By Jonathan Dorfan
In my April 6 All Hands, I spoke of the
pioneering science that we will do at SLAC using ultra fast x-rays. I
introduced you all to the Stanford
Ultrafast Science Center that we have set up at SLAC to recruit and focus the world’s best talent to make astounding
discoveries using the Sub-Picosecond Pulse Source (SPPS) facility in the
Final Focus Test Beam (FFTB) and, beginning in 2009, the Linac Coherent Light
Source (LCLS). The lead story in the
October 21 TIP
recruitment of Professor Phil Bucksbaum as the first Director of the
Ultrafast Science Center.
(Photo by Diana Rogers)
The SPPS facility is already breaking new ground,
providing the hitherto unknown scientific basis for processes as
fundamental as how substances melt. When a snowball melts, you can tell
it has achieved a liquid state when the water drips through your
fingers. But if you could follow the melting process, driven by the heat
of your hand, from its very first instant—the first trillionth of
second—would you be able to point to the exact moment the snowflake
crystals disorder into liquid H2O?
That is one of the many challenging questions
facing researchers using the SPPS to probe the activities of materials
on ultra fast timescales. SPPS makes intense x-ray pulses lasting
quadrillionths of a second (called ‘femtoseconds’), by taking the
electron beam from the linac, compressing it, and passing it through an
undulator magnet in the FFTB. These pulses enable researchers to
directly monitor through diffraction of the ultra fast x-rays the
earliest atomic changes during melting.
One of the first SPPS experiments looked at the
laser-driven melting of a semiconductor material similar to silicon.
When the laser light strikes the semiconductor crystal, it first
disrupts the electrons in the crystal, allowing the atoms to break from
their constrictive bonds and move freely with their inertial energy. The
experiment showed that in the first 500 femtoseconds, the atoms start
moving away from their initial positions, spreading out into a larger
volume like ripples from a stone tossed into a puddle but retaining the
overall crystal shape.
New follow-on research has extended the time range
and shown more. When the incredibly short period of 500 femtoseconds has
passed, the atoms start to bump into their neighbors. The collisions
produce random, diffusive motion, which breaks down the tetrahedral
shape of the crystal that characterizes its solid phase. The data
suggests that it’s the collisions between atoms that is the crucial
mechanism for turning a solid into a liquid.
The study also shows that direction matters. The
distance that atoms travel before hitting boundaries (namely the other
atoms) depends on which direction an atom is traveling. Thus the crystal
disorders, and a liquid state is formed, at different rates in different
Congratulations to the SPPS team for this new,
exciting science. These measurements demonstrate the importance of high
brightness, ultra fast x-ray probes for studying how reactions and
structural changes occur at the level of the atoms on time scales that
are ‘natural’ for the events.
We can all look forward to more remarkable
discoveries at SLAC when the full power of ultra fast science is
unleashed with the start-up of the LCLS in 2009. The first step in the
LCLS construction has begun with the preparations for the injector at
Sector 20 of the linac, and this spring the construction will start in
earnest. LCLS construction was fully funded in the 2006 Energy and Water
Appropriations Bill at $83 million. Many thanks to our colleagues in the
DOE’s Office of Science for their exceptionally strong support of LCLS
and their hard work in securing this funding which is so important to
the future of science at SLAC.