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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/09/29 01:23:01」(JST)

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Photoionization is the process that makes to glow once-invisible filaments in deep space.^[1]

Photoionization is the physical process in which an ion is formed from the interaction of a photon with an atom or molecule.^[2]

Cross section

Not every photon which encounters an atom or ion will photoionize it. The probability of photoionization is related to the photoionization cross-section, which depends on the energy of the photon and the target being considered. 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. At even higher intensities (around 10¹⁵ – 10¹⁶ W/cm² of infrared or visible light), non-perturbative phenomena such as barrier suppression ionization^[3] and rescattering ionization^[4] 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 (below about 10¹⁴ W/cm² at optical frequencies), the probability of absorbing N photons depends on the laser-light intensity I as I^N .^[5] For higher intensities, this dependence becomes invalid due to the then occurring AC Stark effect.^[6]

Resonance-enhanced multiphoton ionization (REMPI) 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 (ATI) ^[7] 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 (lowest possible number of photons) ionization case. The electrons released from the target will have approximately an integer number of photon-energies more kinetic energy.^{[citation needed]}

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.

References

^ "Hubble finds ghosts of quasars past". ESA/Hubble Press Release. Retrieved 23 April 2015.
^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "photoionization".
^ Delone, N. B.; Krainov, V. P. (1998). "Tunneling and barrier-suppression ionization of atoms and ions in a laser radiation field". Physics-Uspekhi 41 (5): 469–485. Bibcode:1998PhyU...41..469D. doi:10.1070/PU1998v041n05ABEH000393.
^ Dichiara, A.; et al. (2005). "Cross-shell multielectron ionization of xenon by an ultrastrong laser field". Proceedings of the Quantum Electronics and Laser Science Conference 3. Optical Society of America. pp. 1974–1976. doi:10.1109/QELS.2005.1549346. ISBN 1-55752-796-2.
^ Deng, Z.; Eberly, J. H. (1985). "Multiphoton absorption above ionization threshold by atoms in strong laser fields". Journal of the Optical Society of America B 2 (3): 491. Bibcode:1985JOSAB...2..486D. doi:10.1364/JOSAB.2.000486.
^ Protopapas, M; Keitel, C H; Knight, P L (1 April 1997). "Atomic physics with super-high intensity lasers". Reports on Progress in Physics 60 (4): 389–486. doi:10.1088/0034-4885/60/4/001. Retrieved 19 August 2013.
^ Agostini, P.; et al. (1979). "Free-Free Transitions Following Six-Photon Ionization of Xenon Atoms". Physical Review Letters 42 (17): 1127–1130. Bibcode:1979PhRvL..42.1127A. doi:10.1103/PhysRevLett.42.1127.

English Journal

Determination of stimulants using gas chromatography/high-resolution time-of-flight mass spectrometry and a soft ionization source.

Lopez-Avila V, Cooley J, Urdahl R, Thevis M.SourceAgilent Technologies, Santa Clara, CA, 95051, USA. viorica_lopez-avila@agilent.com.
Rapid communications in mass spectrometry : RCM.Rapid Commun Mass Spectrom.2012 Dec 15;26(23):2714-24. doi: 10.1002/rcm.6398.
RATIONALE: The aim of this study was to investigate the mass spectral fragmentation of a small set of stimulants in a high-resolution time-of-flight mass spectrometer equipped with a soft ionization source using vacuum ultraviolet (VUV) photons emitted from different plasma gases. It was postulated
PMID 23124661

Photoionization Cross Sections and Asymmetry Parameters for Valence Shell of Methanol.

Lavín C, Vega MV, Velasco AM.AbstractTheoretical cross sections for photoionization of the methanol valence orbitals in covering a region up to 80 eV beyond the first ionization potential are reported. The molecular quantum defect orbital, MQDO, method, which has proved to be reliable in previous applications to molecular photoionization, has been used. To our knowledge, predictions of electronic partial cross section profiles on this molecule are made here for the first time and we are not aware of any reported experimental data. Partial cross sections for production of parent and fragment ions of methanol have also been calculated and compared with previous measurements. In addition, the MQDO method has been used to calculate the angular distribution of photoelectrons for the valence orbitals of methanol over the 11-50 eV photon energy range. Our results are compared with experimental data showing a good agreement in most cases. We hope that the present results might be of use in atmospheric and interstellar chemistry, where this molecule plays an important role.
The journal of physical chemistry. A.J Phys Chem A.2012 Nov 15. [Epub ahead of print]
Theoretical cross sections for photoionization of the methanol valence orbitals in covering a region up to 80 eV beyond the first ionization potential are reported. The molecular quantum defect orbital, MQDO, method, which has proved to be reliable in previous applications to molecular photoionizati
PMID 23153107