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Exquisite Control

Roald Hoffmann

Moving in Two Worlds

The degree of control that chemical dynamicists are beginning to exercise is startling. I love the understanding that emerges. There is also something else that appeals to me personally, both as a quantum chemist and as a builder of conceptual bridges; each of these experiments is not only a tour de force of high vacuum technology, optics and electronics—but also a wonderful blend of quantum and classical mechanics. There is no way to understand the chemistry of molecules reacting without moving in both worlds.

Let me elaborate: The molecular beam machines (I've spared you the cost of these) operate on classical billiard-ball logic; the same way of thinking lets us see molecules colliding, fragments bouncing off in certain directions. Simple and foolproof conservation of energy was at work in the Houston experiment. But at the same time, to understand these ingenious experiments one must think in the beautiful and sometimes mysterious logic of quantum physics—think of the uncertainty relationship shaping the wave packets and time resolution in femtosecond spectroscopy, the quantum helicity I described in one Marginalium, the tunneling current that makes STM possible. The laser, that tunable jack-of-all-trades source of strong light, owes its coherence and power to a quantum effect; the energy level spacing that is behind the laser also explains state selection.

Chemical dynamics, the kinetic art, exemplifies in every one of its experiments the intersection of the classical and quantum worlds. It is the realm of the quantum mechanic, just as much as it is of the clever instrument builders. Who are getting to know, finally, what makes chemistry run.

© Roald Hoffmann

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