to be independent of l j and values.
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.
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.
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 electron energy distribution function (EEDF) in laser-excited Rb vapour at the first resonance transition 5S–5P and 5p– nl transitions is calculated numerically from the Boltzmann equation under the experimental conditions of Barbier and Cheret ( 1987 J. Phys. B: At. Mol. Phys. 20 1229 ). The calculations included all the interactions between electrons and ground or excited states of atomic rubidium. We have also studied the time evolution and the laser power dependences of the EEDF and the electron density which is created during the interactions. The energy spectra of the electrons emerging from the interaction contain a number of peaks corresponding to the low-energy electrons produced by the associative and Hornbeck–Molnar ionization in addition to the electrons created from the various excited states by the ionizing collisions (Penning ionization). The low-energy electrons are then heated by one or more superelastic collisions leading to further ionization. However the calculations indicated that, under the conditions of low power laser intensity and relatively low atom density, the dominant processes are collisional ionization and collisional excitation.