A Wigner-distribution-like (WDL) function is used to study the time-energy distribution of a photoelectron emitted from an atom in few-cycle laser fields with different frequencies. The WDL function shows the dependence of ionization probability on time and explicitly demonstrates the transition of the ionization process from the multiphoton regime to the tunneling regime with decreasing laser frequency. Accompanying this transition, the semiclassical relationship between the final drift energy and the ionization moment becomes more and more valid and is in good agreement with the time-energy distribution in the tunneling regime. Meanwhile, the photoelectron energy spectrum shows a fine structure, which is more and more dependent on the direction of the ejection of the photoelectron and the carrier-envelope phase of the laser field. Moreover, the WDL function enables us to explicitly attribute the fine structure in the tunneling regime to interference between electrons with the same kinetic energy ejected at different times.

• Received 21 August 2012

DOI:https://doi.org/10.1103/PhysRevA.86.053409