Multiphoton-ionization of hydrogen atoms in intense laserfields

The interaction of hydrogen atoms with strong laser fields at intensities up to some 10¹³ W cm^?2 was studied experimentally at the wavelengths λ=355 nm, 532 nm and 1064 nm. The ion yield, the energy spectrum of the photoelectrons and their angular distributions were measured. The angular distributions at λ=355 nm and λ=532 nm provide a sensitive test for theoretical calculations. Comparison with the calculations available shows that perturbation theory with proper inclusion of atomic structure yields results which agree with experiment. Intensity dependent changes of angular distributions at λ=532 nm are observed, which indicate that at 10¹³ W cm^?2 higher order processes become noticable. At λ=1064 nm the situation is more complicated, experimentally as well as theoretically. Intensities of some 10¹³ W cm^?2 are necessary to observe ionization. Strong distortions of the atomic structure can be expected. Presently only qualitative aspects of the angular distributions can be discussed.