This proposal sets out to answer the following astrophysically important questions: Where are oxygen,

silicon, and iron found in the universe? What are their abundances and physical and chemical forms? A

complete set of atomic, molecular, and solid-state photoabsorption data. We propose to generate such

data through a combination of the following theoretical techniques. Firstly, R-matrix calculations will

be carried out to obtain the K-shell photoabsorption of atomic silicon and the L-shell of iron; in this

respect atomic oxygen has already been extensively treated by us. Secondly, the UK molecular R-matrix

(UKRmol) package will be used to compute the photoabsorption of molecular oxygen (O2), carbon monoxide

(CO), carbon dioxide (CO2). And thirdly, we will implement multiple scattering theory in tandem with an

atomic R-matrix treatment for each atom to compute photoabsorption cross sections for condensed-matter

systems such as oxides, silicates, and other compounds comprising interstellar dust and ice.

A final model will be developed in a fashion consistent with the photoabsorption in all environments -

atomic, molecular, and solid-state. To this end, a consistent model for all cases is developed that (1)

preserves the oscillator strength sum rule per electron, and (2) exhibits the expected identical

absorption away from the inner-shell thresholds. Such a model allows for a controlled measure of the

quantitative differences in the near-edge structure of atomic, molecular, and solid-state X-ray spectral

observations.

For oxygen, the atomic neutral and ionic cross sections are now well established from our recent work,

and we will use the UKRmol codes to compute K-shell photoabsorption cross sections for O2, CO, and H2O.

For more complicated systems, we will perform R-matrix calculations for the individual atoms and utilize

multiple scattering theory to compute the photoabsorption cross section.

For Si, atomic R-matrix calculations will be performed for the K-shell atomic cross sections, and then

multiple scattering theory will be used to treat more complex systems.

For the more complex case of iron L-shell absorption, we will perform large-scale atomic R-matrix

calculations, using three approaches: a non-relativistic LS-coupled Hamiltonian, a Breit-Pauli

Hamiltonian, and a Dirac-Fock Hamiltonian, the latter two to include the important fine-structure

splitting of thresholds. Multiple scattering theory will be used with the atomic R-matrix information to

treat photoabsorption in solid-state environments, and a consistent atomic, molecular, and solid-state

absorption model will be developed.

By determining atomic, molecular and solid state cross sections on the same footing, we will use

available experimental and astronomical (Chandra) observations, in the case of atomic oxygen, and

experimental cross sections, in the case of silicates, to calibrate the exact position of all K-shell

thresholds, as well as the absolute cross sections. Moreover, current experimental cross sections for a

few silicate compositions will help us study the effects of chemical binding on the position and shape

of the cross sections across inner-shell thresholds. Thus, we will be able to provide self-consistent

cross sections for all forms of oxygen, silicon, and iron in the X-ray region accessible to Chandra and

XMM-Newton.

The derived data and analytical model will be made available to the astrophysics community, and will be

incorporated into the XSTAR database for x-ray spectral modeling analysis. From observed x-ray spectra

near the K-edge of O and Si, and the L-edge of Fe, we will infer the compositions of each of these three

elements and help answer the question posed initially: in what forms and abundances are oxygen, silicon,

and iron found in the universe?