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

I. Atoms and ions of C, N, and O

¹ Department of Physics, Lund University, PO Box 118, 22100 Lund, Sweden e-mail: Vladilen.Letokhov@fysik.lu.se
² Institute of Spectroscopy, Russian Academy of Sciences, Troitsk, Moscow region, 142190, Russia

Corresponding author: S. Johansson, sveneric.johansson@fysik.lu.se

Received: 8 January 2001
Accepted: 21 May 2001

Abstract

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 photoionization 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.