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For two decades Tantalus produced hundreds of experiments
and was a testing ground for many of the synchrotron techniques
used today. Its administrative home, the University of
Wisconsin Synchrotron Radiation Center, was located in a
bucolic environment more than 13 miles from the Madison
campus. The relative isolation facilitated strong bonds
among users. The SRC’s annual users meeting became an important
event; figure 3 pictures Brown and Rowe at one of
the first gatherings, around 1969.
Today’s dedicated synchrotron facilities can be as large
as a city block. But Tantalus was no bigger than a dinner table,
and its small building, even after a substantial expansion in
1972, was incredibly crowded with equipment and researchers.
Users worked in very close quarters. The close
proximity made cross fertilization of ideas unavoidable. The
atmosphere was open, friendly, informal, and exciting.
It was not particularly comfortable physically, though.
For one thing, the system that heated the control room did
not work in an adjoining washroom. So, to avoid frozen
pipes, users just left the door wide open. After someone
posted a sign alerting users to the policy, an international contest
began, with each person translating the message into his
own language. Acopy of the cosmopolitan sign, shown in figure
4, eventually became part of an NSF funding request as
evidence of Tantalus’s growing international impact.
That impact was truly remarkable. After struggling with
synchrotrons, users came from many countries to discover in
Tantalus an easy-to-use light source. Research during those
early years was dominated by optical spectroscopy of atoms,
molecules, and solids. The broad band of available wavelengthswavelengths
made that a good choice. The photon energies
reached the core-level thresholds in many materials and allowed
researchers to investigate a wealth of phenomena,
most notably electron-correlation effects. Moreover, Tantalus
brought a new dimension to optical experiments. For example,
it supported thermomodulation and electromodulation
studies of solids,9 and thereby expanded the scope of modulation
spectroscopy, a leading field at that time. By using, say,
an oscillating electric or thermal field to perturb a semiconductor,
researchers could extract hidden features from the
optical spectra. The approach solved important issues about
the band structure of gallium arsenide and other materials.
In the mid-1970s the center of gravity at Tantalus gradually
shifted toward photoemission experiments, thanks
largely to a steady improvement of the emitted intensity,
which increased with the beam current circulating in the ring.
The initial Tantalus injector was the old FFAG synchrotron;
only one electron bunch was injected in the ring, which
yielded a current between 1 and 2 mA—three orders of magnitude
below what can be achieved today. The advent of multiple
bunches in 1973 increased the current to 50 mA. Injection
of electrons using a 40-meV accelerator known as
a microtron in 1974 pushed current levels still higher—to
150 mA in 1974 and to an amazing 260 mA in 1977.
In 1971 Dean Eastman and Warren Grobman of IBM produced
the first photoelectron spectra using Tantalus (see figure
5), a result that revealed momentum conserved in photoemission
and changes in the lineshape of gold with photon
energy.10 The demonstration was a milestone in the development
of photoemission as a research tool. The tunability of
synchrotron light allowed researchers to disentangle a
material’s ground-state electronic properties—their main
objective—from its final-states effects, transition probabilities,
and other factors.Between 1974 and 1975, Tantalus reached an intensity
level sufficient for angle-resolved photoemission. A joint
Bell Labs–Montana team led by Neville Smith, Morton
Traum, and Lapeyre conducted the earliest experiments.13
Figure 6 illustrates the impressive first results: The angular
intensity patterns revealed the crystal symmetry of a layered
compound.
As an experimental technique, angle-resolved photoemission
developed rapidly and had an important conceptual
impact on condensed-matter physics.in gas-phase spectroscopy was yet another pillar
of success at SRC, starting from the early absorption studies
of noble gases14 and silane.15 Throughout the 1970s and
early 1980s, Thomas Carlson and Manfred Krause of Oak
Ridge National Laboratory and others produced important
results on Tantalus concerning auto-ionization, shape resonances,
Cooper minima, and several other phenomena.16
James Taylor’s team from the University of Wisconsin–
Madison inaugurated gas-phase photoemission in 1972.17
The results of their studies revealed strong photon-energy
effects that required, for example, a careful reanalysis of
previous benzene data.
The SRC produced more than a flow of experimental results.
It was also the source of advanced optical instrumentation
such as focusing devices and monochromators. In 1973
Ed Rowe, Mills, and Walter Trzeciak even tested insertion devices,
arrays of magnets that produce highly collimated and
very intense beams of light by transversely “wiggling” the
electrons passing through them.
The cases discussed here are merely a fra
<ref name="Marg">{{Cite journal
|last=Margaritondo |first=Giorgio
|year=2008
|title=The evolution of a dedicated synchrotron light source
|journal=Physics Today
|volume=61 |pages=37-43
|doi=10.1063/1.2930734
}}</ref>
===Tantalus: 1968-1985===
With the new Aladdin storage ring operating, Tantalus was officially decommissioned in 1987, although it was run for six weeks in the summer of 1988 for experiments in atomic and molecular fluorescence. The storage ring was disassembled in 1995, and half the ring, the RF cavity and one of the original beamlines are now in storage at the Smithsonian Institution.<ref name="Tant" />
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