Multiphoton absorption by quasifree electrons

4 Other names have been suggested, such as excess photon ionisation, but the simple designation ATI has now been adopted and is difficult to change. 

This effect can be thought of as a generalisation of Einstein s equation for the photoelectric effect. For the kinetic energy of the escaping electron, we can now wite  

in the description just given, one might think of the absorption processes in the continuum occurring sequentially or, alternatively, that all the ATI peaks are produced simultaneously inside the laser pulse (which is the picture developed below). 

Freeman [491] has proposed an experimental test of which picture is correct he argues that, if ATI peaks are observed even for negative ions, where the escaping electron cannot readily interact since the core is not ionic, then the process should be simultaneous rather than sequential. Indeed, several ATI peaks are observed [492] even for negative ions, which suggests that the process as a whole is simultaneous. 

We now address the question how much atomic physics needs to be included in order to account for ATI In fig. 9.6, we show experimental data for ATI from [493], obtained at several laser intensities. One of the important properties of ATI peaks, referred to as peak suppression, is that the relative intensity of the first ATI peaks above threshold does not increase uniformly with laser field strength, but actually begins to decrease in intensity relative to higher energy peaks as the laser field strength increases. Such behaviour cannot be explained in a perturbative scheme, in which interactions must decrease monotonically order by order as the number of photons involved increases, but can be accounted for in terms of the AC Stark shift of the ionisation potential in the presence of the laser field. In ATI experiments, the ionisation potential appears to shift by an average amount nearly equal to the ponderomotive potential, so that prominent, discrete ATI peaks are seen despite the many different intensities present during the laser pulse. However, ATI peaks closest to the ionisation limit become suppressed as the amplitude of the laser field oscillations increases and the ionisation threshold sweeps past them (a different effect which also suppresses ionisation near threshold is discussed in section 9.24.1). 

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