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We discuss resonance-enhanced two-photon ionization (RETPI) and present schemes of successive RETPI of the elements C, N, and O in nebulae. RETPI is activated by intrinsic radiation stored in the form of trapped spectral lines of HI, HeI, and HeII in the optically thick nebula. The rate of this two-step photoion- ization is comparable with or exceeds the low recombination rate of the photoions formed in the process. This leads to an accumulation of photoions and subsequent RETPI until such highly charged ions are formed that they cannot further be ionized in this way by the intrinsic radiation from the strong spectral lines of HI, HeI, and HeII.

Successsive resonance-enhanced two-photon ionization of elements abundant in nebulae