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2.2. Photoionization Cross Sections

The availability of a very large number of photoionization cross sections from the Opacity Project is likely to influence considerably the modeling of radiative and collisional plasmas. There are two main features of these data: (i) inclusion of autoionizing resonances in the near-threshold region, (ii) cross sections for many excited states, typically several hundred bound states for each atom or ion. Prior to the Opacity Project, such data was available mainly for the ground states of relatively few ions, and the excited states were treated in either the hydrogenic approximation or with some Coulomb screening (such as in the quantum defect method).

Fig. 3 shows the photoionization cross section of a complex system, Fe II, illustrating the effect of resonances and the covergence of the close coupling wavefunction expansion. A total of 83 states of the core ion Fe III were included in the R-matrix (Nahar and Pradhan 1994a). In comparison, the central field calculations of Reilman and Manson (1979) underestimate the cross sections by several factors in the near threshold region, although at higher energy the discrepancy is relatively small. Comparison is also made with previous R-matrix calculations of Sawey and Berrington (1992) whose wavefunction expansion had not converged as it did not include the channels that contribute to the phtoionization of the dominant 3d shell; consequently their values are up to two orders of magnitude lower.

Figure 3

Figure 3. Photoionization cross section of the ground state 3d64s(6D) of Fe II (Nahar and Pradhan 1994a); filles circles - Reilman and Manson (1979); dashed line - Sawey and Berrington (1992).

Another important effect in photoionization, first described by Yu and Seaton (1987), is the formation of large resonances due to photoexcitation-of-core (PEC). The PEC resonances appear most strongly in the photoionization of excited bound states along a Rydberg series, characterized by a core ion state and a Rydberg electron, i.e. (ScLc)nl. At incident photon frequencies corresponding to dipole core transitions a doubly excited autoionizing state is formed, resulting in a large resonance. Fig. 4 illustrates the magnitude of PEC resonances in the photoionization of two excited Rydberg states of Fe III (Nahar, private communication).

Figure 4

Figure 4. Large resonances in photoionization cross sections due to photoexcitation-of-core (PEC). The arrow denotes the peak position of the PEC resonance, at the energy of the dipole transition 3d5(6S) -> 3d44p(6Po) in the core ion S IV.

We have

Equation 6

The dipole transition 6S -> 6Po between the Fe IV core states is responsible for the PEC resonance, while the Rydberg electron essentially remains a ``spectator'' owing to the weak interaction with the core states (the process is basically the same as in di-electronic recombination). The PEC resonances attenuate the photoionization cross sections by orders of magnitude, and the cross sections for apparently hydrogenic excited states deviate substantially from what may otherwise by a hydrogenic background.

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