The Ionization Of Atom,Molecule,and Vacuum Driven By Strong Laser Fields

Lasers generating ultra-short ultra-intense light pulses have come of age since the invention of laser mode locking and chirped pulse amplification.Ultra-short ultra-intense lasers are widely used in biology,chemistry,materialogy,medicine,and physics.In the physical science,modern ultra-short ultra-intense light pulses allow us to study ultrafast processes in the microscope world at attosecond time scale,while simultaneously pushing the study of light-matter interactions to an unprecedented intensity level.The electric field of moderate laser is in the same magnitude of the Coulomb field experienced by outer-shell electrons in atoms or molecules,so that ionization occurs when atoms and molecules are exposed to such lasers,producing free electrons and ions.Ionization process is the core of diverse research directions in atomic and molecular physics,such as high harmonic generation,nonsequential double ionization,shake-off and shake-up.One can manipulate these ultrafast dynamics by controlling the motion of electrons in real time.The laser-matter interaction has been traditionally researched by studying with the re-sponses of atoms,molecules,and plasmas to an external laser field.However,with the rapid development of laser technology,the peak intensity of laser has exceeded 10²²W/cm²,and with the development of blossoming worldwide high-power laser projects such as ELI,XCELS,HiPER,peak intensity of 10²⁶W/cm²is no longer out of reach.Strong laser pulses would open the door to study QED process.The vacuum breaks down when the external field reaches the Schwinger threshold(?10²⁹W/cm²),producing electrons and positrons.This paper is devoted to two series of theoretical researches with ultra-short ultra-intense light pulses,one is atomic and molecular process in moderate intense femtosecond or attosecond laser fields,and the other is the ionization of vacuum by an extremely intense laser field.The main topics of this article are as follows.First,we introduced numerical and analytical methods applied to atom,molecule,and vac-uum irradiated by external fields.To deal with the dynamics of atom and molecule in a laser field,we introduced the time-dependent Schr¨odinger equation and the corresponding solutions based on the time-dependent perturbation theory,the strong field approximation,and the ab initio numerical simulation.To deal with the ionization of vacuum,we introduced the com-putational quantum field theory,including perturbation solutions and exact solutions for the time-dependent Dirac equation.In the second part,we studied the ionization of atoms in strong laser fields,including sin-gle ionization of He⁺ and double ionization of He.The first work investigated the interference of two single ionization channels in He⁺driven by a two-color XUV laser field.We observed asymmetric photoelectron momentum spectra and found that the position of the interference minima strongly depends on the carrier-envelope-phase,photon energy,intensity,pulse dura-tion,and rotation direction of the two-color external field.Rabi oscillations between the ground and the first excited state can be formed by increasing the intensity or pulse duration of the laser field whose photon energy resonant to the energy difference between these two states.The influ-ence of Rabi oscillations on the interference of two ionization channels was studied.The second work investigated sequential and non-sequential double ionization of He in an EUV pulse in the presence of an extremely short MIR pulse.After setting a proper time delay between these two pulses,the electron of sequential and non-sequential double ionization may have the same final momenta,leading to peculiar interference patterns in the electron joint momentum distributions.In the third part,we studied the ionization of molecules in strong laser fields with H₂as a target.The first work focused on the electron-electron correlation in the single ionization process of H₂.We discussed the response of bound electron when an XUV or IR laser field is applied.Our simulation results show that two electrons are strongly correlated in the tunneling ionization process of H₂.One electron tunnels through the laser-dressed Coulomb potential and the other electron has enough time to adapt to the new potential of H₂⁺,especially when the laser polarization direction is perpendicular to the molecular axis,and thus the ion is prone to stay in the ground state of H₂⁺ after the interaction.However,in one-photon single ionization,the correlation during the single ionization process is negligible.The second work investigated the effect of the movement of nuclei in H₂ on the harmonic phase.The difference in the mass of the nuclei causes a stable relative phase difference between the high harmonics of D₂ and H₂.Through the spatial density distribution of the real and imaginary parts of the harmonics,the cause of phase differences can be analyzed.The fourth part discussed the ionization of vacuum.By solving the time-dependent Dirac equation numerically,the rate of created electron–positron pairs can be obtained.Different pair creation regimes can be characterized by the value of the parameter??=?c/E?,here E and?are the amplitude and frequency of the external field,and c is the speed of light.When??1,this regime is called as quasistatic regime and the pair creation mechanism is mainly tunneling.In contrast,in the condition of??1,the presence of the laser field can be taken into account perturbatively,equivalently the mechanism in pair creation process can be viewed as multi-photon absorption.Data from these numerical calculations permit us to make a sys-tematic comparison with results of other analytical methods.If parameters E/c³ and?/c² are both less than 1 or?is much less than 1,the analytical formulas based on WKB method predict the true creation rate well.In the multi-photon region,perturbative methods become valid while the WKB-based description fails.In the most interesting in-between regime of??1,only nu-merical simulation gives reliable results.In addition,we analyzed the pair creation dynamics in detail with the time evolution of particle numbers and spatial densities of electron and positron.