The threshold region of the double-photoionization cross section of lithium was investigated using monochromatized synchrotron radiation and ion time-of-flight spectrometry. While the overall energy dependence can be described by the Wannier power law, we found oscillations in the cross section which are in good agreement with a modulated threshold law as proposed by Temkin [Phys. Rev. Lett. 49, 365 (1982)]. This behavior may be due to the unequal binding energies of the electrons involved in the double-photoionization process.

• Received 25 February 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.093002