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Pulse, Pump & Probe

Roald Hoffmann

The essential change implicit in the arrow in a chemical reaction, A + B → C + D, may be rapid, it may be slow. I drop a small chunk of potassium into water, and the show is over in less than a second. The majestic copper beech outside my window was not noticeably smaller yesterday, but a photograph of it 75 years ago shows it just 10 feet high. Many biochemical reactions went into that slow growth. They tell me the universe is 15 billion years old, they tell me that the primary photochemical act in vision—a change in shape of a retinal molecule—transpires in 200 femtoseconds. (One femtosecond, fs, is 10–15 seconds—the name of the unit a relatively rare intrusion of a nice Scandinavian root into a scientific nomenclature generally too anxious to impress in Greek and Latin.) I sort of trust the former, I believe the latter, and in this third Marginalia on the new kinetic chemistry, I would like to tell you why.

When events happen in the time frame defined by our senses, we can easily gauge their rapidity. We have no trouble hearing the difference between a pulse rate of 55 and 155 beats per minute. And we can make out, a bit less distinctly, the number of years elapsed between a frame of Ingrid Bergman in her early Intermezzo and late Autumn Sonata. But as we move to shorter and longer time intervals we must abandon the direct and sometimes fallible evidence of the senses and rely on instruments and theory.

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