Low energy peak features in atomic ionization by mid-infrared laser pulses

The low energy structures which appear in atomic ionization by strong mid-infrared laser pulses are investigated using quasi-classical and quantum-mechanical approaches. We show that the position and the width of the low energy peak in the spectrum is sensitive to the laser field parameters: intensity, wave length and pulse duration. Our quasi-classical analysis reveals that these features are due to electrons within a small angular momentum window which are born within a very narrow tunneling time interval thus defining a time-focusing effect.