Probing the longitudinal momentum spread of the electron wave packet at   the tunnel exit
  We present an ellipticity resolved study of momentum distributions arising from strong-field ionization of Helium at constant intensity. The influence of the ion potential on the departing electron is considered within a semi-classical model consisting of an initial tunneling step and subsequent classical propagation. We find that the momentum distribution can be explained by the presence of a longitudinal momentum spread of the electron at the exit from the tunnel. Our combined experimental and theoretical study provides an estimate of this momentum spread. 
Adrian N. Pfeiffer, Claudio Cirelli, Alexandra S. Landsman, Mathias   Smolarski, Darko Dimitrovski, Lars B. Madsen, Ursula K...
physics.atom-ph