Accurate wavelengths for X-ray spectroscopy and the NIST
Hydrogen and Hydrogen-Like ion databases

S. A. Kotochigova1;2, K. P. Kirby3, N. S. Brickhouse4

1National Institute of Standards and Technology, 100 Bureau Drive, stop 8423, Gaithersburg, Maryland 20899 USA
2Department of Physics, Temple University, Philadelphia, PA 19122 USA
3Institute for Theoretical Atomic and Molecular Physics, Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138 USA
4Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138 USA

We describe our ab initio relativistic approach that is developed to provide accurate wavelengths for X-ray spectroscopy. This approach is based on computational techniques that are well suited to treat relativistic and correlation effects in highly charged ions. We use the large-scale configuration-interaction (CI) Dirac-Fock method to solve the Dirac-Coulomb-Breit equation and calculate energies, transition frequencies and probabilities, using non-orthogonal bases. To generate CI basis sets we solve Dirac-Fock equations for occupied orbitals and the Dirac-Fock-Sturm equations for virtual or non-occupied orbitals numerically. Since the Dirac-Fock-Sturm operator has a complete and discrete set of eigenfunctions, the basis set includes the continuous part of the spectrum.

The performance of our computational technique is tested by comparing our calculations of the p - s and p - d transitions in Fe XVIII and Fe XIX to known experimental results. An estimate of the Lamb shift for these transitions shows that it may start to play a role at this level of 10-3Å accuracy and therefore it has to be included for more precise calculations.

Finally, I will present my work on NIST databases. In particular, I will show two interactive databases designed to calculate total energies and transition frequencies of hydrogen and deuterium as well as hydrogen-like ions. The values are highly accurate and based on current knowledge of the relevant theory, which includes relativistic, quantum electrodynamic, recoil, and nuclear size effects. The hydrogen-like ion database is possibly of interest for X-ray spectroscopy, whereas the hydrogen and deuterium data base lays the theoretical groundwork for precise determination of ground and excited energies.