J.J. Rehr , U. of Washington

The availability of major synchrotron radiation photon sources (i.e., sources of light ranging from ultra-violet to x-rays) combined with advances in the understanding of various spectroscopies (e.g., x-ray absorption spectroscopy (XAS), photoelectron diffraction (PD), etc.) has provided new and powerful high precision tools to elucidate the properties of complex materials. However to use such high precision tools effectively requires theory and analysis of comparable precision. Fortunately, there is a remarkable symbiosis between fundamental theory and synchrotron radiation experiments. Theory and experiment are generally mutually beneficial and, for high precision scientific research, each is essential to the development and impact of the other. For example, advances in theory have permitted the analysis of experimental data to a precision that was not feasible of a decade ago with more phenomenological theories. An example is the development of the XAFS analysis codes FEFF by our group at Univ. of Washington, which has revolutionized the field. This advance has led to a paradigm shift in the way absorption spectra are now treated by the experimental XAS community. Similar advances should be feasible in many other spectroscopies, opening the possibility of great opportunities for scientific and technological advances. Such advances are important to make the synchrotron facilities accessible to a wide range of scientists, including biophysicists, chemists, geophysicists and industrial scientists. Thus, we feel that a modest investment both in theory and analysis tools would significantly enhance the scientific output of synchrotron sources. Thus one of the conclusions of the recent LBNL Workshop on Theory and Computation Synchrotron Applications was the national need for partnerships between universities, national laboratories, and light sources, i.e., a ``Virtual Center," devoted to Photon Spectroscopy Theory and Analysis.

We stress the need for user input in various theoretical developments for synchrotron applications. For example design of the FEFF codes took advantage of significant input from experimental users (e.g., the groups of E. Stern at UW, K. Hodgson and Gordon Brown at SSRL and Stanford, D. Sayers at NCSU, F. Bridges at UCSC and K. Baberschke at Berlin) both in the design of various options and in the design of the analysis tools. At the same time experimental input from users provided high precision tests of new theories. Thus we believe that a successful Theory Center must have significant user representation and a mission that is partly dedicated to the analysis of present and future light-source related experiments. Especially needed are fast, automated analysis tools to deal with the copious data from synchrotron experiments, preferably on-line in a user-friendly manner.

In conclusion we feel that a Photon Spectroscopy Theory Center should address the following major areas:

1. fundamental theory of various photon spectroscopies. This includes theory both at the forefront and at practical levels connected to experiment. Needed are expertise both in ground state and excited state calculations for all hard and soft x-ray applications (e.g., XAS, XPS, PD, etc) at all the major synchrotron centers, (i.e., APS, NSLS, SSRL, and ALS).
2. experimental analysis tools and theoretical support for users (e.g., software, documentation, graphical interfaces, etc) linking on-line theory and experiments.
3. periodic workshops devoted to particular spectroscopies or the use of various theoretical and analysis codes should also be part of the Center's mission.