The problem
RECENT IMPROVEMENTS IN technology have allowed scientists to accelerate electrons in ways that create high-energy, extremely bright, and short laser pulses. Before the invention of these lasers, scientists could not study how high-intensity ultraviolet and X-ray light interacts with matter. Now that such lasers exist, everybody’s dying to know what happens when you blast stuff with short, high-intensity, high-energy laser beams.
The obvious answer is that you blow shit up, and that’s cool and all, but the potential applications of these beams are much greater than that. High-intensity ultraviolet and X-ray lasers might be able to image materials that are currently challenging to study. But before scientists can use these lasers, they have to understand this completely unexplored area of light-matter interaction.
The researcher
Lora Ramunno studies computational photonics at the University of Ottawa. Using her parallel supercomputer (equivalent to about 600 desktops), Ramunno studies nonlinear optical imaging and the interaction between matter and intense laser beams.
The project
Ramunno decided to look at how tiny clusters of matter interact with a high-intensity laser pulse by simulating each and every one of the atoms. When atoms are hit by a photon of light there is some probability that they will absorb the photon and eject one of their electrons. This leaves the atom as a positively charged ion. At every step of her simulation, Ramunno’s computer program must stop and evaluate the quantum probabilities that give these rates before it can move on to the next step.
The key
Before the laser blows up the cluster of atoms, electrons escape from their atomic orbitals and the cluster becomes a plasma. The first few electrons that are ejected simply fly away and leave behind a charged cluster of ions. However, the electrons emitted later find themselves in this charged environment that they can’t escape from.
These electrons are free to zip around, but can’t leave the cluster, and from time to time they collide with unionized atoms. They usually don’t have enough energy to free an orbiting electron from the atom they collided with, but they can excite one of the atom’s electrons up to higher energy. Ramunno found that if a second free electron collides with the same atom that had been energized by an earlier collision, it has a better chance of releasing the orbiting electron. When pairs of free electrons work together like this the cluster charges more quickly than if the laser didn’t have any help and so the cluster explodes in a shorter period of time. Ramunno calls this process Augmented Collision Ionization.
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