Photon vs. charged-particle ionization

Here we apply the coupled-channel method to calculate photo ionization of atomic hydrogen by short (femtosecond) laser pulses at high power densities (up to 5 X 10 W/cm ). A classical electro-dynamical field approximates the laser/atom interaction, according to (in the Coulomb gauge) 

The time dependent shape of this field is given by a cos-function with amplitude Eq enveloped by a Gaussian centered at the time t = 0 with full width at half maximum (FWHM) At representing the laser pulse length. 

Resonant and also non-resonant multiphoton transitions are well reproduced with the program. This was tested by changing the wavelength from 200 to 260 nm. 

The following restrictions have been found to the application of coupled-channel calculations for the computation of pulsed-laser ionization. The dipole approximation restricts the photon energy to 1 keV in the current treatment. This, however, does not pose a strict condition since a partial-wave expansion of the laser field may be used, similar to as in the case of screened Coulomb potentials. In comparison to ion/atom collisions, typical photon/atom interaction times are extremely long. An upper limit of the pulse width AZp = 100 fs at intermediate laser-power densities follows from the numerically restricted density of continuum states. 

High power densities (3 10 W/cm ) and small laser frequencies (A 600 run) are related to extremely high orders of perturhation theory. This requires basis sets extending to high values of (/ 3 15) and high ejected-electron energies (e 20 eV). With the help pf P/Q space methods [60] the range of validity of coupled-channel calculations may he extended in this case. 

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