Announcements - Twanging Molecular Slinky with Ultrafast Laser Gives Researchers Snapshot of Electrons in Action

November 23, 2008 — Ottawa, Ontario

Cover image for Science Magazine, created by Greg Kuebler, JILA/ University of Colorado

In the quest to slow down and ultimately understand chemistry at the level of atoms and electrons, a team from the University of Colorado (CU) and the National Research Council Canada (NRC) has found a new way to peer into a molecule that allows them to see how its electrons rearrange as the molecule changes shape.

Understanding how electrons rearrange during chemical reactions could lead to breakthroughs in materials research as well as in fields like catalysis and alternative energy, according to CU professors Margaret Murnane and Henry Kapteyn, and NRC scientists Albert Stolow and Serguei Patchkovskii who led the research efforts.

"The Holy Grail in molecular sciences would be to be able to look at all aspects of a chemical reaction and to see how atoms are moving and how electrons are rearranging themselves as this happens," Stolow said. "We're not there yet, but this is one step toward that goal."

To be able to chart a chemical reaction, scientists need to be able to see how bonds are formed or broken between atoms in a molecule during chemical reactions. The problem is - there are very limited tools available to capture the rapidly changing electron cloud that surrounds a molecule as the atoms move around, Murnane said. Changes in the electron cloud can happen on timescales of less than a femtosecond, or one quadrillionth of a second, representing some of the fastest processes in the natural world.

The team's paper entitled "Time-Resolved Dynamics in N₂O₄ Probed Using High Harmonic Generation" made it as the cover story in the November 21^st issue of Science Magazine. The paper describes how they excited a molecule, N₂O₄, with a short burst of laser light to induce very large oscillations within the molecule. They then used a second laser to produce an x-ray, which was used to map the electron energy levels of the molecule, and most importantly, to understand how these electron energy levels rearrange as the molecule changes its shape, according to Kapteyn.

"This is a fundamentally new way of looking at molecules," Kapteyn said. "This process allowed us to freeze the motion of electrons in a system, to capture their dizzying dance."

The researchers describe their process of stretching the N₂O₄ molecule as being similar to pulling on a slinky and then letting it go and watching it vibrate. They used the N₂O₄ molecule because it vibrates more slowly compared to other molecules, which allowed them to see what was going on.

In many ways, molecules are like tiny masses connected by tiny springs of differing strengths, Murnane said. These springs are the chemical bonds, made up of shared electrons, which hold all matter together. In this experiment they used ultrafast laser pulses to twang these springs, making the nanoscale molecular slinkies vibrate. However, unlike real life springs, in vibrating molecules their properties can change.

And it is precisely the changing properties that they wanted to see. Being able to watch and understand why the electrons did what they did is very useful in fields like alternative energy.

"If we understand the nature of these processes, in the future we can then translate that knowledge into better technology, such as creating more efficient light harvesting molecules or catalysis or perhaps even solar cells," Murnane said.