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Photoionization

Photoionization rates are obtained by convolving the radiation field with the photoionization cross section. Cross sections are included for all levels of every ion for a wide range of photon energies occurring in our model. The cross sections where taken from the Opacity Project ([Seaton, et al., 1993], [Cunto et al., 1993]), then averaged over resonances as in [Bautista et al. 1997] and split over fine structure according to statistical weights.

For inner shell photoionization not yet available from the opacity project we use the cross sections of [Verner and Yakovlev 1995]. Inner shell ionization in X-ray illuminated clouds is enhanced by Auger cascades. This process can result in the ejection of up to eight extra electrons (in the case of iron) in addition to the original photoelectron. We include this effect by treating each inner shell ionization/auger event as a rate connecting the ground state of one ion with another level of an ion (in general not adjacent to the initial ion). The rates for inner shell ionization/auger processes are calculated using the relative probabilities of the various possible outcomes of an inner shell ionization event from [Kaastra and Mewe 1989]. These yields are multiplied by the appropriate inner shell photoionization cross section in order to calculate a rate for each inner shell ionization/Auger cascade individually. Our level scheme also includes levels with inner shell vacancies, which are populated by inner shell/Auger events. Populations of these levels are calculated in the customary way, and the decays from these levels produce inner shell flourescence lines.


next up previous contents
Next: Collisional Ionization Up: Atomic Processes Previous: Atomic Processes   Contents
Tim Kallman 2014-06-30