Computation of ATI spectra Keldysh-Faisal-Rees theory

3 Computation of ATI spectra Keldysh-Faisal-Rees theory Keldysh [494] obtains the transition rate in a strong field from the ansatz 

These equations allow the main features of observed ATI spectra to be computed in a simple way. The method is referred to as the Keldysh-Faisal-Reiss (KFR) theory. Note that the final states are Volkov states, and therefore contain no information about the atom. In this sense, we are dealing with a model of type (i) in section 9.18. Very little atomic physics is required only the ionisation potential of the atom, the angular momentum acquired by the emerging electron and the properties of the 

The method works particularly well for ATI spectra excited by circularly polarised light. The reason for this is as follows an atom which absorbs N photons then acquires Nh units of angular momentum. The emerging electron is then subject to a repulsive centrifugal barrier (see chapter 5) and does not therefore penetrate into the core. Consequently, most atomic effects are suppressed, and a final state representation as a Volkov wavefunction is a reasonable approximation. This is also why intensity suppression occurs near threshold in this case the effect is very similar to delayed onset in single-photon ionisation to continua of high angular momentum. 

A comparison between the predictions of KFR theory [496], in which ponderomotive and angular momentum effects are included, and experimental data [497] is shown in fig. 9.7. 

The ATI spectra described above are found for pulse durations of about 1 ps or longer. For much shorter pulses, there is a qualitative change which is further discussed in section 9.25 it turns out that the minimum atomic physics option is then no longer sufficient. 

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