Multiphoton excitation and ionization of He atoms in an intense laser beam

Transition amplitudes corresponding to bound-bound and bound-free multiphoton transitions for an He atom in the presence of an intense laser beam have been evaluated. Results show that for multiphoton excitation the transition amplitude for small values of N (the number of photons being absorbed) decreases with increasing intensity. However, for higher values of N the amplitude continues to rise with increasing intensity. A particular feature of the present method of calculating multiphoton ionization is that the transition amplitude increases linearly for small values of the intensity, but for higher values, the slope of the curve decreases, showing an important nonlinear effect.