Investigation Of Ultrafast Dynamics Of Frustrated Double Ionization Electrons Of Molecules In Strong Laser Fields

Many nonlinear photoionization phenomena are generated when ultrafast laser interacts with atoms and molecules.For example,sequential double ionization(SDI),non-sequential double ionization(NSDI)and frustrated double ionization(FDI).At present,the details of electron emission dynamics underlying FDI of strong field molecules need to be clarified.Moreover,compared with atoms,the molecular structure is more complex(such as molecular alignment,internuclear distance),which makes the electron emission dynamics of FDI molecules have more degrees of freedom.In order to explore the details of molecular FDI electron emission dynamics,we study the linear polarized double ionization of nitrogen molecules by using a three-dimensional classical model.This paper carried out the following research work:(1)The dependence of molecular FDI electron emission dynamics on laser wavelength is studied.Two mechanisms of single-recollision and multiple-recollision are found by tracing the classical trajectories.When the laser wavelength is shorter,the two collision mechanisms contribute equally to the FDI yield.However,in the case of longer wavelength,the contribution of single-recollision mechanism to FDI yield plays a dominant role.At different wavelengths,we also explored the effect of the angle between the molecular axis and the polarization direction of the laser field on the molecular FDI electron emission dynamics,and found that with the increase of the angle,the yield of FDI decreases under the same wavelength.In particular,in molecular FDI,compared with atomic FDI,the ionization-exit momentum of the recaptured electron matches the laser vector potential to a large extent.(2)The FDI electron dynamics of molecules with laser intensity at recollision threshold intensity is studied.The inversion analysis shows that FDI of nitrogen molecules is mainly generated through two kinds of ionization channels,and the corresponding ionization exit velocity distribution deviates greatly or slightly from the vector potential line.The final momentum distribution of ionized electrons corresponding to the two ionization channels corresponds to the large momentum and small momentum regions respectively.With the increase of laser intensity,the large momentum electron yield decreases,resulting in the change of the final momentum distributions of ionized electrons with the laser intensity,showing a double-hump structure or a four-hump structure.(3)The dependence of nitrogen molecular FDI on internuclear distance under different molecular alignment.The results showed that the yield of FDI with internuclear distance changes in the same trend under different molecular alignment.Compared with DI,with the increase of internuclear distance,the yield of FDI also increases rapidly first and then decreases slowly.The inverse analysis shows that molecular FDI is mainly generated through recollision excitation with subsequent ionization.With the increase of internuclear distance,the ionization exit velocity of the final small-momentum electron is close to the vector potential,while the ionization exit velocity of the final large-momentum electron is far away from the vector potential.As a result,the momentum distribution of the ionized electrons in FDI changes from a four-hump structure to a double-hump structure with the increase of internuclear distance.