Above threshold ionization
Within atomic, molecular, and optical physics above threshold ionization (ATI), which was first observed in 1979, is a multi-photon effect where an atom is ionized with more than the required number of photons.
In the case of ATI the peaks should appear at
where the integer represents the minimum number of photons absorbed and the integer s represents the number of additional photons absorbed. is the ionization energy and is the electron kinetic energy of the peak corresponding to s additional photons being absorbed.
It typically has a very strong maxima at the minimum number of photons to ionize the system, with successive peaks (known as ATI peaks) separated by the photon energy and thus corresponding to higher numbers of photons being absorbed.
In the non-perturbative regime the bound states are dressed with the electric field, shifting the ionization energy. If the ponderomotive energy of the field is greater than the photon energy then the first peak disappears.
Features from Ultrashort pulses
High intensity Ultrashort laser pulses can create ATI features with 20 or more peaks. The photoelectron spectrum of electron energies is continuous since actual light sources contain a spread of energies.
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