Theoretical Study On Atomic Field-ionization And Electronic Acceleration In Ultrafast Intense Laser Field

The interaction of an ultra-intense femtosecond laser pulse with atoms andmolecules gives rise to a variety of nonlinear phenomena, including above-thresholdionization, the generation of high-order harmonics (HHG), etc. The field-ionizationplays a key role in these processes. When a laser pulse interacts with the gas atoms,these atoms may ionize according to different ionization mechanisms. Phenomenalike atomic ionization and electron acceleration caused by ultrafast laser are studied inthis thesis.Several kinds of one-dimensional atomic potential which appear in manyimportant literatures are displayed. Based on the soft-core Coulomb potential, thecriticalal electric field strength for atom ionization is calculated under the tunnelingcondition which is determined in the way that Keldysh parameter is determined in.For several kinds of noble gas atoms, the width of the potential barrier and theionization position of the ionized electron are calculated according to the criticalelectric field strength. The ionized electrons will continue to absorb energies fromlaser field and be accelerated by laser field combined with Coulomb field. Thecorresponding equation of motion of the electron is presented.By solving the motion of the electron numerically, the ionization of several kindsof noble gas atoms and hydrogen atom is investigated, and the displacements andkinetic energies of ionized electrons are calculated. The schematics are presented inthis thesis. The final position of an ionized electron which is the electron’s positionwhen laser electric field reverses twice after it was ionized, together with its birthposition, indicates that only the electrons whose birth position and final position havethe inverse sign are relevant to the generation of high-order harmonics. One can seethat these are the kind of the electrons that appear on the lowering sides of the peaksof laser electric field in the schematics, which would come back near to their parentions after they ionized.A software is made to describe the ionization and motion of an electron in strongfemtosecond laser pulse according to the model built in this thesis. This software cansimulate the trajectory of an electron dynamically and give the kinetics energy when itpasses by its parent ion.