Population trapping in the laser excitation of a realistic autoionising level

Photoionisation by a strong laser tuned to the neighbourhood of an autoionising level is discussed. An explicit model for the continuum dipole strength is used which reproduces the important features of a real atom. Within the context of this model the problem is solved exactly. The exact solution is related to the usual pole approximation via a Taylor expansion which also yields an improved pole approximation. Population trapping is observed to occur and the general condition for it is derived. An interpretation of population trapping as the strong field relic of the Fano minimum is given. Some remarks are made on the consequences of population trapping for electron-ion scattering in the presence of a laser.