Successsive resonance-enhanced two-photon ionization of elements abundant in nebulae. I. Atoms and ions of C, N, and O

Abstract:
We discuss resonance-enhanced two-photon ionization (RETPI) and presentschemes of successive RETPI of the elements C, N, and O in nebulae.RETPI is activated by intrinsic radiation stored in the form of trappedspectral lines of HI, HeI, and HeII in the optically thick nebula. Therate of this two-step photoionization is comparable with or exceeds thelow recombination rate of the photoions formed in the process. Thisleads to an accumulation of photoions and subsequent RETPI until suchhighly charged ions are formed that they cannot further be ionized inthis way by the intrinsic radiation from the strong spectral lines ofHI, HeI, and HeII. Erratum: http://lu-research.lub.lu.se/php/gateway.php?who=lr&method=getfile&file=archive/00017447/