Not every interaction between a photon and an atom, or molecule, will result in photoionization. The probability of photoionization is related to the photoionization cross-section of the species, which depends on the energy of the photon and the species being considered. In the case of molecules, the photoionization cross-section can be estimated by examination of Franck-Condonfactors between a ground-state molecule and the target ion. For photon energies below the ionizationthreshold, the photoionization cross-section is near zero. But with the development of pulsed lasers it has become possible to create extremely intense, coherent light where multi-photon ionization may occur. At even higher intensities, non-perturbative phenomena such as barrier suppression ionization and rescattering ionization are observed.
Multi-photon ionization
Several photons of energy below the ionization threshold may actually combine their energies to ionize an atom. This probability decreases rapidly with the number of photons required, but the development of very intense, pulsed lasers still makes it possible. In the perturbative regime, the probability of absorbing N photons depends on the laser-light intensity I as IN. For higher intensities, this dependence becomes invalid due to the then occurring AC Stark effect. Resonance-enhanced multiphoton ionization is a technique applied to the spectroscopy of atoms and small molecules in which a tunable laser can be used to access an excited intermediate state. Above-threshold ionization is an extension of multi-photon ionization where even more photons are absorbed than actually would be necessary to ionize the atom. The excess energy gives the released electron higher kinetic energy than the usual case of just-above threshold ionization. More precisely, The system will have multiple peaks in its photoelectron spectrum which are separated by the photon energies, this indicates that the emitted electron has more kinetic energy than in the normal ionization case. The electrons released from the target will have approximately an integer number of photon-energies more kinetic energy.
Tunnel ionization
When either the laser intensity is further increased or a longer wavelength is applied as compared with the regime in which multi-photon ionization takes place, a quasi-stationary approach can be used and results in the distortion of the atomic potential in such a way that only a relatively low and narrow barrier between a bound state and the continuum states remains. Then, the electron can tunnel through or for larger distortions even overcome this barrier. These phenomena are called tunnel ionization and over-the-barrier ionization, respectively.
