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re for the musical composition see ionisation varèse the solar wind moving through the magnetosphere alters the movements of charged particles in the earth s thermosphere or exosphere and the resulting ionization of these particles causes them to emit light of varying color thus forming auroras near the polar regions 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 is called an ion ionization can result from the loss of an electron after collisions with subatomic particles collisions with other atoms molecules electrons positrons 1 protons antiprotons 2 and ions 3 4 5 6 7 8 9 10 or through the interaction with electromagnetic radiation 11 heterolytic bond cleavage and heterolytic substitution reactions can result in the formation of ion pairs ionization can occur through radioactive decay by the internal conversion process in which an excited nucleus transfers its energy to one of the inner shell electrons causing it to be ejected uses edit everyday examples of gas ionization occur within a fluorescent lamp or other electrical discharge lamps it is also used in radiation detectors such as the geiger müller counter or the ionization chamber the ionization process is widely used in a variety of equipment in fundamental science e g mass spectrometry and in medical treatment e g radiation therapy it is also widely used for air purification though studies have shown harmful effects of this application 12 13 production of ions edit avalanche effect in an electric field created between two electrodes the original ionization event liberates one electron and each subsequent collision liberates a further electron so two electrons emerge from each collision the ionizing electron and the liberated electron negatively charged ions 14 are produced when a free electron collides with an atom and is subsequently trapped inside the electric potential barrier releasing any excess energy the process is known as electron capture ionization positively charged ions are produced by transferring an amount of energy to a bound electron in a collision with charged particles e g ions electrons or positrons or with photons the threshold amount of the required energy is known as ionization energy the study of such collisions is of fundamental importance with regard to the few body problem which is one of the major unsolved problems in physics kinematically complete experiments 15 i e experiments in which the complete momentum vector of all collision fragments the scattered projectile the recoiling target ion and the ejected electron are determined have contributed to major advances in the theoretical understanding of the few body problem in recent years adiabatic ionization edit adiabatic ionization is a form of ionization in which an electron is removed from or added to an atom or molecule in its lowest energy state to form an ion in its lowest energy state 16 the townsend discharge is a good example of the creation of positive ions and free electrons due to ion impact it is a cascade reaction involving electrons in a region with a sufficiently high electric field in a gaseous medium that can be ionized such as air following an original ionization event due to such as ionizing radiation the positive ion drifts towards the cathode while the free electron drifts towards the anode of the device if the electric field is strong enough the free electron gains sufficient energy to liberate a further electron when it next collides with another molecule the two free electrons then travel towards the anode and gain sufficient energy from the electric field to cause impact ionization when the next collisions occur and so on this is effectively a chain reaction of electron generation and is dependent on the free electrons gaining sufficient energy between collisions to sustain the avalanche 17 ionization efficiency is the ratio of the number of ions formed to the number of electrons or photons used 18 19 ionization energy of atoms edit this section does not cite any sources please help improve this section by adding citations to reliable sources unsourced material may be challenged and removed november 2025 learn how and when to remove this message ionization energies of neutral elements predicted beyond 104 the trend in the ionization energy of atoms is often used to demonstrate the periodic behavior of atoms with respect to the atomic number as summarized by ordering atoms in mendeleev s table this is a valuable tool for establishing and understanding the ordering of electrons in atomic orbitals without going into the details of wave functions or the ionization process an example is presented in the figure to the right the periodic abrupt decrease in ionization