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.

Photoelectrons

In the case of ATI the photoelectron peaks should appear at :

E_s = (n + s) \hbar \omega - W,

where the integer ''n'' represents the minimal number of photons absorbed, and the integer ''s'' represents the number of additional photons absorbed. ''W'' is the

ionization energy Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...

, and

E_s

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

Structure

It typically has a strong maximum at the minimal 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 In strong-field laser physics, ponderomotive energy is the cycle-averaged quiver energy of a free electron in an electromagnetic field. Equation The ponderomotive energy is given by :U_p = , where e is the electron charge, E is the linearly pol ...

of the field is greater than the photon energy

\omega

, then the first peak disappears.

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.

References

External links

Above threshold ionization photoelectron spectra
{{Lasers Atomic Quantum mechanics