A modified trajectory method of evaluation of multiphoton ionization probability

Trajectory methods are successfully applied for description of the photoionization process in a strong laser field and, in particular, for multiphoton ionization. A trajectory method that we developed before is one of them. Testing using the method on such objects as hydrogen, helium, and lithium showed that it allows calculating of photoionization probabilities with separation of contributions of different multiplicities for multielectron systems in a very wide range of photopulse parameters. However, for a weak field, the method does not reproduce a correct dependence of multiphoton ionization probability on field intensity if the photon energy is below the ionization threshold. We present a procedure that makes it possible to overcome this drawback. The modified method works in a wide range of field intensities, including superatomic fields. We calculated the photoionization probability of a hydrogen atom as a simple example of its application.