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 announced the recruitment of Professor Phil Bucksbaum as the first Director of the Ultrafast Science Center.

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 directions.

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.