Rationale for a Virtual Center
A. Background
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 a powerful tool to elucidate the properties of
complex materials.
There is a remarkable symbiosis between fundamental theory and synchrotron
radiation experiments. Theory and experiment are always mutually beneficial
and, for high precision scientific research, each is
essential to the development and impact of the other. Thus the availability
of third-generation synchrotron x-ray sources has led to experimental data
of unprecedented precision. The analysis of this data therefore requires
theories of correspondingly increased precision for an adequate treatment.
Conversely, advances in theory have permitted the analysis of experimental
data to a precision that was not feasible of a decade ago.
As an example, advances in the theory of XAFS by our group at Univ. of
Washington have revolutionized the field in several respects:
1) these advances have permitted a far more detailed analysis than possible
before, for example enabling accurate structure determinations considerably
beyond the 1st coordination shell; 2) they have led to greatly improved
efficiencies in data collection and analysis by eliminating the need for
reference compounds and semi-automating the analysis; and 3) they have
put state of the art theoretical tools in the hands of the users,
essentially eliminating the need for a theoretical guru to interpret
experimental data.
This last step has led to a paradigm shift in the way absorption spectra
are now treated by the experimental community.
We believe that similar advances are 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. At present, however, the investment in
theoretical understanding considerably lags that in the facilities
themselves. 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.
B. Historical Precedents
Historically, the Theory Center established at Daresbury
Laboratories by John Pendry and others in the early 70's was
very successful in advancing the theory of various spectroscopies,
particularly XAS and LEED. Although Daresbury is still active.
the only major Center devoted to photon spectroscopy theory is that
at the ESRF in Grenoble. There is, as yet, no US counterpart, despite
the fact that the US is a world leader in experimental synchrotron
facilities. This is unfortunate, given 1) the major investment in modern
synchrotron radiation light sources and their operation 2) the growing
interest in these facilities by scientists in many fields; 3)
the consequent needs for theoretical analysis tools, and 4)
the substantial benefits both to users and theorists of such theoretical
research.
In our view, the powerful 3rd generation experimental facilities will not live
up to their potential if the available experimental analysis is based on
underdeveloped theory. Moreover, advances in theory tend to
parallel advances in computational resources. These are taking place quite
rapidly at present, presenting opportunities for new scientific
developments that were not practicable a few years ago.
For comparison, there has always been a significant theoretical component
to complement the experimental particle physics effort in the US.
C. Cost Effectiveness
There is ample evidence that a Theory Center would be a
cost-effective advantage to experimental science at synchrotron sources.
A concrete example is the successful development of the general
theoretical x-ray spectroscopy codes FEFF by our group at UW. [FEFF
stands for the effective scattering amplitude f_eff,
an important ingredient in the theory of EXAFS (extended x-ray absorption
fine structure)]. The FEFF codes are now one of the most important theoretical
tools in EXAFS spectroscopy and have revolutionized the field by making
state of the art theory widely available to researchers.
For example, instead of comparing to empirical standards - which seldom
works well - scientists can use fast theoretical calculations to analyze
EXAFS in terms of local atomic structure with confidence and reliability.
Thus the codes shortened measurement times and also lead to a more effective
and detailed analysis.
This expert programming effort also avoided duplicate programming and
development, for example by non-expert experimental post-docs.
D. User Representation
We stress the need for user input in these theoretical developments. The
design of the FEFF codes took advantage of significant input from
experimental users (e.g., the groups of E. Stern at UW, G. Brown
at 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 envisage 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.
E. Composition and Goals of a Theory and Analysis Center
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.
The estimated man-power needs for such a center will be discussed
at the Workshop. In my view a nominal effort would include 1 director
(or 2 co-directors in the case of a Virtual Center with two main hubs),
2-3 research scientist positions, about 5 postdoc positions and perhaps
10 graduate students. A university or national laboratory setting for
a Center with some proximity to synchrotron light sources might
be preferable, e.g., Berkeley, Stanford, LBNL or ANL.
However, we believe that there would be a number of advantages to
a Virtual Center which could efficiently combine theoretical efforts
at several universities and national laboratories, even far from light
sources, without the need for substantial new
space or administrative needs. Electronic links could then connect the center
to all the major synchrotron facilities and other research laboratories. Some
of the postdocs could just as well be located at the synchrotron facilities
themselves so as to provide both theoretical and analysis support to
users. Indeed just the availability of an interested theorist can have an
intangible positive benefit to synchrotron based research efforts.
As noted in the LBNL Workshop Findings, such a Theory and Analysis Center
might fit well into
a DOE-2000 type project philosophy, both with regard to the concept of
collaboration between universities and national laboratories and advanced
simulation of expensive experiments.
Return to the front page