USC Participates in Chemical First

A team from six research institutions in Canada, the United States and the United Kingdom has become the first to watch a molecule from the inside as it falls apart.

Results of the experiment will be published in the Jan. 13 issue of Science, the world’s leading scientific journal.

Hanna Reisler, Anna Krylov and doctoral student Sergey Levchenko of the department of chemistry in the USC College of Letters, Arts and Sciences collaborated with researchers from the Steacie Institute for Molecular Sciences, the University of Regina and Queen’s University in Canada; the Sandia National Laboratories in the U.S.; and the Open University in the U.K.

The lead author was Albert Stolow, senior research scientist at the Steacie Institute.

In the experiment, performed at multiple sites, a laser pulse caused a dinitrogen dioxide molecule, known as the NO dimer, to break apart into nitrogen oxide fragments.

The entire reaction lasts approximately a thousand femtoseconds, or one millionth of a millionth of a second. Using a one femtosecond laser pulse as a starter’s pistol, subsequent laser pulses were used both to clock the chemical reaction and to knock off an electron with each pulse.

The information in the ejected electrons enabled researchers to reconstruct snapshots of the chemical reaction in progress. An innovative use of this technique allowed observation of the reaction from the so-called “molecular frame” viewpoint, as if from a molecule-mounted camera.

“You can almost see how the electronic structure of the molecule evolves in time the way the molecule would see it, and you can observe it directly,” Reisler said.

Reisler’s group contributed experimental data about the endpoints of the reaction. The Canadian team’s ultrashort laser pulses could not pinpoint the energetics of the process because a law of quantum physics, the Heisenberg Uncertainty Principle, limits how accurately one can determine both the time and the energy of molecular-scale processes.

The femtosecond laser pulse experiments determined the timing accurately, but provided only a range for energies.

Long-pulse lasers in Reisler’s laboratory “interrogated” the initial and final states of the reaction, providing accurate measurements of the distribution of energy among the reaction products.

Krylov’s group then computed changes in the arrangements of electrons and nuclei in the molecule during the reaction.

“Our calculations characterized electronic states of the dimer involved in the process and helped to develop an explanation of what actually happens during the reaction,” Krylov said.

The result of the six-institution collaboration was not only an experimental breakthrough, but a source of new insights into the electronic charge and energy flow changes that form the basis of all chemical reactions.