The ponderomotive potential of high intensity light and its role in the multiphoton ionization of atoms

At very high laser light intensities, several unexpected phenomena appear in the electron spectra of multiphoton ionization (MPI) of even the simplest atom. Chief among these is the appearance of multiple electron peaks separated by the photon energy, a result now generally referred to as above-threshold ionization (ATI). Detailed investigations have shown that the relative amplitudes and angular distributions of these ATI peaks depend on the intensity and polarization of the laser light. In addition, the peak widths and positions are a function not only of the laser intensity, but of the temporal width of the laser pulse as well. Surprisingly, for extremely short pulse excitation the energies and momenta of the ATI electrons are independent of either the laser energy or pulse duration. All of these light-induced changes in the ATI peaks arise from a single physical concept: the ponderomotive potential associated with the high-intensity laser light used for multiphoton ionization of the atom