Application of a numerical-basis-state method to strong-field excitation and ionization of hydrogen atoms

We apply a numerical-basis-state method to study dynamical processes in the interaction of atoms with strong laser pulses. The method is based on the numerical representation of finite-space energy eigenstates of the field-free atomic Hamiltonian in a box on a grid and the expansion of the solution of the full time-dependent Schrodinger equation, including the interaction with the field, in this numerical basis. We apply the method to the hydrogen atom and present results for excitation and ionization probabilities as well as photoelectron momentum distributions. Convergence of the results with respect to the size of the basis as well as the parallel efficiency of the numerical algorithm is studied. The results of the numerical-basis-state method are in good agreement with those of two-dimensional numerical grid calculations. The computation times for the numerical-basis-state method usually are found to be significantly smaller than those for the two-dimensional grid calculations, whereas, even higher excited states can be well represented. We further apply this method to study a few recently reported phenomena related to strong-field excitation of atoms, such as the dependence of the excitation probabilities on the carrier-envelope phase in ultrashort pulses as well as the so-called frustrated ionization.