Multiphoton ionization

Multiphoton ionization is the process of ionization of an atom , molecule or ion in the field of an electromagnetic wave by absorption of two or more photons . It is a special case of multiphoton absorption .

Multiphoton ionization is a particular limiting case of a more general ionization process in an alternating field (the opposite limiting case, when the dependence of the external field on time can be neglected, is tunnel ionization ).

Physics of Phenomenon

A necessary condition for observing multiphoton ionization is the fulfillment of the condition $h\nu <eI_{p}$ ${\ displaystyle h \ nu <eI_ {p}}$ where $h\nu$ ${\ displaystyle h \ nu}$ Is the photon energy ( $\nu$ ${\ displaystyle \ nu}$ - frequency of electromagnetic radiation), $eI_{p}$ ${\ displaystyle eI_ {p}}$ - ionization energy ( $I_{p}$ ${\ displaystyle I_ {p}}$ - ionization potential ). At the same time, the total energy of the absorbed photons $Nh\nu$ ${\ displaystyle Nh \ nu}$ ( $N$ ${\ displaystyle N}$ - the number of photons absorbed in one elementary event) should be greater than the ionization energy.

Ionization probability $w$ ${\ displaystyle w}$ depends on radiation intensity $I$ ${\ displaystyle I}$ and is connected with it by a power law:

w=\alpha _{N}I^{N}

{\ displaystyle w = \ alpha _ {N} I ^ {N}}

Thus, multiphoton ionization is a nonlinear process with respect to radiation intensity. Constant $\alpha _{N}$ ${\ displaystyle \ alpha _ {N}}$ depends on the type of ionized atom, as well as on the frequency and polarization (for non-spherical molecules) of the radiation. For addiction $\alpha _{N}(\omega )$ ${\ displaystyle \ alpha _ {N} (\ omega)}$ resonances associated with the coincidence of the radiation frequency with one of the transitions in the spectrum of an atom or molecule are characteristic (in a strong field, the Stark effect must also be taken into account).

Multiphoton ionization, generally speaking, is not a threshold effect, that is, in principle, it can be observed at arbitrarily small radiation intensities. In practice, however, experimental observation of the effect requires relatively high intensities, achievable only with the use of lasers . In addition, multiphoton ionization can be obtained only in rarefied gases. In dense gases (at pressures of more than a few torr ), avalanche ionization dominates.

Application

The multiphoton ionization process underlies the method of multiphoton resonance spectroscopy , which is characterized by a high degree of frequency selectivity and a high detection efficiency.

Literature

H. B. Delaunay . Physical Encyclopedia : [in 5 volumes] / Ch. ed. A.M. Prokhorov . - M .: Big Russian Encyclopedia, 1992. - T. 3: Magnetoplasma - Poynting's theorem. - S. 165 .-- 672 p. - 48,000 copies. - ISBN 5-85270-019-3 .

Multiphoton ionization

Physics of Phenomenon

Application

Literature

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