Complex Mechanism Of Nonsequential Double Ionization Of Atoms In Intense Laser Fields

With the development of laser technology,many new physical phenomena can happen,when ultra-short and intense laser pulses interact with atoms or molecules.These phenomena include multiphoton ionization(MPI),above-threshold ionization(ATI),sequential double ionization(SDI),nonsequential double ionization(NSDI),high-orde harmonic generation(HHG),attosecond pulses generation(APG),et al.,which have received wide attention from both theoretical research and practical application.Between electron and electron in NSDI process are highly correlated,so that NSDI becomes complicated topic among these new phenomena.NSDI reveals fundamental and complex electron dynamics in atomic and molecular physics,which is a great subject of research value.Since an obvious “V” structure has been observed in the correlated electron momentum distribution of He atom,which indicates that it is not only collision ionization in NSDI,but also a more complex microscopic dynamic in recollision process.A large number of experimental work and theoretical researches have been studied NSDI of rare gas atoms in intense laser fields,such as NSDI of Ar atom in linearly polarized(LP)laser fields.However,few experiments and theories focus on NSDI process of Mg atoms,relatively.In addition,constantly updated laser technology has promoted the research of complex mechanism in NSDI,when different laser fields interact with atoms.In 2016,NSDI in counter-rotating two-color circularly polarized(TCCP)laser fields has been observed for the first time.Moreover,the double ionization probability was enhanced or suppressed by adjusting the laser field relative intensity ratio.Since the TCCP laser field has many adjustable parameters,such as combined electron field amplitude ratio,relative phase and elliptical polarization,this allows us to shape different forms of laser pulses to control the dynamic behavior of electrons in NSDI.Therefore,the electron trajectory in counter-rotating TCCP laser fields becomes complicated,and the significant change depends on the shape of the driving fields.The observations and theoretical analysis have inspired a wave of research.In this paper,we explore the complicated mechanism of NSDI for atoms in circularly polarized(CP)laser fields and counter-rotating TCCP laser field,respectively,which bases on the new theoretical researches and experimental works.The main results are summarized as follows:The temporal correlation and momentum correlation of NSDI for Mg atoms are investigated more detail in CP laser fields.We perform the statistical analysis of the momentum distribution of fast and slow electrons,and the temporal relation of the ‘side-by-side emission' and the ‘back-to-back emission',respectively.The electron trajectories from recollision-induced ionization(RII)and recollision-induced excitation with subsequent ionization(RESI)mechanisms are distinguished through the different time delay.The electron-electron correlation is also investigated at the end of the laser fields by analysis of the ion momentum distribution and the distribution of the angle between the emission directions of the two electrons.Two ionized electrons of the RESI mechanism are more likely to be ionized in opposite directions,which indicates that the anticorrelation between two electrons is predominant.However,the two ionized electrons of the RII mechanism are more likely to be ionized in the same direction,which indicates that the correlation between two electrons is predominant.We perform the statistical analysis of the trajectories of NSDI of Mg atoms in 8-cycle CP laser fields.Simulations are implemented for the time evolution of the electron energy distribution,the time evolution of the repulsion energy distribution III between two electrons in the DI process and the time evolution of the distance distribution between the nucleus and the two electrons,and for the trajectories of ionized process.The dependence of the electron momentum distribution on the angle between the momentum and the force of laser field at the time of the first electron is also investigated.The theoretical results indicate that a single recollision leads to the NSDI process.In addition,the initial transverse velocity to compensate the drift velocity will decrease,as the angle between the momentum and the force of laser field at the time of the first electron decrease,leading the different recollision time of the released electron.The recollision dynamics in strong-field NSDI of Ar atoms by counter-rotating TCCP laser fields is investigated.With the analysis of the NSDI trajectories,we find that not only double-recollision(DR)but also single-recollision(SR)processes occur in the double ionization events.Furthermore,the multiple-recollition and single-recollision processes both undergo the RESI and RII.The ion momentum distribution shows a prominent anticorrelated behavior in the RESI and RII mechanisms for single-recollision and multiple-recollision event,which is different form the phenomenon in LP laser fields.However,the RESI and RII mechanisms follows that the anticorrelated phenomenon is prominent for the RESI mechanism and correlated phenomenon is prominent for the RII mechanism.The related reasons for single-and double-recollision in NSDI are discussed under the same laser field.The probability of the SR and DR as a function of the delay time is shown,which indicate that the double ionization of the SR events occur more quickly.We demonstrate the probability distribution of the recollision energy by tracing the energy transfer during the recollision processes.The energy transfer during the recollision for the SR is slight larger than that for the DR.Thus,the double-recollision NSDI event occurs a little slower than the single-recollision NSDI event.From the statistical analysis of the angle between the momentum and the force of the laser field and the momentum at the recollision moment,we found that the NSDI occur in DR evens with the decrease of angle and the increase of momentum.We theoretically investigate the effect of elliptically polarizations on NSDI of Ar and Mg atoms in two-color elliptically polarized laser fields.Comparing the theoretical results,we found that the difference physical phenomenon between the two atoms as follows:(a)the probabilities of the double ionization versus laser intensity for Ar atoms increase as the ellipticity increases,but it is almost unchanged for Mg atoms;(b)the correlated behavior is influenced by the ellipticity by analysis of the ion momentum and the correlated electron momentum along the x-direction distribution of Ar atoms;(c)The correlated momentums along the x-direction distributions at the time of recollision are shown,and it indicates that the ellipticity and the travelling time can affect the emitted directions of the both electrons.