Double Photoionization of Helium Down to 100 meV above Threshold

Double photoionization of helium has been studied using a new instrument which allows one to detect the two emitted electrons over $4\pi$ steradians and measure their individual momenta. The experiments have been performed at 0.2 and 0.1 eV above threshold, and both double and triple differential cross sections have been extracted. In comparison with higher energy data, the results show that the strength of angular correlation between the two electrons increases towards threshold.