Beating bromine

by Tyler Shendruk

Published: Mar 2

The problem
THE QUANTUM WORLD works quite contrary to our own concrete and everyday existence. When we are first taught about atoms, we are shown a solar system-like model with electrons orbiting the nucleus like planets orbit the sun. But physicists have known for nearly a hundred years that this picture is too simple.
Electrons are both a particle and a wave at the same time. So electrons shouldn’t just be thought of like planets, but also like a vibrating guitar string. These so-called wavefunctions can be experimentally probed and scientists understand them very well. But for more complicated molecules—combinations of more than just one atom—it becomes very difficult to directly see that theory and reality are the same.

The researcher
On top of being a professor in the physics department at the University of Ottawa, Paul Corkum heads the Attosecond Science Laboratory at the National Research Council. He is renowned for using a laser pulse to accelerate an electron out of its atom, turn the electron around, and drive it back into its orbital. When the electron recollides with the atom, short bursts of light are given off that tells Corkum about the environment in which the electron settles.
Using very short laser pulses, Corkum was able to take a high definition snap shot of the quantum cloud that defines where the electrons are around the atom.

The project
Taking high-resolution pictures of quantum orbitals is one thing, but filming a movie of the wavefunctions during a chemical reaction is another altogether. And yet, this is exactly the challenge Corkum set for his lab. Using the same technology he invented for imaging orbitals, Corkum wanted to watch a single molecule of bromine disassociate into two separate bromine atoms.

The key
Corkum blasted bromine gas with blue light. Blue is exactly the right colour to excite bromine molecules. Immediately after the blue light excites the bromine, the short laser pulse that knocks an electron out and drives it back in is shot at the gas.
Corkum saw that the blue light had not excited all the bromine molecules. This turned out to be an advantage since the resulting bursts of light from the recollision of the excited and the non-excited molecules mixed into beats.
The non-excited bromine acted exactly like a tuning fork: Corkum could use the beating between the bursts of light from molecules of bromine and from excited, separate atoms to see the difference between the two.