**4.3. Photoionization Cross Sections**

A comprehensive bibliography has been compiled by LeDourneuf (ZLD, 1991). General data sources are as follows.

1. TOPBASE provides detailed photoionization cross sections for Z = 1-14 isoelectronic sequences up to Iron. As mentioned above, these cross sections may not be accurate in the high energy, non-resonant region where they have been fitted to a simple analytic form that does not correspond to actual photoionization calculations. The photoionization data for some of the heavier elements in low ionization stages (e.g. Fe I - V) is being updated, as the older data is not accurate (users are advised to check the NEWS file).

2. ``Subshell photoionization cross sections and ionization energies of atoms and ions from He to Zn'', have been computed by Verner et al. (1993) using the Hartee-Dirac-Slater method, analogous to the central field calculations but including some relativistic effects. Ground state photoionization cross sections are tabulated in analytic form up to high energies, E 50 kev, including inner-shell ionization edges.

3. ``Analytic fits for partial photoionization cross sections'', by Verner and Yakovlev (1994, A&A in press), gives partial cross sections for the ground state shells of all atoms from H to Zn up to 100 kev.

4. ``Photoabsorption cross sections for positive atomic ions with Z 30'', by Reilman and Manson (1979) are obtained through Hartree-Slater central-field calculations. These include only the ground state data and do not include autoionizing resonances, or coupling effects, and consequently are not generally accurate in the low energy region of photoionization. However, at high energies and for multiply charged ions these data reproduce well the background cross sections, compared to TOPBASE or other R-matrix calculations.

5. ``X-ray interactions: photoionization, scattering, transmission, and reflection at E = 50-30,000 eV, Z = 1-92'', by Henke et al. (1993) describes the absorption and scattering of X-rays in terms of atomic scattering factors calculated from photoabsorption cross sections (both of which are tabulated). This work should be useful in applications involving high energy photoabsorption.

For astrophysical applications, a combination of 1. and 2. is recommended, for low and high energies respectively, until more accurate calculations are completed in the high energy range and incorporated into TOPBASE.

6. H-ions: A computer program for analytic calculations for H-like ions has been written by Storey and Hummer (1991).

7. Photoionization of heavy atomic systems is discussed by Kelly (1990).