potential after rare gas atoms for instance indicates the emergence of a new shell in alkali metals in addition the local maximums in the ionization energy plot moving from left to right in a row are indicative of s p d and f sub shells semi classical description of ionization edit this section does not cite any sources please help improve this section by adding citations to reliable sources unsourced material may be challenged and removed november 2025 learn how and when to remove this message classical physics and the bohr model of the atom can qualitatively explain photoionization and collision mediated ionization in these cases during the ionization process the energy of the electron exceeds the energy difference of the potential barrier it is trying to pass the classical description however cannot describe tunnel ionization since the process involves the passage of electron through a classically forbidden potential barrier quantum mechanical description of ionization edit the interaction of atoms and molecules with sufficiently strong laser pulses or with other charged particles leads to the ionization to singly or multiply charged ions the ionization rate i e the ionization probability in unit time can be calculated using quantum mechanics there are classical methods available also like the classical trajectory monte carlo method ctmc 20 21 but it is not overall accepted and often criticized by the community two quantum mechanical methods exist perturbative and non perturbative methods like time dependent coupled channel or time independent close coupling 22 where the wave function is expanded in a finite basis set are examples of non perturbative methods and numerous options exist e g b splines 23 generalized sturmians 24 or coulomb wave packets 25 26 another non perturbative method is to solve the corresponding schrödinger equation fully numerically on a lattice 27 in general analytic solutions are not available and the approximations required for manageable numerical calculations do not provide acceptably accurate results however when the laser intensity is sufficiently high the detailed structure of the atom or molecule can be ignored and an analytic solution for the ionization rate is possible tunnel ionization edit combined potential of an atom and a uniform laser field at distances r r 0 the potential of the laser can be neglected while at distances with r r 0 the coulomb potential is negligible compared to the potential of the laser field the electron emerges from under the barrier at r r c e i is the ionization potential of the atom 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 the probability of an electron s tunneling through the barrier drops off exponentially with the width of the potential barrier therefore an electron with a higher energy can make it further up the potential barrier leaving a much thinner barrier to tunnel through and thus a greater chance to do so 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 keldysh 28 modeled the mpi process as a transition of the electron from the ground state of the atom to the volkov states 29 in this model the perturbation of the ground state by the laser field is neglected and the details of atomic structure in determining the ionization probability are not taken into account the major difficulty with keldysh s model was its neglect of the effects of coulomb interaction on the final state of the electron as it is observed from figure the coulomb field is not very small in magnitude compared to the potential of the laser at larger distances from the nucleus this is in contrast to the approximation made by neglecting the potential of the laser at regions near the nucleus perelomov et al 30 31 included the coulomb interaction at larger internuclear distances their model which we call the ppt model was derived for short range potential and includes the effect of the long range coulomb interaction through the first order correction in the quasi classical action larochelle et al 32 have compared the theoretically predicted ion versus intensity curves of rare gas atoms interacting with a ti sapphire laser with experimental measurement they have shown that the total ionization rate predicted by the ppt model fit very well the experimental ion yields for all rare gases in the intermediate regime of the keldysh parameter the rate of mpi on atom with an ionization potential e i displaystyle e_ i in a linearly polarized laser with frequency ω displaystyle omega is given by w p p t c n l 2 6 π f l m e i 2 f 2 e i 3 2 2 n m 3 2 1 γ 2 m 2 3 4 a m ω γ e 2 f 2 e i 3 2 g γ displaystyle w_ ppt left c_ n l right 2 sqrt frac 6 pi f_ lm e_ i left frac 2 f left 2e_ i right frac 3 2 right 2n m frac 3 2 left 1 gamma 2 right left frac