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Journal of Experimental and Theoretical Physics

Probability of the two-electron mechanism of the formation of doubly charged barium ions as a function of laser radiation intensity

І. І. Бондар, В. В. Суран, Denys I. Bondar

Published Jun 1, 2013• 1 citation
Publisher

Abstract

The formation of singly and doubly charged ions upon nonlinear ionization of barium atoms is studied as a function of the frequency and intensity of a 8800-8920-cm−1 IR color center laser. Barium atoms are ionized via the four-photon resonance with the strongly perturbed bound 6p 2 1 D 2 state. Doubly charged barium ions are produced by a two-electron mechanism. It is found that the probability of the formation of doubly charged ions under these conditions linearly depends on the laser radiation intensity.

DOI
10.1134/s1063776113060034

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The formation of singly and doubly charged ions upon nonlinear ionization of barium atoms is studied as a function of the frequency and intensity of a 8800-8920-cm−1 IR color center laser. Barium atoms are ionized via the four-photon resonance with the strongly perturbed bound 6p 2 1 D 2 state. Doubly charged barium ions are produced by a two-electron mechanism. It is found that the probability of the formation of doubly charged ions under these conditions linearly depends on the laser radiation intensity.
The process of formation of doubly charged ions upon multiphoton ionization of barium atoms by linearly and circularly polarized radiation of dye lasers is studied in the frequency range from 16800 to 18700 cm −1 . A large number of resonance maxima in the yield of Ba + and Ba 2+ ions were observed upon tuning the radiation frequency. It is unambiguously proved that most resonance maxima in the yield of Ba 2+ ions are associated with the resonance transitions in the spectrum of Ba + ions. This fact confirms the cascade mechanism of formation of Ba 2+ ions in the frequency range of localization of these maxima.
The process of formation of doubly charged ions upon multiphoton ionization of barium atoms by linearly and circularly polarized radiation of dye lasers is studied in the frequency range from 16 600 to 17 900 cm-1. A large number of resonance maxima in the yield of Ba+ and Ba2+ ions were observed upon tuning the radiation frequency. It is unambiguously proved that most resonance maxima in the yield of Ba2+ ions are associated with the resonance transitions in the spectrum of Ba+ ions. This fact confirms the cascade mechanism of formation of Ba2+ ions in the frequency range of localization of these maxima. But the number of the resonant maxima in Ba2+ ions yield do not identified by transitions both in spectra of Ba atom and Ba+ ion. This fact indicates on realization of the noncascade mechanism of Ba2+ ions formation in the frequency range of these maxima.
Despite inherently complex multiphoton dynamics, our observations show that Ba double ionization with an infrared laser (8800--8920 cm${}^{\ensuremath{-}1}$) resembles a single-photon process; namely, its probability is proportional to the laser intensity. In this regime, single-electron ionization is due to a four-photon resonant transition through the highly perturbed state 6p21D26{p}^{2}\phantom{\rule{0.16em}{0ex}}{}^{1}{D}_{2}6p21D2​, whereas double ionization is realized by the two-electron mechanism. Furthermore, we argue that these conclusions are valid for other alkaline-earth-metal atoms and other parameters of laser radiation.
The influence of an additional perturbation (the ac Stark effect) of neutral barium atoms on the formation of doubly charged ions in the course of ionisation of these atoms by visible light (by the second harmonic radiation of an yttrium-aluminium garnet laser) was investigated experimentally. The frequency-tunable radiation of a colour-centre laser (ω = 8500 — 8950 cm -1 ) was used in these investigations to effect the additional perturbation of barium atoms. It was found that this perturbation changed the mechanism of formation of the doubly charged Ba 2+ ions from a stepwise to a two-electron mechanism. The yield of the Ba 2+ ions increased sharply under these conditions.