Above-threshold ionization

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.

[2] In the case of ATI the photoelectron peaks should appear at where the integer n represents the minimal number of photons absorbed, and the integer s represents the number of additional photons absorbed.

is the electron kinetic energy of the peak corresponding to s additional photons being absorbed.

[1][4] In the non-perturbative regime the bound states are dressed with the electric field, shifting the ionization energy.

[3] High intensity ultrashort pulse lasers can create ATI features with 20 or more peaks.

The angle-integrated photoelectron spectrum resulting from a laser interacting with a hydrogen atom. The x axis marks the electron kinetic energies in eV , whilst the y axis is the differential probability. The first three above-threshold ionization peaks are visible in the image.