ABSOLUTE CROSS-SECTIONS FOR DOUBLE PHOTOIONIZATION OF HELIUM AT ENERGIES FROM 0 TO 80 EV ABOVE-THRESHOLD

We have performed, using an ab initio basis-set method, calculations o f the absolute cross section for double photoionization of helium for energies from 2 to 80 eV above threshold, and we have extended the res ults down to 0 eV through spline interpolation. Our results are consis tent with the Wannier threshold law, and agree well with the near-thre shold mesurements. At higher energies (20 eV or so above threshold) si gnificant discrepancies with experiment are evident. We find a simple relation between the integrated cross section for double photoionizati on and the singly differential cross section at the midpoint of the en ergy distribution.