The lab of Poul B. Petersen, Chemistry and Chemical Biology, is a finely controlled space, designed to maintain consistent temperature and humidity. “It’s the best air-conditioned room on campus,” says Petersen, as he points to a walled-off room within a room. That’s because the room houses the latest infrared (IR) laser technologies and hundreds of pieces of optics equipment—lenses, mirrors, cameras, photo detectors—all mounted to a two-ton floating metal optics table. Any environmental disturbance would throw off the lasers and the measurements they take.
Specifically, Petersen’s goal is to capture ultrafast molecular processes using new techniques that his lab develops. Many molecular processes happen on the femtosecond timescale. As a point of reference, as many femtoseconds occur in a single second as do seconds in 32 million years. Simply put, these processes happen incredibly fast.
Petersen works toward developing new technologies that directly measure these dynamics and capture the processes as they happen, to essentially create a “molecular movie” of a given process.
New Techniques in Ultrafast Infrared Spectroscopy
The focus of Petersen’s lab is time-resolved IR spectroscopy, which involves an initial ultrafast laser pulse to initiate a process and a second ultrafast IR laser pulse that probes molecular vibrations as a function of time. The vibrations provide a direct, specific look at the molecular structure and how bonds break and form during the chemical process.
The Petersen lab conducts research in molecular dynamics in bulk liquids, such as water and DNA, as well as at interfaces, such as at the surface of water or the surface of nanoparticles used for solar energy applications.
Ultrafast IR spectroscopy is a technologically difficult technique. The lab’s work, therefore, is often technologically driven.
“In the history of this type of research, what really has led to new breakthroughs scientifically [are] new developments in the technique,” says Petersen. “You develop a new method and all of a sudden you can study things that you couldn’t study before.”
To that end, the researchers spend a part of their time pushing techniques to perform experiments that have never been performed before.
“We’re doing really hard experiments, we’re pushing the envelope of what’s possible,” says Petersen. “We develop new, better technologies and if we can successfully perform those experiments—and we’ve shown that we can—it means we can directly measure the process that we want to look at instead of relying on indirect methods that are technologically easier.”
For example, during his postdoctoral fellowship, Petersen developed a new way to make femtosecond IR laser pulses that contain the entire vibrational frequency range in a single laser pulse. Now at Cornell, his lab is applying these laser pulses in time-resolved experiments, called ultrafast continuum mid-IR spectroscopy, to see proton transfer and other processes in action.
To Study How Protons Move
Proton transfer is a fundamental chemical reaction that also drives most of the processes in our bodies. Although many theories have been proposed on how protons are transferred, there has been no direct observation to validate any theory.
One theory suggests that protons transfer along a chain of water molecules, known as a proton wire. The wire could shuttle protons very efficiently, each proton moving along the steps of the wire. It’s the dominant theory when it comes to biological systems, although no one has actually seen it happen. Another theory suggests that protons move more slowly and individually in the solvent. Again, this has not been observed at the molecular level.
“The key reason we don’t have a good understanding of proton transfer is because we currently don’t have a good way of studying how proton move specifically,” says Petersen. “That’s where we come in.”
As a proton is being transferred, the molecular bond vibrations that contain the proton undergo very wide frequency changes. Regular femtosecond IR laser pulses, otherwise known as broadband pulses, can measure only a small fraction of the entire range, says Petersen. With the femtosecond, ultrafast continuum mid-IR laser in hand, the lab is now able to cover the whole frequency range associated with proton transfer, making it possible to follow the protons in real time as they move.
We develop a new method and all of a sudden we can study things that we couldn’t study before, Petersen says.
Seeing proton transfer happen, and validating one of the major theories, will affect how other researchers might approach addressing various diseases or implementing biomimicry in technology applications.
For Petersen, however, the beauty is in the fundamentals. “Personally, what drives me is just that I want to see the protons move,” he says. “I want to understand what makes them move and how they move.”
Charges Trapped in Nanoparticles
In another project, Petersen’s group looks at charge transfer in nanoparticles. Researchers working in the field of new materials have become very good at making nanomaterials with various properties. For these materials to have applicable uses, they need to conduct charges. Yet some have trouble doing this, and charges become trapped in the material instead of going in and out.
Like proton transfer, the charges in nanoparticles absorb light across the mid-IR range. It’s another ideal experiment for Petersen’s lab, which, thanks to a seed grant from the Cornell Center for Materials Research (CCMR), was able to start the research.
Petersen’s group has so far collected preliminary data that show that IR spectroscopy can see the charges trapped in the nanoparticles. The next steps are to collect and analyze data to figure out what causes the traps and how they function.
Delving into Water Behavior at Surfaces
Petersen first fell in love with ultrafast lasers as an undergraduate in Denmark studying water. He expanded his research on water as a graduate student at UC Berkeley and, later, as a postdoctoral fellow at MIT.
Today, a large part of his group still studies water, particularly how it behaves at surfaces. To study the surface of water, Petersen is developing another new technique, known as coherent surface-specific 2D-IR (ss2D-IR) spectroscopy. Using ss2D-IR, Petersen’s lab is able to measure the ultrafast dynamics of aqueous surfaces using four different laser pulses.
The multiple laser pulses make it possible to selectively measure the dynamics of surface water molecules with high time and spectral resolution, so the researchers can capture exactly how water molecules behave at varying surfaces. Petersen’s lab has recently figured out how to take measurements of a model of water at a biological surface. The ultimate goal is to develop methods to directly measure the dynamics of water in real biological systems.
Though IR spectroscopy is at the center of his work, Petersen is always open to exploring various avenues for its use. His advice to graduate students: “Don’t be too focused on a plan, that you let good opportunities go. You can’t plan for the unexpected events and opportunities that you are going to come across along the way.”