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A Really Moving Story

Roald Hoffmann





Chemistry with a Hammer


Figure 1. Argon crashesClick to Enlarge Image



The Quantum World

Though mysteries remain aplenty in the falling apart of molecules at surfaces, the beautiful story I just told was pretty classical. Sometimes the dynamics of chemical reactions have truly nonclassical features, ones that derive from the quantum nature of molecules. For molecules are truly betwixt and between—part classical objects, sculpted in their multifarious richness as macroscopic structures are, part quantum objects with the seemingly arcane consequences of the quantum world.

A remarkable report recently appeared from Richard Zare's Stanford University group that exemplifies quantum mysteries, even as it has a partially classical explanation. Diatomic molecules can be induced to fall apart by either heat or light. The Stanford group looked at a simple diatomic, ICl (the mixed partner to familiar diatomic chlorine, Cl2, and iodine, I2) and its photodissociation by a beam of linearly polarized green light. Off come Cl and I atom fragments; the Cl atom is the one they studied.

Figure 2. Light can be used to split apart a moleculeClick to Enlarge Image

The electron in the Cl atom has two kinds of angular momentum—one is its intrinsic spin, the other the angular momentum of its orbital motion. The two kinds of Cl atoms in which (as the Cl sails away) the odd electron is rotating clockwise or counterclockwise (thus with "topspin" or "backspin" with respect to the direction of motion) can be experimentally distinguished.

The remarkable outcome of the quantum nature of the bond-breaking process is that this "handedness" (helicity) of the electronic angular momentum oscillates between backspin and topspin, depending on the wavelength (thus energy) of the dissociating light! The explanation comes only from the wavelike properties of the material molecule, more precisely from the interference of wave-like descriptions of the mixture of different electronic states accessed by the absorption of that light.

Figure 3. TopspinClick to Enlarge Image

Actually, a classical model gets us part of the way to understanding what transpires. The absorption of light starts the electron cloud in the ICl molecule oscillating. Normally, such oscillations have the same symmetry as the molecule; that is, the oscillation occurs either along the bond or at right angles to the bond. In the particular case that Zare's group studies, the excitation of the electron cloud is to a mixed state in which the electron cloud oscillates both along the bond axis and at right angles to it because of quantum-interference effects. The resulting electron motion is a circulation (the helicity) whose sense of rotation depends on how the two different motions are combined. The fragments carry away in the topspin or backspin the memory of that interference.

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.

The Arrow in a New Light

The light is that of lasers, in the service of transforming change itself. Chemists have always been interested in mechanism, the answer to the simple question "How? How does it happen?" The response comes out of looking closer at the arrow in A + B → C + D, finding the many sequential arrows hidden in the gross transformation. The newer work, both femtosecond spectroscopy (to be discussed in a future Marginalia) and the two studies mentioned here, consistently uses lasers. They deliver precise dollops of energy, to disrupt or characterize bonds and motions in a molecule. In the laser's bright light, we are seeing signposts, I think, to a new dynamics.

© Roald Hoffmann

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