Beyond the dipole approximation: Angular distribution effects in the is photoemission from small molecules

Over the past two decades, the dipole approximation has facilitated a basic understanding of the photoionization process in atoms and molecules. Recen t experiments on the Is inner shells of small molecules at relatively low p hoton energies (less than or equal to 1000 eV) show strong nondipole effect s. They are significant and measurable at energies close to threshold, in c onflict with a common assumption that the dipole approximation is valid for photon energies below 1 keV.