Abstract
Non-linear photoionization of molecules in the 10–50eV range is a prerequisite for pump-probe measurements with sub-femtosecond resolution, but hitherto has been limited to femtosecond resolution, low repetition rate and high photon flux laser systems. We demonstrate two-photon single ionization of helium atoms using 100pJ, 1.34fs pulses (main peak FWHM=680 as) at 1kHz repetition rate with a central photon energy of 19.6eV. We obtained an exponent of 2.27±0.21 for the intensity dependence of the signal and a two-photon ionization cross-section of 5.0±0.5×10−50cm4s. Our work opens the possibility of attosecond pump-probe measurements of ultrafast molecular processes.
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