A generalization of the analytical theory of high harmonic generation in the long wavelength limit and in the single active electron approximation is developed taking into account the magnetic dipole and electric quadrupole interaction. Quantum mechanical and classical theories are found to be in excellent agreement, which allows one to explain the influence of multipole effects in terms of an intuitive picture. For Ti:S lasers ( 0.8 &mgr;m) multipole contributions are found to be small below an intensity of about 10(17) W/cm(2), at which harmonic radiation with photon energies of several keV is generated. This promises the extension of high harmonic generation well into the sub-nm wavelength regime.

The spin dynamics and its reaction on the particle motion are investigated for free and bound electrons in intense linearly polarized laser fields. Employing both classical and quantum treatments we analytically evaluate the spin oscillation of free electrons in intense laser fields and indicate the effect of spin-orbit coupling on the motion of the electron. In Mott scattering an estimation for the spin oscillation is derived. In intense laser ion dynamics spin signatures are studied in detail with emphasis on high-order harmonic generation in the tunneling regime. First-and second-order calculations in the ratio of electron velocity and the speed of light show spin signatures in the radiation spectrum and spin-orbit effects in the electron polarization.

We investigate analytically the spin-laser coupling of an electron in an ultra-intense linearly polarized plane laser field. Particular emphasis is placed on the reaction of the spin on the electron motion which, in contrast to a recently applied approach, is calculated by means of a quantum mechanical description. An additional oscillation in the direction of the magnetic field is predicted and the classical results are confirmed up to a numerical constant. By comparison with Klein-Gordon results we show that this effect is doubtless a signature of the spin, and isolate the leading second-order correction terms in the ratio of electron and light velocity.

The emission of harmonic radiation by an electron oscillating anharmonically in a linearly polarized plane ultra-intense laser field is studied. The electron is described as a classical relativistic particle with a magnetic moment in order to include the spin-laser interaction. In contrast to ordinary Maxwell dynamics the motion can become three-dimensional depending on the orientation of the spin. Additional harmonics are visible in the direction of the electric field of the laser where no harmonics of polarization along the magnetic-field direction are to be expected.

We present a novel approach towards strong-laser-field–atom interaction based on the semiclassical Feynman path integral. The semiclassical propagator opens new ways for numerically and analytically describing strong-laser–matter interaction.