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Journal of Physics B Atomic and Molecular Physics

Hornbeck-Molnar ionisation in a K-Rb mixture

M. T. Djerad, M. Chéret, F. Gounand

Published Aug 14, 1987• 9 citations
Publisher

Abstract

Rate coefficients for Hornbeck-Molnar ionisation in a K-Rb mixture at thermal energy are measured using CW laser excitation of the atoms. Special attention is paid to the formation of the KRb + molecular ion in Rb(nl)-K(4S) and K(nl)-Rb(5S) collisions. The absolute rate constant values are deduced from the analysis of collisional currents by mass spectrometry techniques. They are compared with the theoretical values calculated using a model recently developed by Janev and Mihajlov (1980).

DOI
10.1088/0022-3700/20/15/024

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Exchange and direct mechanisms in heteronuclear alkali Penning ionisation

M. T. Djerad, M. Chéret, F. Gounand

The authors measure rate coefficients for Penning ionisation occurring in a vapour mixture of rubidium and potassium at thermal energy. The experimental technique uses the mass spectrometry analysis of collisional currents resulting from continuous laser excitation of the atoms. They report the first experimental evidence concerning the non-occurrence of an exchange-type mechanism. Although a second exchange-type reaction cannot be discarded by their experimental results, it seems that Penning ionisation, involving low-lying excited alkali atoms, most probably proceeds via the direct mechanism.

Journal of Physics B Atomic and Molecular Physics•Aug 14, 1987•4 citations
Rate coefficients for Hornbeck-Molnar ionisation in a K-Rb mixture at thermal energy are measured using CW laser excitation of the atoms. Special attention is paid to the formation of the KRb + molecular ion in Rb(nl)-K(4S) and K(nl)-Rb(5S) collisions. The absolute rate constant values are deduced from the analysis of collisional currents by mass spectrometry techniques. They are compared with the theoretical values calculated using a model recently developed by Janev and Mihajlov (1980).
An investigation of the 'Hornbeck-Molnar ionization' process between laser-excited Cs and ground state Na atoms has been performed in a crossed beam apparatus. Caesium atoms in the 8d, 10s, 9d, 11s and 10d levels respectively have been obtained by two-step resonant CW laser excitation. The relative collisional cross sections and the dissociation energy of the NaCs + molecular ion have been obtained from quantitative analysis of the produced atomic and molecular ions. The experimental results are compared with the theoretical values calculated using a model developed by Janev and Mihajlov (1980).
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.
Cette etude porte essentiellement sur les collisions ionisantes mettant en jeu des etats excites dans un melange de vapeurs saturantes K-Rb, a energie thermique. La methode experimentale consiste en l'excitation laser continue et resonnante en deux etapes des etats de Rydberg intermediaires (n 10) et l'analyse par spectrometrie de masse du courant d'ions. La relaxation radiative et colli­ sionnelle des atomes engendre un milieu complexe. Les processus collisionnels les plus efficaces sont l'ionisation Penning et l'ionisation Hornbeck-Molnar. Dans le systeme heteronucleaire Rb(nl) + K(4P) on pourrait s'attendre aux voies de sortie : Rb(nl) + K(4P) -- + e+ K + e + Rb + e. Les mesures montrent que la premiere voie a une section efficace moyenne de 10-13 cm2. Celles des autres sont au moins trois ordres de grandeurs plus petites et sont donc comparativement negligeables. L'ensemble des resultats obtenus a partir des niveaux 50 a lOS permet de montrer que le flux de la voie d'entree s'ionise a grande separation entre Rb(nl) et K(4P). Le processus d'ionisation est domine par les forces de polarisation, les forces d'echange etant comparativement negligeables. Dans le present melange, l'ionisation Hornbeck- Molnar conduit a la formation d'ions homonucleaires K2+, Rb2+ et heteronucleaires KRb+. Nous avons mesure les taux de reaction dans les systemes K ( nl) + Rb--KRb+ + e Rb(nl) + K--KRb+ + e-. Les taux de reaction augmentent avec l'enersie d'excitation de l'etat nl, ils ne presentent pas de differences fondamentales avec ceux connus dans les alcalins en phase vapeur pure.
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.
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 authors measure rate coefficients for Penning ionisation occurring in a vapour mixture of rubidium and potassium at thermal energy. The experimental technique uses the mass spectrometry analysis of collisional currents resulting from continuous laser excitation of the atoms. They report the first experimental evidence concerning the non-occurrence of an exchange-type mechanism. Although a second exchange-type reaction cannot be discarded by their experimental results, it seems that Penning ionisation, involving low-lying excited alkali atoms, most probably proceeds via the direct mechanism.