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Science 26 January 2001:
Vol. 291. no. 5504, pp. 625 - 627
DOI: 10.1126/science.291.5504.625
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Reports

Resonance-Enhanced Two-Photon Ionization of Ions by Lyman alpha  Radiation in Gaseous Nebulae

Sveneric Johansson,1 Vladilen Letokhov12

One of the mysteries of nebulae in the vicinity of bright stars is the appearance of bright emission spectral lines of ions, which imply fairly high excitation temperatures. We suggest that an ion formation mechanism, based on resonance-enhanced two-photon ionization (RETPI) by intense H Lyman alpha  radiation (wavelength of 1215 angstroms) trapped inside optically thick nebulae, can produce these spectral lines. The rate of such an ionization process is high enough for rarefied gaseous media where the recombination rate of the ions formed can be 10-6 to 10-8 per second for an electron density of 103 to 105 per cubic centimeter in the nebula. Under such conditions, the photo-ions formed may subsequently undergo further RETPI, catalyzed by intense He I and He II radiation, which also gets enhanced in optically thick nebulae that contain enough helium.

1 Institute of Physics, Lund University, Lund S-221 00, Sweden.
2 Institute of Spectroscopy, Russian Academy of Sciences, Troitsk, Moscow region, 142190, Russia.

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Science. ISSN 0036-8075 (print), 1095-9203 (online)