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Journal of Physics B Atomic Molecular and Optical Physics

The two-electron mechanism of the formation of doubly charged ions by the multiphoton ionization of Ba atoms with two-laser radiation

І. І. Бондар, В. В. Суран

Published Aug 7, 2002• 12 citations
Publisher

Abstract

The experimental investigation of the formation of doubly charged ions by the multiphoton ionization of Ba atoms by simultaneous irradiation with the fundamental radiation of a colour centres laser (ω = 8800–8880 cm −1 ) and its second harmonics was carried out. A two-electron mechanism was observed to be responsible for the formation of these ions; they are formed as a result of the excitation and subsequent ionization of strongly perturbed states of neutral Ba atoms.

DOI
10.1088/0953-4075/35/16/302

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Journal of Experimental and Theoretical Physics•Jun 1, 2013•1 citation
The experimental investigation of the formation of doubly charged ions by the multiphoton ionization of Ba atoms by simultaneous irradiation with the fundamental radiation of a colour centres laser (ω = 8800–8880 cm −1 ) and its second harmonics was carried out. A two-electron mechanism was observed to be responsible for the formation of these ions; they are formed as a result of the excitation and subsequent ionization of strongly perturbed states of neutral Ba atoms.
Summary form only given. Beginning with first experiments, where the effect of doubly-charged ions formation was discovered, intensive investigations to determine the mechanism leading to the production of doubly-charged ions in different spectral ranges are carried out. The main goal of our work is the creation of conditions for realization of direct two-electron mechanism of Ba/sup 2+/ ions formation in the visible spectral range. The experimental setup consists in following: during ionization of Ba atoms by visible laser radiation we realized strong additional perturbation of atoms by laser radiation of another frequency range.
Experimental investigations of resonant structure in doubly charged ion (A 2+ ) formation were carried out during Sr and Ba atom ionization by infrared laser radiation (ω = 8400-9100 cm -1 ). It has been shown that in this case the mechanism of ion formation is a two-electron process - the ions are produced immediately from the neutral atoms. It has been found for the first time that the resonant structure in A 2+ ion formation in such conditions is due to the excitation and ionization of neutral-atom mixed states. The atomic-state mixing in this case results from the Stark effect provided that the polarizability of these states is very high.
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 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.
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.