Ionization of the hydrogen atom by intense ultrashort laser pulses

The ionization of atomic hydrogen in intense laser fields is studied theoretically by both quantum-mechanical and classical approaches. In the quantum- mechanical treatment we apply a momentum- space strong- field approximation ( MSSFA ) and the Coulomb potential is taken into account as a perturbation. The classical calculations are performed within the framework of the classical trajectory Monte Carlo method. The energy and angular distributions of the ionization probabilities of the photoelectrons are presented for different laser pulses. While for the case of low electron energies larger discrepancies can be observed between the theories in the double- differential ionization probabilities, at high electron energies the agreement is excellent. This indicates that the generation of low- energy electrons is of quantum type and it is strongly influenced by the Coulomb potential, while the production of high- energy electrons is of classical type and it is less influenced by the Coulomb interaction. Our MSSFA results are in good agreement with the most reliable calculations based on a numerical solution of the time- dependent Schrodinger equation for high momentum transfers.