MARGINALIA
Pulse, Pump & Probe
Roald Hoffmann
Pump and Probe
Still another piece of the logic of femtosecond experiments is the pump and probe technique. Two bursts of laser power are required; the first pumps the molecule, excites it, starts the reaction going. A precise and precisely variable time thereafter, the second pulse probes the action.
As in comedy, it's all about timing. Often the pump-and-probe pulses are sent on their appointed rounds at exactly the same time; that is easy to arrange. But the probe pulse light is routed a longer way. "Longer" doesn't mean much longer; the velocity of light is 3 x 108 m/s, so a 3.3-fs delay is caused by a path difference of only 1 x 10–6 m, one-thousandth of a millimeter. Can one arrange such a precise path-length difference for the pump-and-probe pulses? No sweat, at least today. You can't do it on top of a New York subway grate, but the same kind of piezoelectric controls that in scanning tunneling microscopy move a tungsten tip a few angstroms left or right serve in this case.
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