Newswise — It all started when a high school chemistry teacher encouraged Amy Cordones-Hahn to leapfrog her regular classroom assignments and do experiments in his lab.
Fifteen years later, with a PhD in physical chemistry from UC Berkeley, she still spends a lot of time at a lab bench. But now her work goes far beyond classic beaker-and-test-tube chemistry into a new frontier, where scientists use lasers and X-rays to watch electrons dance to a femtosecond beat.
These electron movements drive all chemistry by making, breaking, stretching and squeezing the bonds between atoms. Cordones-Hahn especially wants to understand how electrons behave when light hits particular kinds of molecules—the first step in generating electricity and fuel from sunlight, and in photosynthesis that is the basis for most life on Earth.
Tell us more about what led you to chemistry research.
Growing up in Durham, North Carolina I had a really great high school chemistry teacher. We had an open period, sort of like study hall, and my teacher allowed me to use that time to do experiments with him in his lab. I did some reading beyond what we had learned in basic chemistry class and learned more about how to put together an experiment.
Then I went to Brandeis University, which has a great program getting undergraduate students to do research, and I worked in a biochemistry lab. In graduate school at UC-Berkeley I got interested in how electrons move around in nanocrystals and in studying the processes that impede the efficient use of nanocrystals in solar cells and LEDs. That’s where I started working with ultrafast lasers.
After a postdoc at Lawrence Berkeley National Laboratory, where I learned how to use X-ray techniques to study how electrons move around in molecules, I came to SLAC in September 2015.
What do you look at on femtosecond timescales?
One class of reactions we’ve been studying converts sunlight to electricity or fuel. In biology this is photosynthesis, which plants use to convert sunlight into fuel or sugar. In chemistry, people are trying to mimic those natural processes to create materials or molecules that can absorb sunlight and convert that into a fuel source – hydrogen, for example – or into electricity.
In our group we are really looking at fundamental processes, not trying to create a device. We look at the very fast, initial steps where a single photon, or particle of light, is absorbed by a single molecule.
In general, chemical changes are driven by moving electrons. We use light to give energy to a molecule’s outermost electrons so they can move around and reconfigure themselves. As the electrons move, the force fields felt by the atoms change and the atoms move in response to this change. This causes the bonds between atoms to break and form or just to change, bend and stretch.
With X-rays you can measure where the electrons are sitting with atomic resolution and see how their distribution changes. You are seeing chemistry happen in real time, and you are seeing exactly what the mechanism of a chemical reaction is.
Almost like watching a movie.
Exactly. In fact, we use SLAC’s X-ray laser, the Linac Coherent Light Source (LCLS), to make “molecular movies” that show how molecules respond to light.
First we hit the molecule with a pulse from an optical laser. The energy from the light pulse makes the electrons and the bonds between the atoms shift around. Then we immediately hit the molecule with an X-ray laser pulse from LCLS. The way the X-rays interact with the molecule gives us a snapshot of how its electron configuration and atomic structure changed in response to the light.
By making a series of these X-ray snapshots, about 50 femtoseconds apart, we get a nice picture—like frames of a movie—of how the light-struck molecule evolves over time. It’s like having a camera with an incredibly bright flash that shoots 20,000,000,000,000 frames per second.
Do these experiments have any practical benefit?
They do have potential benefits down the road.
For example, our group has been doing a lot of work on dyes that are added to solar cells. They allow the cell to absorb a bigger fraction of the sun’s energy and generate more electricity. The most common dyes are based on the metal ruthenium, which is really efficient but also rare and expensive. We would like to use iron instead, which is cheaper and more abundant, but dyes made with iron don’t work as well; they quickly become deactivated. We are trying to find out why, and these X-ray snapshots are helping us unravel the deactivation process.
We’re also doing work on photocatalysts—molecules that absorb light and use that energy to convert hydrogen ions that are floating in a solution into hydrogen gas for fuel. Other groups have shown that nickel-based photocatalysts can carry out this reaction; now we are trying to find out exactly how it proceeds.
Our hope is that the knowledge we gain from these femtosecond studies will help researchers find ways to make these processes work better.