Chemistry: Making Things More Exact

"Most chemical reactions," says Caltech Chemical Physicist Aron Kuppermann, "are unholy messes." The neat formulas and precise equations of the textbooks may be all right as far as they go, but they present an incomplete picture of what is really "a large number of processes, some of which are not fully understood."

In an effort to clear up the confusion, Kuppermann and Chemist John White have taken an impressive step toward making chemistry exact and predictable: they have made the first direct measurement of the minimum energy required to cause one of the simplest chemical reactions known to science. An absolute minimum of one-third of an electron volt is needed, they discovered, to split a hydrogen molecule into two hydrogen atoms and to combine one of them with a deuterium atom to form deuterium hydride. An addition of any less energy and the reaction will not occur.

Rainbow of Colors. To measure this tiny quantity—less than a millionth of the energy needed to split the nucleus of an atom—the scientists devised an ingenious technique. Light from a 200-watt mercury vapor lamp was focused on a diffraction grating, which, like a prism, broke up the beam into its constituent rainbow of colors, its separate wave lengths of light. By rotating the grating to a carefully calculated angle, the scientists were able to reflect light of a single, specific wave length at a target. Knowing the wave length, they were able to determine precisely the energy of the photons—or bits of light—they were using.

Beginning with the light of shortest wave length (and thus the highest energy), they aimed a beam from the grating through a Pyrex cylinder containing hydrogen and deuterium iodide gas, which breaks down when exposed to light. When molecules of deuterium iodide were struck by photons in the light beam, they split into fast-moving atoms of deuterium and sluggish, heavier atoms of iodine. Some of the speeding deuterium atoms in turn collided with hydrogen molecules in the cylinder, knocking off one of the hydrogen atoms and combining with the other to form deuterium hydride.

Newtonian or Quantum? After exposing the cylinder to light of a uniform wave length for periods ranging from half an hour to ten hours, the scientists analyzed its contents to detect molecules of deuterium hydride. The process was repeated, each time with a light beam of longer wave length and lower energy, until they failed to find molecules of deuterium hydride in the cylinder—no matter how long the gases had been exposed to the light. At this particular wave length, it seemed clear, the deuterium atoms had not been given enough velocity to split the hydrogen molecules and combine with the freed hydrogen atoms.

To Kuppermann and White, this suggested that the wave length of light used in the previous exposure provided the minimum energy needed to cause the reaction. They then determined the energy carried by a photon at that wave length and calculated how much of it had been imparted to the deuterium atom when the deuterium iodide molecule was split. Their result: one-third of an electron volt.