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Journal of Physics B Atomic and Molecular Physics

Collisional ionisation probabilities of highly excited S1/2, D3/2and D5/2states of caesium

P P Herrmann, John A. Hoffnagle, Nick Schlumpf, D. Taqqu, V. L. Telegdi, Antoine Weis

Published Jun 14, 1986• 6 citations
Publisher

Abstract

The authors present experimental results on the ionisation probabilities of highly excited Rydberg S and D states of caesium in collisions with ground-state Cs atoms. The measurements were done in a thermionic diode using two-photon excitation of Rydberg states with principal quantum numbers in the range 18 to 60. Over the investigated range (0.17 to 12 mTorr) a simple exponential dependence of the ionisation probabilities on binding energy and pressure was found. The authors also measured the intensity ratios of the fine-structure doublets in 6S 1/2 to nD two-photon transitions and find good agreement with theoretical calculations. From the results they conclude that the collisional ionisation probabilities are independent of orbital and total angular momentum.

DOI
10.1088/0022-3700/19/11/021

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The authors present experimental results on the ionisation probabilities of highly excited Rydberg S and D states of caesium in collisions with ground-state Cs atoms. The measurements were done in a thermionic diode using two-photon excitation of Rydberg states with principal quantum numbers in the range 18 to 60. Over the investigated range (0.17 to 12 mTorr) a simple exponential dependence of the ionisation probabilities on binding energy and pressure was found. The authors also measured the intensity ratios of the fine-structure doublets in 6S 1/2 to nD two-photon transitions and find good agreement with theoretical calculations. From the results they conclude that the collisional ionisation probabilities are independent of orbital and total angular momentum.
The ionisation of highly excited Rydberg atoms in collisions with ground-state atoms is considered. The stationary diffusion-like mechanism of the ionisation is presented. The investigation is based on the Fokker-Planck equation. Analytical expressions for the probabilities of stationary diffusive ionisation are obtained and applied for the explanation of experimental results on the ionisation of highly excited Rydberg states of caesium in collisions with ground-state Cs atoms. It is concluded that the diffusive mechanism is the main mechanism for collisional ionisation of Rydberg atoms with n>or approximately=25-30.
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.
Applying the cw laser absorption and fluorescence method the cross sections for the fine-structure mixing and quenching of the caesium 5D J states have been measured in pure caesium vapour. Caesium atoms were optically excited to the 5D 5/2 state via the quadrupole-allowed 6S 1/2 →5D 5/2 transition. The ground-state caesium density N 0 , obtained from the absorption at the pumped quadrupole transition, and the fluorescence intensity I 689 of the sensitized 5D 3/2 →6S 1/2 emission were measured simultaneously in the range 3×10 14 cm -3 ⩽ N 0 ⩽1×10 16 cm -3 at constant temperature T = 585 K. It was found that the quantity N 0 2 / I 689 exhibited a parabolic dependence on N 0 , confirming that the quenching of the Cs 5D J states is due to collisions with Cs ground-state atoms, not molecules. The coefficients of the second-order polynomial fitted through the measured data yielded the cross sections σ 5/2→3/2 = (57±19)×10 -16 cm 2 and σ D = (35±10)×10 -16 cm 2 for the Cs 5D J fine-structure mixing and quenching, respectively, due to collisions with caesium ground-state atoms. Using recently calculated Cs*+Cs potentials we performed an analysis which shows good agreement between the measured values and the theoretical predictions.
Collisional transfer of excitation energy from cadmium to caesium was investigated using methods of sensitized fluorescence. A mixture of cadmium and caesium vapors at low pressure was irradiated with Cd 3263.0 Å resonance radiation producing a population of Cd (53P1) atoms whose inelastic collisions with ground-state Cs atoms resulted in a transfer of excitation energy to a close-lying S, P, D and F states in caesium. The subsequent decay of these states manifested itself in the emission of sensitized fluorescence spectrum. Intensity measurements on the components of the spectrum by means of energy calibrated detector yielded some 50 cross-section values whose magnitudes range from 0.2 Å2 to 30.0 Å2 and which exhibit a pronounced resonances with ΔE the energy defect between Cd (53P1) and the appropriate level in caesium.
By using a perturbative treatment based on the central field approximation, the authors have calculated multiphoton ionisation probabilities in several cases of experimental interest (two-, three- and four-photon ionisation of the caesium and potassium ground states at the doubled Nd glass laser frequency and one-, two- and three-photon ionisation of some s, p, d and caesium Rydberg states). Some of these probabilities have been calculated beyond the minimum non-vanishing order. They have also performed some calculations for hydrogen. The comparison of their results with previous ones obtained by analytical methods provides a good test of the numerical techniques involved in the calculations.
In the two-photon ionisation of Cs atoms resonances may be expected which correspond to a quadrupole 6s-nd transition associated with the first photon. At the 6D 5/2 and 6D 3/2 resonances the ratio of photoionisation cross sections for circularly polarised and for linearly polarised light (ion asymmetry) and the spin polarisation of the photoelectrons ejected by circularly polarised light have been measured. The ratios of the radial transition matrix elements from the 6D to the epsilon P and epsilon F continuum states were derived from the experimental results.