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

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Science  26 Jan 2001:
Vol. 291, Issue 5504, pp. 625-627
DOI: 10.1126/science.291.5504.625

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Abstract

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 α 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 Hei and He ii radiation, which also gets enhanced in optically thick nebulae that contain enough helium.

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