  We derive the photoionisation cross section in dipole approximation for
many-electron atoms and ions for neutron star magnetic field strengths in the
range of 10^7 to 10^9 T. Both bound and continuum states are treated in
adiabatic approximation in a self-consistent way. Bound states are calculated
by solving the Hartree-Fock-Roothaan equations using finite-element and
B-spline techniques while the continuum orbital is calculated by direct
integration of the Hartree-Fock equations in the mean-field potential of the
remaining bound orbitals. We take into account mass and photon density in the
neutron star's atmosphere, finite nuclear mass as well as thermal occupation of
the levels. The data may be of importance for the quantitative interpretation
of observed x-ray spectra that originate from the thermal emission of isolated
neutron stars. They can serve as input for modeling neutron star atmospheres as
regards chemical composition, magnetic field strength, temperature, and
redshift. Our main focus in this paper lies on helium and helium-like oxygen.
These two-electron systems are simple enough to calculate all possible
transitions when limiting the quantum numbers and should show all the basic
structures and behaviour of other two-electron systems up to iron.
