Introduction

The creation of doubly charged ions upon the multiphoton ionization of atoms and molecules was observed for the first time in[1]. Discovery of the effect of the creation of doubly charged ions upon multiphoton atomic ionization laid the foundation of a new branch in the physics of nonlinear ionization processes that involved the creation of multicharged ions due to the multiphoton or tunneling ionization of atoms and molecules.

In later experiments, the creation of doubly charged ions was revealed in experiments with atomic barium, samarium, europium, and ytterbium[2−4]. The creation of doubly charged ions was also observed upon multiphoton ionization of inert gas atoms and other atoms and molecules[5]. Experimental results have demonstrated the general character of the creation of doubly charged ions upon multiphoton ionization of various groups of atoms.

In this chapter, we will discuss the creation of singly and doubly charged ions upon multiphoton atomic ionization of alkaline-earth elements using laser radiation with nanosecond pulse duration at laser field strengths ϵ = 5 x 106 − 109 V/cm, which are significantly less than the intra-atomic field ϵa = 5x109 V/cm. The results of using this field strength significantly differ from the recent results obtained at the laser field strength ϵ ˂ 5 x 109 V/cm or ϵ > 5 x 109 V/cm in experiments with femtoseconds and atto second laser pulses[6]. The differences include the domination of the tunneling ionization of atoms and molecules under such conditions and the creation of doubly charged ions. The main processes are electron rescattering, shaking, etc.[6].

The processes that lead to the creation of doubly charged ions upon multiphoton ionization of the alkaline-earth elements under the given conditions differ from the above processes. Two effects (cascade and two-electron)were proposed in[7, 8] to account for the creation of doubly charged ions upon the multiphoton ionization of alkaline-earth elements. In the cascade effect, doubly charged ions (A2+) are created owing to themultiphoton ionization of singly charged ions (A+) that appear in the presence of the same laser pulse due to themultiphoton ionization of atoms A : A+K0 ħω−A++ e and A+ +K1 ħω−A2+ +e.