The above-threshold ionization (ATI) process of argon in intensive circularly polarized laser field has been investigated by solving the time-dependent Schrödinger equation in length gauge with emphasis on pulse duration effect. It is shown that the influence of pulse duration is obvious on the transition between multiple-photon and tunneling ionization regimes when the Keldysh parameter is no more than 3.25. That is, photoelectron ionization by shorter pulse occurs in the tunneling regime mainly, and yet the multiple-photon absorption appears more easily with longer pulse. However, such influence disappears when the Keldysh parameter is sufficiently large and the ATI always appears in the multiple-photon ionization regime.
In this paper, we provide a detailed derivation of the strong-field ionization rates originally developed by Keldysh who introduced the strong-field approximation. Numerical results are presented to examine the saddle-point approximation, the low photoelectron energy approximation, and neglecting the interference terms, which are necessary to arrive at the simple quasistatic result, when the Keldysh parameter becomes very small.
Identification of tunneling and multiphoton ionization in intermediate Keldysh parameter regime