An extensive study of the threshold law for the cross section of double photoionization in helium is presented. It provides quantitative information about the Wannier exponent α, which agrees with the theoretical prediction. In addition, the hitherto unknown constant of proportionality ${\sigma }_0$ for the cross section ${\sigma }^{++}$ was determined as well as the energy range of validity, which was found to be smaller than expected.

• Received 15 September 1987

DOI:https://doi.org/10.1103/PhysRevLett.60.1266