Editing Photoionization (section)

==Cross section==
Not every interaction between a photon and an atom, or molecule, will result in photoionization. The probability of photoionization is related to the [[Photoionisation cross section|photoionization cross section]] of the species – the probability of an ionization event conceptualized as a hypothetical cross-sectional area. This cross section depends on the energy of the photon (proportional to its wavenumber) and the species being considered i.e. it depends on the structure of the molecular species. In the case of molecules, the photoionization cross-section can be estimated by examination of Franck-Condon factors between a ground-state molecule and the target ion. This can be initialized by computing the vibrations of a molecule and associated cation (post ionization) using quantum chemical software e.g. QChem. For photon energies below the ionization threshold, 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 via sequences of excitations and relaxations. At even higher intensities (around {{nowrap|10<sup>15</sup> – 10<sup>16</sup> W/cm<sup>2</sup>}} of infrared or visible light), [[non-perturbative]] phenomena such as ''barrier suppression ionization''<ref>{{cite journal | last1=Delone | first1=N. B. | last2=Krainov | first2=V. P. | year=1998 | title=Tunneling and barrier-suppression ionization of atoms and ions in a laser radiation field | journal=[[Physics-Uspekhi]] | volume=41 | issue=5 | pages=469–485 | doi=10.1070/PU1998v041n05ABEH000393 |bibcode = 1998PhyU...41..469D | s2cid=94362581 }}</ref> and ''rescattering ionization''<ref>{{cite conference |last1=Dichiara |first1=A. |title=2005 Quantum Electronics and Laser Science Conference |display-authors=etal |year=2005 |chapter=Cross-shell multielectron ionization of xenon by an ultrastrong laser field |book-title=Proceedings of the Quantum Electronics and Laser Science Conference |volume=3 |pages=1974–1976 |publisher=[[Optical Society of America]] |doi=10.1109/QELS.2005.1549346 |isbn=1-55752-796-2 }}</ref> are observed.