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Science

Multiphoton Ionization of Atoms

C. K. Rhodes

Published Sep 27, 1985• 148 citations
Publisher

Abstract

Studies of multiphoton ionization of atoms have revealed several unexpected characteristics. The confluence of the experimental evidence leads to the hypothesis that the basic character of the atomic response involves highly organized, coherent motions of entire atomic shells. The important regime, for which the radiative field strength is greater than an atomic unit (e/a(2)(0)), can be viewed in approximate correspondence with the physics of fast (approximately 10 MeV per atomic mass unit) atom-atom scattering. This physical picture provides a basis for the expectation that stimulated emission in the x-ray range can be produced by direct, highly nonlinear coupling of ultraviolet radiation to atoms.

DOI
10.1126/science.229.4720.1345

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Abstract : Studies of multiphoton ionization of atoms have revealed several unexpected characteristics. The confluence of the experimental evidence leads to the hypothesis that the basic character of the atomic response involves highly organized, coherent motions of entire atomic shells. The important regime, for which the radiative field strength E is greater than an atomic unit, can be viewed in approximate correspondence with the physics of fast atom-atom scattering. This physical picture provides a basis for the expectation that stimulation emission in the x-ray range can be produced by direct highly nonlinear coupling of ultraviolet radiation to atoms. Keywords: Ions, Multiphoton, Photons.

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Multiphoton ionization (MPI) of alkali atoms has played a pivotal role in our understanding of the interaction of intense electromagnetic radiation with matter. The "hydrogen-like" energy levels and absence of low-lying autoionizing states make the alkali atom theoretically tractable. The low ionization potential of these atomic species and the ease with which one can produce atomic beams of alkali atoms facilitate the experimental studies. Many of the early MPI studies involved high-powered fixed-frequency lasers. Presently, the availability of tunable dye lasers with wavelength extension (i.e., frequency doubling, Raman shifting, etc.) makes it possible to study one- to six-photon ionization of the alkali atom with continuously tunable radiation. Also, harmonic generation in these systems which may be present during ionization can be recorded more easily than in other systems since the radiation generated is in the visible or near ultraviolet spectral region. The ability to tune the laser to high Rydberg states with even or odd numbers of photons accesses electronic states of both parities. Stepwise excitation is capable of selectively producing states of high orbital angular momentum. Experiments in which highly excited states are produced in external electric or magnetic fields allow one to test fundamental ideas about the electron–Coulomb system in the quasi–continuum.3

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Studies of multiphoton ionization of atoms have revealed several unexpected characteristics. The confluence of the experimental evidence leads to the hypothesis that the basic character of the atomic response involves highly organized, coherent motions of entire atomic shells. The important regime, for which the radiative field strength is greater than an atomic unit (e/a(2)(0)), can be viewed in approximate correspondence with the physics of fast (approximately 10 MeV per atomic mass unit) atom-atom scattering. This physical picture provides a basis for the expectation that stimulated emission in the x-ray range can be produced by direct, highly nonlinear coupling of ultraviolet radiation to atoms.
Abstract : Studies of multiphoton ionization of atoms have revealed several unexpected characteristics. The confluence of the experimental evidence leads to the hypothesis that the basic character of the atomic response involves highly organized, coherent motions of entire atomic shells. The important regime, for which the radiative field strength E is greater than an atomic unit, can be viewed in approximate correspondence with the physics of fast atom-atom scattering. This physical picture provides a basis for the expectation that stimulation emission in the x-ray range can be produced by direct highly nonlinear coupling of ultraviolet radiation to atoms. Keywords: Ions, Multiphoton, Photons.
In recent years a strong interest has developed in the study of the basic coupling mechanisms governing high order multiphoton processes in atomic systems.1 Such interest has been stimulated by the rapid advances in short pulse generation techniques which allow atomic interactions at an intense field strength of the order of atomic field to be studied. Furthermore, good understanding of the highly nonlinear photon interactions is necessary to evaluate the possibility of using multiphoton processes2 as a means to couple energy into excited states emitting in the x-ray region.
It has been conjectured that atomic motions fundamentally different from those presently observed may be driven under extreme conditions of irradiation. Indeed, it has been suggested1-4 that ordered many-electron motions of outer-shell electrons could lead to enhanced rates of coupling from the radiation field to an atom. In order to study the conditions necessary for new atomic responses to occur and to determine the influence of possible damping mechanisms, the properties of ion charge state distributions,5 electron energy spectra,6 and harmonic radiation7 produced by irradiation of atoms with ultra-violet radiation with different pulse lengths, wavelengths, and intensities have been investigated.
The non linear interaction between an intense laser radiation and atoms leads to ionization through the absorption of N photons from the laser radiation via laser-induced virtual states. The multiphoton ionization rate varies as a function of the laser intensity I as IN. We discuss the two most important effects which govern multiphoton ionization processes : resonance effects and laser-coherence effects. In a moderate laser intensity range (107 - 109 W cm-2) corresponding to the two, three or four-photon ionization of atoms, resonance effects play a dominant role while the photon statistics of the laser radiation are relatively unimportant. On the contrary, in the very high intensity range (1012 - 1013 W cm-2) required to observe large Nth - order ionization processes, coherence effects play a dominant role through N! enhancement in the N-photon ionization rate, while resonance effects are completely damped due to the high laser intensity. We conclude by reviewing the possible applications of multiphoton absorption processes in other fields.
The most important results of study of multiphoton ionization of atoms are reviewed. Discussions are presented of specific features of the process, theoretical methods of description of ionization of atoms in a strong light field, the basic principles of design of experiments in which intense laser radiation is used, results of investigation of direct and resonance processes of multiphoton ionization, and the mutual relation between multiphoton processes (multiphoton excitation and ionization) and various phenomena leading to perturbation of the spectrum of bound states of an electron in an atom under the influence of a strong light field.
Multiphoton ionization (MPI) of alkali atoms has played a pivotal role in our understanding of the interaction of intense electromagnetic radiation with matter. The "hydrogen-like" energy levels and absence of low-lying autoionizing states make the alkali atom theoretically tractable. The low ionization potential of these atomic species and the ease with which one can produce atomic beams of alkali atoms facilitate the experimental studies. Many of the early MPI studies involved high-powered fixed-frequency lasers. Presently, the availability of tunable dye lasers with wavelength extension (i.e., frequency doubling, Raman shifting, etc.) makes it possible to study one- to six-photon ionization of the alkali atom with continuously tunable radiation. Also, harmonic generation in these systems which may be present during ionization can be recorded more easily than in other systems since the radiation generated is in the visible or near ultraviolet spectral region. The ability to tune the laser to high Rydberg states with even or odd numbers of photons accesses electronic states of both parities. Stepwise excitation is capable of selectively producing states of high orbital angular momentum. Experiments in which highly excited states are produced in external electric or magnetic fields allow one to test fundamental ideas about the electron–Coulomb system in the quasi–continuum.3