Study On Double Ionization Of Strontium And Calcium Atoms In High Intensity Laser Field

The non-sequential double ionization of atoms and molecules with strong laser field is of great significance to the study of strong field physics.It reveals the electronic correlation effect in the double ionization process and provides a simple method for people to study the electronic correlation in nature.Double ionization mechanism can be distinguished into sequential double ionization(SDI)and non-sequential double ionization(NSDI).Metal atoms have shells consisting of active electrons,and their double ionization process still has a large margin in the academic field,which has always been a hot spot for scientific researchers.Research of NSDI nowadays on Mg atoms has been conducted,including electron correlation microdynamics,and the dependence of NSDI on laser wavelength and laser polarization.It has been discovered that the metal atom Mg has a "knee" structure under circularly polarized laser,which is consistent with Heavy impact ionization mechanism.But researches on other metal atoms is not in-depth,especially the experimental exploration of NSDI.Theoretical exploration of NSDI of atoms and molecules has formed a system.There are four common methods:the classical ensemble model,the semi-classical ensemble model,numerically solving the Schrodinger equation,and the quantum Monte Carlo method.NSDI belongs to the category of quantum physics,but because NSDI has two electronic systems,it is necessary to follow the time-dependent Schrodinger equation in three-dimensional space.If the quantum Monte Carlo method is used to explain NSDI,a lot of calculations are required.Therefore,in this paper,we use the classical ensemble model,a method with high computational efficiency and less data for calculating the strong field ionization,compared with our experimental results,to further explore the ionization mechanism of NSDI and fill the margin in the experiment of NSDI.We have empirical studied the ionization process of alkaline-earth metal atoms Sr and Ca with strong laser.Sr and C a are typical alkaline-earth metal atoms with two active electrons in the outermost layer.The experiment uses 50 fs,800 nm and 400 nm lasers interacting with metal atom beams combined with a velocity imaging system to detect ions and electrons during the ionization process.The main work and innovations of this paper are as follows:(1)Explore the applicability of the classic ensemble model to describe the NSDI process.We compared the experimental results with theoretical calculations based on the classical ensemble model.We found that the double ionization yield of Ca atoms in the 800 nm laser varies with light intensity well in line with the results given by the classical model,and the mechanism of NSDI is analyzed from the two directions of threshold ionization and energy trajectory.At the same time,through the comparative analysis of experimental results and PPT theoretical calculations,we quantitatively distinguished SDI and NSDI.(2)Explore the ionization dynamics of NSDI in a circularly polarized laser field.It is observed that under the conditions of 400 nm and 800 nm circularly polarized light of Sr and Ca atoms,the experimental results have a "knee" structure,but the area is very small,and the existence of the "knee" structure has been proved in the classical ensemble simulation and PPT theory.NSDI occurs in two ways:double-impact excitation reionization and collision-induced direct ionization.At low laser intensity,NSDI mainly occurs through double-impact excitation reionization.(3)NSDI of alkaline-earth metal atoms driven by elliptically polarized light.Experiments show that the yields of monovalent and divalent ions of metal Sr and Ca under different ellipsometry conditions change with light intensity.We found that with the increase of ellipsometry,the ionization yield gradually decreases,indicating that electron re-scattering is the key mechanism to cause double ionization.With low light intensity,the change of ion yield with ellipticity is more obvious,indicating that double-impact ionization plays an important role in double ionization.