PerspectiveULTRAFAST X-RAY DIFFRACTION

Watching Matter Rearrange

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Science  12 Nov 1999:
Vol. 286, Issue 5443, pp. 1310-1311
DOI: 10.1126/science.286.5443.1310

Can we “watch” the elementary molecular or collective motions that lead to chemical or structural change as they take place (see the figure below)? On page 1340 of this issue, Siders et al. (1) report an important step toward this goal. Using ultrafast pulses of x-ray radiation as the probe, they monitor the melting of a semiconductor sample after irradiation by an intense, ultrafast visible pulse. The experiment (see the second figure below) allows the direct measurement of the changes in crystal lattice structure and loss of crystalline order associated with melting. The results represent the early fruits of a worldwide effort to generate ultrashort hard x-ray pulses and use them for the characterization of ultrafast events through time-resolved x-ray diffraction (2-5).

Molecular movies.

Time-resolved x-ray diffraction can record transient structures of materials as they undergo rapid change initiated by an ultrashort optical pulse.

CREDIT: C.W. SIDERS
Now you see it…

An intense, ultrashort optical pulse leads to melting, which is monitored through time-resolved x-ray diffraction.

In earlier work, the same group was able to monitor light-driven sound waves that give rise to time-dependent changes in unit cell volume (6). The next step, namely to watch the sample undergo collective structural change during a solid-liquid phase transition (1), introduced numerous experimental difficulties, not the least of which is that each laser shot destroys the irradiated region of the single-crystal thin-film sample. Still, enough data could be collected to reveal a precipitous drop in x-ray diffraction intensity within the first few picoseconds of intense optical excitation. The results indicate that the excited region of the sample, which contains a very high density of energetic electrons, melts quickly—long before the electrons have given up most of their energy to lattice vibrations—and homogeneously, in contrast to the more usual case (observed in less highly excited sample regions) of surface melting followed by propagation of the melt front into the sample at a fraction of the speed of sound.

Of course, we can already monitor ultrafast time-dependent motion in chemical reactions and structural rearrangements using femtosecond optical pulses (7). This year's Nobel Prize in Chemistry, awarded to Ahmed Zewail for the development of “femtochemistry,” recognizes remarkable achievements in monitoring chemical bond breakage in photochemically reactive molecules (8). These events are monitored with light in the visible or nearby spectral regions, rather than x-rays, and the probe pulse therefore “watches” the valence electrons, rather than the positions of atomic nuclei. That is fine for photochemistry, where the valence electrons are involved in chemical change and their features in the visible spectrum can often be associated reliably with the molecular geometry, especially for small, well-studied molecules. But the connection between valence electronic spectral properties and collective structure is not easily made, and it is therefore often difficult to infer the evolution of condensed matter structure from transient optical spectroscopic features. Also, light can often not penetrate the samples sufficiently. In fact, the melting transition, now monitored with x-rays, was observed earlier with visible light (9, 10). Fast melting was indicated, but it was uncertain whether it was homogeneous because the light did not penetrate the sample.

The development of ultrafast x-ray probes promises new capabilities for monitoring time-dependent structural changes in complex systems including crystalline solids and biological molecules, and is particularly promising for probing collective changes in crystal structure. But its development also issues a challenge. Now that we can watch collective structural evolution, can we devise ways to initiate it in a synchronized, or phase-coherent, manner?

In femtochemistry, absorption of an ultrashort optical pulse launches all the photochemically active molecules along the reaction path at the same time. Only then can probing light pulses be used to record snapshots of all the excited molecules as they pass through a sequence of well-defined transient molecular structures. Ultrashort optical pulses can also launch coherent collective motion of ions or molecules in crystal lattices, in some cases along the paths that lead them part of the way toward—but generally not all the way into—the positions they occupy in new crystalline structures, that is, part of the way along “collective reaction coordinates” (11). There is some evidence that collective structural change can occur as a result (12). Methods permitting more extensive optical control over collective behavior have been demonstrated (13, 14). Only if they can be extended substantially will we truly gain the ability to watch the transformation of condensed matter as it passes through a sequence of well-defined transient collective structures (15-17).

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