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