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Pulse, Pump & Probe

Roald Hoffmann

The Intermediate Identified

You will note above that the femtosecond-scale spectroscopy identifies the intermediacy of a specific molecule (CF2I–CF2) with a certain lifetime. But what is the structure of that molecule? From experience with related molecules, one could imagine the intermediate "non-bridged" (in the position of the I) or bridged, as shown in Figure 4.

Figure 4. Reaction intermediateClick to Enlarge Image

For stable molecules, especially those forming nice little crystals, we have a marvelous technique, x-ray crystallography, to determine the structure. Nuclear magnetic resonance, electron diffraction and microwave spectroscopy also give us geometrical information. But imagine the problem of applying these techniques to a figurative handful of molecules zooming around in high vacuum!

In the most exciting development in recent years, people are beginning to do just that. In January 1999 Jianming Cao, Hyotcherl Ihee and Zewail published an electron diffraction study on CF2I–CF2 in situ, pointing to a non-bridged structure for the intermediate. And in a remarkable paper that is in a March issue of Nature, Kent R. Wilson of the University of California at San Diego and his group succeeded in following a structural change on the femtosecond scale by x-ray diffraction.

Figure 5. Time- and angle-resolved diffraction curvesClick to Enlarge Image

Here's what the San Diego chemists do. They illuminate the surface of a crystal of gallium arsenide (GaAs) crystal with 800-nanometer laser light. This creates electron-hole pairs in the semiconductor. The electronic excitation is quickly transformed into heat, and the heat wave at the surface dissipates into the interior. The structural change at the GaAs surface may be complex, but its most important component is simply heat-induced expansion of the lattice.

That expansion is probed by x rays generated at a copper wire by that 75-mJ, 20-fs laser pulse that was energetic enough to light up Milan. What one sees very clearly in Figure 5 is a motion of a crucial x-ray reflection, a function of the lattice spacing, as the surface is heated. Corresponding to an expansion at the surface of only 0.0082 ?.

Not much that is surprising, you will say; the structural information is meager, the molecules small. But Kent Wilson's x-ray observations, as Zewail's electron diffraction, are proof-of-principle experiments. I think within the next decade we will see ultrafast structural studies defining precisely the many transient species we now know exist but whose identity is so far hidden from us.

© Roald Hoffmann

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