m 2 right frac 3 4 a_ m omega gamma e frac 2 f left 2e_ i right frac 3 2 g left gamma right where γ ω 2 e i f displaystyle gamma frac omega sqrt 2e_ i f is the keldysh parameter n 2 e i z 2 displaystyle n frac sqrt 2e_ i z 2 f displaystyle f is the peak electric field of the laser and l n 1 displaystyle l n 1 the coefficients f l m displaystyle f_ lm g γ displaystyle g gamma and c n l displaystyle c_ n l are given by f l m 2 l 1 l m 2 m m l m g γ 3 2 γ 1 1 2 γ 2 sinh 1 γ 1 γ 2 2 γ c n l 2 2 2 n n γ n l 1 γ n l displaystyle begin aligned f_ lm frac 2l 1 l m 2 m m l m g gamma frac 3 2 gamma left left 1 frac 1 2 gamma 2 right sinh 1 gamma frac sqrt 1 gamma 2 2 gamma right c_ n l 2 frac 2 2n n gamma n l 1 gamma n l end aligned the coefficient a m ω γ displaystyle a_ m omega gamma is given by a m ω γ 4 3 π 1 m γ 2 1 γ 2 n v e n v α γ w m 2 γ 1 γ 2 n v displaystyle a_ m omega gamma frac 4 3 pi frac 1 m frac gamma 2 1 gamma 2 sum _ n v infty e n v alpha gamma w_ m left sqrt frac 2 gamma sqrt 1 gamma 2 n v right where w m x e x 2 0 x x 2 y 2 m e y 2 d y α γ 2 sinh 1 γ γ 1 γ 2 v e i ω 1 1 2 γ 2 displaystyle begin aligned w_ m x e x 2 int _ 0 x x 2 y 2 m e y 2 dy alpha gamma 2 left sinh 1 gamma frac gamma sqrt 1 gamma 2 right v frac e_ i omega left 1 frac 1 2 gamma 2 right end aligned quasi static tunnel ionization edit the quasi static tunneling qst is the ionization whose rate can be satisfactorily predicted by the adk model 33 i e the limit of the ppt model when γ displaystyle gamma approaches zero 34 the rate of qst is given by w a d k c n l 2 6 π f l m e i 2 f 2 e i 3 2 2 n m 3 2 e 2 3 f 2 e i 3 2 displaystyle w_ adk left c_ n l right 2 sqrt frac 6 pi f_ lm e_ i left frac 2 f left 2e_ i right frac 3 2 right 2n m frac 3 2 e frac 2 3f left 2e_ i right frac 3 2 as compared to w p p t displaystyle w_ ppt the absence of summation over n which represent different above threshold ionization ati peaks is remarkable strong field approximation for the ionization rate edit the calculations of ppt are done in the e gauge meaning that the laser field is taken as electromagnetic waves the ionization rate can also be calculated in a gauge which emphasizes the particle nature of light absorbing multiple photons during ionization this approach was adopted by krainov model 35 based on the earlier works of faisal 36 and reiss 37 the resulting rate is given by w k r a n n 2 π ω 2 p n n o s c 2 d ω f t i k a r ψ r 2 j n 2 n f n o s c 2 displaystyle w_ kra sum _ n n infty 2 pi omega 2 p left n n_ mathrm osc right 2 int mathrm d omega left ft left i_ kar psi left mathbf r right right right 2 j_ n 2 left n_ f frac n_ mathrm osc 2 right where n i e i ω displaystyle n_ i e_ i omega n o s c u p ω displaystyle n_ mathrm osc u_ p omega with u p displaystyle u_ p being the ponderomotive energy n n i n o s c displaystyle n n_ i n_ mathrm osc is the minimum number of photons necessary to ionize the atom j n u v displaystyle j_ n u v is the double bessel function p 2 ω n n o s c n i displaystyle p sqrt 2 omega n n_ mathrm osc n_ i n f 2 n o s c ω p cos θ textstyle n_ f 2 sqrt n_ mathrm osc omega p cos theta with θ displaystyle theta the angle between the momentum of the electron p and the electric field of the laser f ft is the three dimensional fourier transform and i k a r 2 z 2 n 2 f r n displaystyle i_ kar left frac 2z 2 n 2 fr right n incorporates the coulomb correction in the sfa model population trapping edit in calculating the rate of mpi of atoms only transitions to the continuum states are considered such an approximation is acceptable as long as there is no multiphoton resonance between the ground state and some excited states however in real situation of interaction with pulsed lasers during the evolution of laser intensity due to different stark shift of the ground and excited states there is a possibility that some excited state go into multiphoton resonance with the ground state within the dressed atom picture the ground state dressed by m displaystyle m photons and the resonant state undergo an avoided crossing at the resonance intensity i r displaystyle i_ r the minimum distance v m displaystyle v_ m at the avoided crossing is proportional to the generalized rabi frequency γ t γ m i t m 2 displaystyle gamma t gamma _ m i t m 2 coupling the two states according to story et al 38 the probability of remaining in the ground state p g displaystyle p_ g is given by p g exp 2 π w m 2 d w d t displaystyle p_ g exp left frac 2 pi w_ m 2 mathrm d w mathrm d t right where w displaystyle w is the time dependent energy difference between the two dressed states in interaction with a short pulse if the dynamic resonance is reached in the rising or the falling part of the pulse the population practically remains in the...