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Journal of Physics B Atomic and Molecular Physics

Penning and associative ionisation of highly excited rubidium atoms

M. Chéret, L. Barbier, W. Lindinger, Robert Deloche

Published Oct 14, 1982• 63 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3700/15/19/015

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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.
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.
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.
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.
Rubidium vapor (1011 - 1014 cm-3) was irradiated by a dye-laser beam (10 kW(DOT)cm-2 - 2 MW(DOT)cm-2, pulse duration approximately 30 nsec, linewidth <EQ 0.25 cm-1), which was tuned to the D1,2-resonance lines. The mutual competition of the two photon ionization, associative ionization and Penning ionization of the laser excited rubidium atoms was investigated. The data of laser intensities and a rubidium vapor concentrations, which results in domination of the each ionization channels were founded. On the basis of the received experimental results, the cross-section values for the multiphoton and collisional ionization channels of laser-excited atoms were determined.
The ionisation yields of the optically excited caesium atoms (9.4<or=n*<or approximately=29) are measured in a vapour cell at a caesium vapour density of about 10 14 cm -3 . Direct ionisation rate constants are given for Rydberg atoms with 9.4<or=n*<or=21.4. Collisions with caesium ground-state atoms and black-body radiation strongly modify the population of Rydberg levels with n*>or approximately=25 so the concept of direct collisional ionisation cannot be applied to those levels. The diffusional ionisation mechanism of Rydberg atoms in vapour cells, atomic beams or buffer gases is discussed.
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.