Abstract

Introduction. Previous studies have showed that the mechanism of doubly charged ions formation is different for different ionization regimes and different laser wavelengths. In particular, the cascade creation of doubly charged ions dominates when the visible radiation interacts with the alkaline-earth atoms. In the case of the multiphoton ionization of the alkaline-earth atoms by the infrared laser radiation, the two-electron process occurs.
Purpose. The aim of this research was to study the formation mechanism of doubly charged multiphoton ionization of the Ba atoms resulting from the simultaneous action of two laser radiations: visible and infrared. 
Methods. The experiments employ the method of crossed atomic and laser beams. The laser radiations are focused into the beam of Ba atoms. The ions are pulled from the beams interaction region by an electric field, separated with respect to the charge and mass in the time-of-flight mass spectrometer, and detected by an electron multiplier. We used the fundamental radiation of laser on yttrium-aluminum garnet (LYAG) (ω = 9395 cm^–1) and the LYAG second harmonic (ω = 18790 cm^–1). 
Results. The experiments on the simultaneous action of the fundamental LYAG radiation and its second harmonic on the atomic barium show that the Ba²⁺ ions yield significantly (by two orders of magnitude) increases in comparison with the ion yields corresponding to the separate action of the two laser beams. We measured the dependence of the Ba²⁺ ions yield on the laser intensity upon the multiphoton ionization of the Ba atoms resulting from the simultaneous action of the mentioned two lasers. The obtained dependence is in good agreements with calculations made in the framework of the two-electron mechanism. 
Conclusion. The investigation results show that the Ba²⁺ ions are produced in accordance with the two-electron mechanism under the simultaneous action of the fundamental and the second harmonic of LYAG radiation

Keywords: multiphoton ionization, doubly charged ions, two-electron mechanism, the dynamic Stark effect

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