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In perturbation theory (QM), i found the graph of ionization rate with various parameter like momentum of ejected electron, intensity, frequency, field strength of incident laser field. But i am unable to extract the physical meaning out of it.

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Ionization is when an EM neutral atom gains EM charge by gaining or losing electrons.

The interaction of atoms and molecules with sufficiently strong laser pulses leads to the ionization to singly or multiply charged ions. The ionization rate, i.e. the ionization probability in unit time, can only be calculated using quantum mechanics. In general, the analytic solutions are not available, and the approximations required for manageable numerical calculations do not provide accurate enough results. However, when the laser intensity is sufficiently high, the detailed structure of the atom or molecule can be ignored and analytic solution for the ionization rate is possible.

There are two types:

  1. Tunnel ionization

Tunnel ionization is ionization due to quantum tunneling. In classical ionization, an electron must have enough energy to make it over the potential barrier, but quantum tunneling allows the electron simply to go through the potential barrier instead of going all the way over it because of the wave nature of the electron.

  1. Multiphoton ionization

In practice, tunnel ionization is observable when the atom or molecule is interacting with near-infrared strong laser pulses. This process can be understood as a process by which a bounded electron, through the absorption of more than one photon from the laser field, is ionized. This picture is generally known as multiphoton ionization (MPI).

In both cases, the electron is ejected and the atom becomes an ion, but in tunneling, the electron needs a sufficiently strong laser field to go through the potential barrier, and with MPI, the electron absorbs multiple photons to overcome the barrier.

Please see here:

https://en.wikipedia.org/wiki/Ionization

https://arxiv.org/ftp/arxiv/papers/1412/1412.7974.pdf

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