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Exquisite Control

Roald Hoffmann

Ogling the Aftermath

In an earlier Marginalium, I wrote of scanning tunneling microscopy (STM) images showing us molecules in stages of catalysis; in another, I described femtosecond spectroscopy "freezing" molecules in the act of falling apart or coming together. Experiments whose output is a graphic image have immense power; the experiment I am about to describe images quite directly the products of a chemical reaction flying apart.

Paul Houston and his colleagues at Cornell have been looking at what happens when ozone is dissociated in the near ultraviolet, just the region in which ozone optimally shields us from much of the sun's ultraviolet. The Cornell chemists follow one mode of decomposition, into a certain spin state (triplet) of both O2 and O:

O3 yields O(3P) + O2(3Sg–)

The fragments travel away from each other, preferentially along a specific direction that is roughly along the extension of what was an unbroken O–O bond of the parent ozone. In this case the oxygen atoms are detected—the brighter (more yellow) the area in Figure 4, left, the more oxygen atoms strike there; the farther from the center of this pattern, the faster moving the oxygen atoms.

Figure 4. Photographic recordClick to Enlarge Image

The peaks at top and bottom in this roughly resolved image were expected—lots of fast moving O atoms. What was startling was that there were also some slow-moving O atoms (the vertical dumbbell near the center).

Next, conservation of energy comes into play. Knowing the energy in the light, the energy needed to break the O–O bond in ozone and the kinetic energy borne by the fleeing O atom, one can calculate the energy left behind in the unobserved O2 (actually that energy can be measured in another experiment). Lots of energy in O (outer peaks in Figure 4, left) means little energy in O2. Little energy in O (inner peaks in Figure 4, left) implies that there's a lot of energy in the O2 molecule left behind.

A lot is a lot—indeed enough to put the O2 into its 27th and 28th overtone, nearly enough to knock it apart. That's remarkable. The fine-resolution image at right in Figure 4 shows well-defined "halos" that are the result of molecular vibration being quantized—one "halo" corresponds to O atoms that have left behind an O2 in the 27th vibrational overtone. The other halo, further in, is due to the slower O atoms that leave behind the O2 in the 28th overtone.

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