The Decoupling Problem in Intense-Field Ionization

Recent experiments on multiphoton ionization in intense laser fields have revealed that the photoelectron spectrum consists of a large number of peaks. Under certain pulse duration and laser intensity conditions, the spectrum appears shifted towards lower energies and the interval between two consecutive peaks is not equal to the photon energy. The theoretical explanation based on a classical description of electron-field decoupling previously proposed in this context is reexamined. For moderate laser intensities, an exact analytical solution giving a linear intensity dependence of the shift effect throughout the spectrum is obtained. At very high laser intensities, the problem is solved numerically. In this case, the shift effect is non-linear with respect to intensity and is considerably reduced in comparison with the predictions of the linear model. Quantitative comparisons between the results of these models and experimental data show the models to be satisfactory. The conditions under which the photoelectrons can be strongly accelerated are then investigated. For optimum acceleration, laser pulse durations and intensities are intimately related. Results of numerical computations show that electron energies at the limit of relativistic corrections could be obtained by present-day lasers.