Molecular divorce gives strange vibes.

Molecular divorce gives strange vibes

Common sense says that the more vigorously you vibrate something, the more likely it will break into parts. But laser-wielding chemists pushing around weakly bound, identical molecular twins are finding that the effects of vibration on the molecular scale may not be as straightforward.

Researchers at the National Bureau of Standards in Gaithersburg, Md., observed that the harder they 'shake' coupled twins, or dimers, of nitric oxide (NO) molecules, the longer it takes to break the weak bond that holds together the two individual NO molecules. Such bonds are governed by what are called van der Waals forces, which are, in part, responsible for the shapes of proteins and enzymes, as well as for some chemical and physical properties of all materials. The scientists' findings, which will appear in the NOV. 15 JOURNAL OF CHEMICAL PHYSICS, run counter to both intuition and existing theories about how and why molecules break into pieces.

In usual earthly situations, molecules rock, rattle and roll in an energetic dance at a dizzying pace. A molecule's constituent atoms rotate around bonds a trillion times every second. With each tick of the clock, these bonds twist and bend about 10 trillion times. The atoms vibrate toward and away from each other at an even faster rate. Meanwhile entire molecules are slam-dancing with their neighbors, making complex exchanges of energy. Twists become bends, which become vibrations that become rotations, or some other such choreography of transitions and motions. In those cases where all of this internal energy becomes too much -- for instance, when solids melt into liquids or liquids change into vapors -- molecules fragment until the smaller pieces are dancing at a pace that allows them to stick around.

When a molecule itself disintegrates or when it dissociates from another molecule to which it is bound, how are the complex internal energies redistributed throughout the fragments involved? Theories exist that fit well with observed patterns of molecular disintegration, but they are based on unexplained assumptions.

In the new research, a quartet of scientists, led by Michael Casassa, used ultra-fast laser pulses to study how two identical and weakly bound NO molecules dissociate, a divorce that occurs in less than a billionth of a second. The researchers opted to study relatively simple NO dimers -- two molecules, each composed of an oxygen atom and a nitrogen atom, which stick together due to van der Waals forces and form a trapezoidal molecule. Compared with more complex molecules, the four-atom NO dimer has a limited repertoire of dance moves. Yet even at room temperatures, motions within NO dimers are extremely complex. To simplify these motions nearly to the point of removing them entirely, Casassa and his co-workers used a special molecular jet that cooled the molecules to near absolute zero, the point at which all internal motions come to a halt.

Then, by using trillionth-of-a-second laser pulses that were tuned to inject precise amounts of energy into the NO dimers, the scientists were able to choreograph from scratch a simple molecular dance. They directed the molecules to vibrate in one of two modes: symmetric vibrations, in which the back and forth oscillations of both nitrogen-oxygen pairs are synchronized; or asymmetric vibrations, in which the atoms of one NO molecule approach each other while the atoms of the other recede from one another.

After setting the dimers vibrating in one of the two vibrational modes, the scientists used another ultrafast laser to see how the dance was going. This laser could in effect take snapshots of the dances at different times after movement began. The researchers observed that the dissociation of the NO pairs took 890 picoseconds (1 picosecond = 1 trillionth of a second), or 40 times longer, to finalize when the scientists made them vibrate in the higher-energy asymmetric mode than when they made the dimers vibrate in the lower-energy, symmetric mode.

Intuition suggests that higher-energy vibrations should break apart weakly bound dimers more easily than can lower-energy vibrations. And a leading theory on how molecules break apart formalizes this intuition. But Casassa's experiments clash with both intuition and theory. Casassa suggests one possible explanation that he has not yet tested. He says that some of the energy from the asymmetric vibrations might be shunted into electronic motions in a way impossible for the lower-energy vibrations, which might vent their energies more readily by causing the dimer to dissociate. Hence, the longer dissociation times for the high-energy vibrations.

Whatever the explanation, the paradoxical molecular behavior that Casassa and his colleagues observed has turned a few heads in the scientific community. Physical chemist Richard Zare of Stanford University calls Casassa's work "very exciting" and praises the experiments as "elegant and beautiful." The next step, say Casassa and other laser chemists, is to critique more molecular choreographies to see if the behavior observed in NO dimers is a general phenomenon of molecules bound by weak van der Waals forces.