Line broadening in multiphoton processes with a resonant intermediate transition

It is found that the line width of the excitation spectrum for multiphoton ionization is broadened much more severely than the cascade fluorescence originating from the resonant intermediate level. These results derive from the mutual effects of the ionizing and resonating transitions, which are not properly accounted for in perturbative treatments. In general, the ionization line shape can be expressed as the sum of two Lorentzian components, each with different degrees of broadening. This line shape reduces to that of the resonating transition in the limit of vanishing intensities. Moreover, the fluorescence line shape may also exhibit a second component whose amplitude may become important under certain conditions.