3-D NUMERICAL CALCULATIONS OF LASER ATOM INTERACTIONS- Subrelativistic and weakly relativistic regime

摘要

Laser-atom interactions at routinely used high laser intensity have defied a complete numerical description because of the very large quiver mo-menta and excursion amplitudes of the electron in the laser field. For nuclear charge Z, laser intensity I, and frequency ω the dimensions of the bound states are ～1/Z while the quiver motion covers distances of ∝ I~(1/2)/ω~2. When I approaches one atomic unit in a Ti:Sapphire laser of λ = 800 nm ≈ 0.057 a.u., the quiver amplitude exceeds atomic dimensions by 300Z, making the simultaneous description of elect ron-ion rescattering and non-bound electronic motion exceedingly difficult Models have stepped in and provided essential plysical insight. Analytic and semi-analytic calculations based on the strong field approximation (SFA) were able to explain experin ental data in great detail, in spite of the fact that several important approximations are made in these models. Numerical calculations were performed for one- and two-dimensional models. From all these calculations a generally accepted picture of strong laser-atom interactions has emerged: an electron is set free from the atom with low initial velocity, it s accelerated in the laser field with hardly any influence of the ionic potential, where the free motion is only disturbed, when the electron happens to return to the nucleus one or several times. If electron velocities become comparable to the velocity of light, the magnetic field component of the laser field Makes itself felt by pushing the electron into the direction of laser propagat on. At present theory cannot give definite answers to the following questions: What is the initial momentum spectrum after release of the electron ? What is the influence of the Coulomb potential on the non-bound electronic motion, is there "Coulomb focusing" of the electronic motion ? How are harmonic generation and non-sequential ionization affected by the suppression of rescattering, when the electron drifts away due to magnetic field effects ? These questions motivate our effort to integrate the successful ideas of the SFA into a complete numerical description of intense laser-atom interaction. In its present form, the method is capable of solving the time-dependent Schroedinger equation for an atom in fields up to 10~(17)W/cm~2 at the Ti:Sapphire wave length of 800 nm. The necessary extensions to include lowest order non-dipole effects will be discussed in detail.