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The production of He^+ and He^2+ by a 160 fs, 780 nm laser has been measured over an unprecedented 12 orders of magnitude in counting range. Enhanced double electron emission, called nonsequential (NS) ionization, was observed over an intensity range where the single ionization dynamics is evolving from multiphoton to pure tunneling. The NS yield is found to scale with the ac-tunneling rate for the neutral, even when tunneling is not the dominant ionization pathway. A rescattering mechanism fails to predict the observed NS threshold or magnitude.
Walker et al. (Mon,) studied this question.