Exact Results in Quantum Theory

The major problem of describing the photoionization of molecules and atoms is the continuum state of the ejected electron. We propose a method of approximating the Coulomb wavefunction in the finite basis set. The deviation of the potential resulting from the structure of ion can be taken into account in this method. We also propose the construction of the final state of the system for the simple case of He atom. Finally, we obtain the prescription for computing the photoionization cross section.