Threshold photo-double ionization processes in helium studied by a photoelectron/photoion coincidence technique

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Abstract
A new type of photoelectron/photoion coincidence spectrometer using a penetrating field technique has been used to investigate photo-double ionization processes in helium. Threshold photoelectron/photoion coincidence measurements have been used to determine the value of the exponent in the Wannier theory and its range of validity. The value obtained is consistent with the Wannier prediction. Photoelectron/photoion coincidence measurements have also been used to investigate the behaviour of the asymmetry parameter, beta , for near threshold photo-double ionization and a near constant value of beta , close to -0.4, has been observed. This result is in disagreement with the predictions of the Wannier theory which appears to underestimate the angular correlation between the two electrons.