Studying the universality of field induced tunnel ionization times via   high-order harmonic spectroscopy
  High-harmonics generation spectroscopy is a promising tool for resolving electron dynamics and structure in atomic and molecular systems. This scheme, commonly described by the strong field approximation, requires a deep insight into the basic mechanism that leads to the harmonics generation. Recently, we have demonstrated the ability to resolve the first stage of the process -- field induced tunnel ionization -- by adding a weak perturbation to the strong fundamental field. Here we generalize this approach and show that the assumptions behind the strong field approximation are valid over a wide range of tunnel ionization conditions. Performing a systematic study -- modifying the fundamental wavelength, intensity and atomic system -- we observed a good agreement with quantum path analysis over a range of Keldysh parameters. The generality of this scheme opens new perspectives in high harmonics spectroscopy, holding the potential of probing large, complex molecular systems. 
Hadas Soifer (1), Barry D. Bruner (1), Matteo Negro (2), Michele   Devetta (2), Davide Faccial\`a (3), Caterina Vozzi (2), S...
physics.atom-ph physics.optics