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In atomic, molecular, and optical physics, above threshold ionization (ATI), is a multi-photon effect where an atom is ionized with more than the energetically required number of photons. It was first observed in 1979.
Contents
Photoelectrons
In the case of ATI the photoelectron peaks should appear at
where the integer
Structure
It typically has a strong maximum 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
Features from ultrashort pulses
High intensity ultrashort pulse lasers 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.