ELIZABETH WILSON

It's a given that a gaseous diatomic molecule struck by another atom will go spinning off like an airborne dumbbell. But now, for the first time, scientists have measured the direction--clockwise or counterclockwise--in which these molecules rotate after a collision. And in doing so, they have also revealed the quantum behavior that prevails at the infinitesimal level of individual molecules.

COLLISION A beam of argon atoms approaches NO molecules (left). As the argon and NO molecules collide, rotationally excited NO molecules recoil at different angles (right). The rotation directions--clockwise or counterclockwise--of NO molecules (shown in small arrows) vary with the scattering angle. The color map shows relative NO intensity, with positive (red) indicating clockwise rotation and negative (violet) indicating counterclockwise rotation.

The question of this "preferred sense of rotation" is important to chemists delving into the nitty-gritty chemical dynamics of gas-phase reactions.

In what some chemists are calling "fascinating" research, associate chemistry professor Joseph Cline of the University of Nevada, Reno; chemist David W. Chandler of Sandia National Laboratory in Livermore, Calif.; and their colleagues take their understanding of these dynamics a step deeper by measuring the rotational direction of NO molecules after they've been bombarded by argon atoms [Science, 293, 2063 (2001)].

When struck by a beam of the Ar atoms, the NO molecules scatter at all angles relative to the beam. Cline, Chandler, and coworkers find that for any final rotation state that the NO molecules end up in, the majority of molecules that spin off in a particular direction will be rotating the same way. So, for example, at a scattering angle of, say, 45°, they might see 70% of the molecules spinning clockwise. And at a deflection angle of 60°, the bulk of the molecules might be going counterclockwise.

Not only that, but as the researchers scanned through all the deflection angles, they found the rotational preference changes back and forth from clockwise to counterclockwise in a complicated, oscillatory pattern. This is due in part to quantum mechanical interference of the different scattering reaction pathways that lead to the same rotational state. And they saw a different oscillating pattern for every final rotational level that the NO molecules populated in the collision.

"A classical calculation using solely repulsions between the molecules can only recover the coarse features of the scattering," F. Fleming Crim, chemistry professor at the University of Wisconsin, Madison, notes in an essay accompanying the Cline paper in Science.

"A classical calculation using solely repulsions between the molecules can only recover the coarse features of the scattering." F. Fleming Crim, chemistry professor at the University of Wisconsin, Madison

Oxford University chemist Mark Brouard says these "beautiful experimental results provide insight about the nature of the scattering process at a microscopic level."

A few groups have measured the preferred sense of rotation of molecules following a photodissociation reaction, but this is the first time it's been observed in inelastic collisions. And though these effects were anticipated to a certain extent, the high percentage of molecules at a scattering angle that prefers one direction to the other surprised the group. "I was amazed at how dramatic it was," Cline says.

The group mapped the subtle effect using circularly polarized laser light as a molecular probe. The electric vector of this type of light rotates either clockwise or counterclockwise as it travels. Molecules with a preferred sense of rotation will interact differently with light that is right circularly polarized than it will with light that is left circularly polarized. The researchers took laser ionization spectra using both the right-handed and left-handed forms of the polarized light and, from the intensity differences, deduced which direction the molecules were rotating.

This new kind of measurement is "exquisitely sensitive" to the dynamical details of molecular scattering, Cline says. "I think rotational orientation in molecular collisions is a universal phenomenon, but only now is it measurable."

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Copyright © 2001 American Chemical Society