New spectroscopy technique probes molecular energy flow

Amid a tangled maze of wires and shiny steel enclosures on the ground floor of Frick Laboratory, Chemistry Professors Kevin Lehmann and Giacinto Scoles are overseeing an investigation into the behavior of energy within individual molecules. At temperatures near absolute zero, the coldest temperature in the universe, the two men are using high frequency lasers and temperature probes accurate to one hundredth of one millionth of one degree to better understand how energy moves from molecular bond to molecular bond.

Their 15-yearlong collaboration, quite unusual in the field of chemistry, earned them the Earl K. Plyler Prize for Molecular Spectroscopy.

"The collaboration has been extremely special," Lehmann said. "We have complimentary strengths: We are more than the sum of the parts."

Given annually by the American Physical Society since 1976, the prize awards $3,000 to the scientist or scientists whose work most significantly advanced the field and our understanding of molecular properties through spectroscopy. This is only the second time the prize has been shared.

"At least in the field of spectroscopy, almost all the major advances have been a particular individual's brilliant idea," said George Schatz, professor of chemistry at Northwestern University and chair of the committee that selected Lehmann and Scoles for the award.

"But in this particular case, they were already well established people coming from very different backgrounds," he said. "They somehow got together and realized that there was this experiment that they could do that brought forth the most exciting things they had going on at the time."

Past winners include Gerhard Herzberg and Ahmed Zewail, both Nobel Laureates in chemistry.

Their powers combined

In molecular spectroscopy, scientists expose molecules to particular frequencies of light. By carefully examining the way the light interacts with the particles, they can infer many properties of the substance, from structure to color.

Spectroscopy is a discipline that straddles the border between chemistry and physics. Scoles said it has been a challenge to place their work into the University's rigid departmental structures.

"It's more dated than I am, which is a lot," said Scoles, who has been in academia for more than 40 years.

Lehmann and Scoles' recent work builds on a technique Scoles developed in the 1970's. Analyzing the light that is transmitted through particles is very difficult, since most of the light is transmitted through the material without being affected. Thus scientists are faced with the challenge of looking for exceedingly small fluctuations in a strong, uniform signal.

Instead of looking at the light that was transmitted through the molecules, Scoles proposed that scientists could get more information by examining the energy the molecules absorbed. If it were possible to remove most of the molecules' energy before exposing them to the light, by cooling them to temperatures near absolute zero, then the signals they would examine would reflect only the energy that was absorbed. Scoles refers to the technique, called optothermal spectroscopy, as his "contribution to science."

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By using this technique in their recent research, Lehmann and Scoles have achieved a signal-to-noise ratio that would have been virtually impossible by looking at transmitted light. Their research focused on how energy is transported within individual molecules. When chemists want to engineer a particular molecule, there are standard techniques they can employ. One of the most common is exciting a certain bond to replace one atom with another.

For example, consider benzene, a ring of six carbon atoms each of which has a hydrogen atom sticking out of it. If a chemist wanted to replace a hydrogen atom with a chlorine atom, he could do so by adding energy to the bond between the carbon and the hydrogen to make that site reactive.

All chemical bonds vibrate at a certain natural frequency, so to add energy to a given bond, all you need to do is hit the molecule with light of that frequency.

But there is a catch. Once the energy is put in a carbon-hydrogen bond, it doesn't stay there — it begins to vibrate the molecule in other ways. Once the energy has left the bond, the bond is no longer reactive.

Lehmann and Scoles were able to precisely describe how long energy stays in certain bonds in several particular molecules. This is the microscopic equivalent to describing how long different kinds of glue take to dry.

In order to measure how quickly energy leaves a bond, Lehmann and Scoles expose a beam of particles to a laser whose frequency was tuned precisely to match the vibration frequency of a particular bond. The beam hits a small temperature detector. As the particles hit the detector, they change its temperature at the same frequency as their bonds are vibrating.

If the detector finds the temperature fluctuates with the same frequency as the light that excited the molecule, then the experimenters know all of the energy stayed in the bond.

If instead some of the energy leaked to a different bond, with a different natural frequency, the detector would record a mixture of the two frequencies.

Lehmann and Scoles measured the time it took for the energy to leak to other bonds by waiting for the mixture of frequencies to appear.

The big chill

In order to lower the temperature of the particles before exposing them to the light, Lehmann and Scoles embedded them in microscopic droplets of liquid helium. The tiny droplets were cooled to 2.2 Kelvin — just over absolute zero — and then passed through a box containing gaseous particles of the substance to be studied.

Scoles compared the process by which the particles wind up in the droplets to a boxcar picking up passengers. As the helium passes through the gaseous region, the particles just hop in and are then free to move around inside the droplets. Once in the "boxcars," the particles are more easily manipulated as the droplets serve as an ideal medium in which to synthesize new compounds, some of which cannot be made in any other way.

It is the hope that this research will help scientists engineer new molecules more efficiently as they come to better understand the way energy is stored within a molecule.

"Physics is one experiment, you do it, you write down the result and you feel you understand something," Scoles said. "In chemistry, one day you look at one [sample], the next day the next, systematically. Then you understand something."