Simultaneous multiphoton ionization and excitation of helium in an intense laser beam

The transition amplitude corresponding to multiphoton absorption resulting in simultaneous ionization and excitation of the helium atom in the presence of an intense laser beam has been evaluated by using the method of Reiss (1970). Results show that reduced transition amplitude varies linearly with initial value of the intensity but for high intensity the slope of the curve decreases, showing an important nonlinear effect. It is found that at the intensity increases the maximum number of photons taking part in the process also increases. Another important feature found is the dominance of the multiphoton ionization process over the multiphoton excitation process at high intensities.