MARGINALIA
A Really Moving Story
Roald Hoffmann
Beam Me Down
Both of the studies described above depend on supersonic molecular beams expanding into a vacuum—in Ceyer's case a beam of neon, argon or krypton seeded in helium, in Zare's case a pulsed beam source of ICl molecules. The velocities of the molecules emerging from these beams can be controlled by heating the expansion nozzle over a wide temperature range. The beams are "skimmed" downstream by sharp-edged openings and collimated by slits, obstructions and otherwise-shaped orifices. The language of handling tenuous gases is one of machined metals—walking down the lanes of exhibitor's wares at a meeting of the American Vacuum Society is a fantastic experience (in the advertisers' wares, I once found a poem). The language is one of intended control. The outcome, if everything is just right (days of alignment and pumping to get it there), is a molecular beam exquisitely well defined in its dimensions and in the speed of the molecules coursing down their appointed paths.
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