When a snowball melts, you can tell it has achieved a liquid state when the frigid water drips through your fingers. But if you could follow the melting process, driven by the heat of your hand, from its very first moments - the first trillionth of second, would you be able to point to the exact moment the snowflake crystals disorder into liquid H2O? That's the challenge facing researchers using the Sub-Picosecond Pulse Source (SPPS) to probe the activities of materials on ultrafast timescales. SPPS makes intense x-ray pulses lasting quadrillionths of a second (femtoseconds), enabling researchers to directly monitor the earliest atomic changes during melting with ultrafast x-ray diffraction.

In one of the first SPPS experiments, SLAC scientists looked at the laser-driven melting of a semiconductor material similar to silicon. In that study, they found that the atoms in the tetrahedrally shaped crystal moved away from their crystal lattice positions, but retained the overall crystal shape in the first 500 femtoseconds (half a picosecond). New follow-on research has extended the time range and shown more: that the atoms move faster parallel to the chemical bonds than they do in the perpendicular direction, opposite of what models have so far predicted. At about 500 femtoseconds, the atoms have moved far enough to bump into their neighbors. The collisions produce random, diffusive motion, no longer preserving the tetrahedral shape. This suggests that collisions are the mechanism for turning a solid into a liquid. This work was led locally by Kelly Gaffney, Aaron Lindenberg, and Jerry Hastings of SSRL and critically depended on the efforts of the SPPS international collaboration.

A. M. Lindenberg, J. Larsson, K. Sokolowski-Tinten, K. J. Gaffney, et al. 'Atomic-Scale Visualization of Inertial Dynamics.' Science 308, 392 (2005).

K. J. Gaffney, A. M. Lindenberg, J. Larsson, K. Sokolowski-Tinten, et al. 'Observation of Structural Anisotropy and the Onset of Liquidlike Motion During the Nonthermal Melting of InSb.' Phys. Rev. Lett., 95, 127501(2005).

(Full author lists for these papers can be found in the technical science highlight)