| Strong field physics is built upon the interaction between the laser field with matter,which contains many physical phenomena such as tunneling ionization(TI),multiphoton ionization(MPI),high harmonic generation(HHG),etc.HHG is a highly nonlinear process,it can be illustrated by the widely used three-step model proposed by Corkum.The electron can be ionized to the continuum state;then the freed electron is accelerated by the driving laser and gains kinetic energy;finally,the electron has chance recolliding with parent ion emitting coherent radiation.It has been a powerful tool for obtaining coherent extreme ultraviolet(XUV)and attosecond pulse.Ionization is one of the fundamental processes within laser-matter interaction,photoelectron momentum distribution(PMD)is widely employed to investigate the electron ionization dynamics as well as the structure of atom or molecule.In the current paper,we sysmetically investigate the HHG of CS2 molecule as well as the PMD of hydrogen atom and H2+ molecule.The detailed researches are listed as follows:The HHG from the CS2 molecule in intense laser fields is investigated using the extended Lewenstein method.The initial state is the highest-occupied molecular orbital of the CS2 molecule,which can be well described by Gaussian wave packet using GAMESS-UK package.The HHG of CS2 molecule under the elliptically polarized laser pulse and the two-color circularly polarized laser pulse are plotted.Besides,the HHG of hydrogen atom is also plotted for comparison.It can be seen that the HHG cutoff can be extended in two-color circularly polarized laser pulse than that in elliptically polarized laser pulse.The time-frequency analysis and classical electron trajectory as well as the ionization yield curve are presented to further explain the underlying mechanism,the results from classical calculation are consistent with that from the quantum calculation.After adding a static electric field with ?=0.35,the cutoff of HHG is extended to 180th order with a supercontinuum spectra.As can be seen from the time-frequency analysis,only one short quantum path located at around t=0.1 O.C.has contribution to HHG for orders above 90.Thus,the single quantum path control is realized.Moreover,an isolated 110 as pulse can be obtained by superposing the harmonics from 130th to 180th order.The multiphoton ionization of hydrogen atom is investigated by strong field approximation(SFA)method under two counter-rotating circularly polarized laser pulse,the vortex structure appears in the PMD.There are two spiral arms in vortex structure with ?=30,70 nm.When ?=140 nm,it shows four spiral arms in vortex structure.The attosecond perturbation ionization theory is employed to illustrate the wavelength effect on PMD.And the simulation results obtained by solving the time-dependent Schrodinger equation(TDSE)provide verification for that under SFA.Then we investigate the change of vortex structure with varying time-delay.The vortex structure rotates as the time-delay increases,and it just rotates 180° when Td=1 O.C.Besides,we investigate the effect of laser polarization on PMD.Result shows that the helicity of the vortex structure follows that of the second time-delayed laser.Besides,the vortex structure disappears when the two co-rotating circularly polarized laser pulse is employed and it presents a "multi-ring" structure which is similar to "Newton's rings" The attosecond perturbation ionization theory is employed to illustrate the above-stated phenomena.We investigate molecular photoelectron momentum distribution(MPMD)and molecular photoelectron angular distribution(MPAD)of oriented H2+molecule under frozen-nuclei approximation driven by elliptically polarized laser pulse by numerically solving a two-dimensional TDSE.The ellipticity effect is investigated first,the photoelectron momentum mainly distributes along the direction that is perpendicular to the molecular axis with a small tilted angle;the distribution becomes more intense as the increase of laser ellipticity,which can be illustrated by the attosecond perturbation ionization theory.And the MPMD and MPAD are sensitive to the internuclear distance.When the internuclear distance is subtly increased,the distribution along molecular axis becomes more and more intense.The initial MPMDs and the time evolution of MPMDs are presented to illustrate this phenomenon.In addition,the MPMD and MPAD are also dependent on laser wavelength.When the laser wavelength reduces from 35 nm to 5 nm,laser-induced electron diffraction(LIED)occurs which can be illustrated by the attosecond perturbation ionization theory.We theoretically study the MPMD of H2+ molecule with varying orientation angles.When the laser ellipticity is ?=0.5 and ?=1.0,the MPMD distribute perpendicular to the axis which is independent of the orientation angle;when the ellipticity is ?=0,the MPMD would follow the rule unless the angle between linearly polarized laser field and the molecular axis is zero.The attosecond perturbation ionization theory can be employed to explain this phenomenon well.However,when the angle between linearly polarized laser field and the molecular axis is zero,the MPMD presents a distinct six-lobe pattern.The evolution of electron wavepacket(EWP)and MPMD are employed to interpret this distinct pattern. |
PDF Full Text Request