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Journal of Physics B Atomic and Molecular Physics

Collisional ionisation of high-Rydberg atoms. Diffusive mechanism

B. Kaulakys, Vitas Švedas

Published Sep 14, 1987• 7 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3700/20/17/005

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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.
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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 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.
A stochastic model of associative ionization in collisions of Rydberg atoms with ground-state atoms is presented. The conventional Duman–Shmatov–Mihajlov–Janev (DSMJ) model treats the ionization as excitation of Rydberg electron to the continuum by the electric-dipole field generated by exchange interaction within the quasi-molecular ion. The stochastic model essentially extends this treatment by taking into account redistribution of population over a range of Rydberg states prior to ionization, which is caused by non-adiabatic processes in overlapping multiple level crossings of quasi-molecular Rydberg states. The redistribution is modelled as diffusion of electrons in the Rydberg energy spectrum using a Fokker–Planck-type equation. The process of l -mixing of Rydberg states at large internuclear distances and twisting of the collision trajectories on attractive potentials are taken into account. The choice of the collision velocity distribution is also shown to be important. Associative ionization rates have been calculated for Na**( nl ) + Na collisions with n = 5–25 and l = 0, 1, 2, and compared with the available experimental data and the calculations performed using the nonlinear DSMJ model. At relatively low n the stochastic model yields a substantially better agreement with the experimental data than the DSMJ model, while the results of both models converge at large n .
The author obtains simple analytical expressions for the cross sections of the state-changing and ionisation collisions of Rydberg atoms with neutral atomic particles due to the Rydberg-electron-perturber interaction. The theoretical investigation is based on the impulse approximation and binary encounter approach for the form factor. The earlier results of Alekseev and Sobel'man (1965) and Omont (1977) turn out to be the limiting cases of the author's expressions. The calculated quenching cross sections for the Rydberg states agree well with experimental results and numerical calculations.