The Radical Comb-Over

Artist’s conception of an infrared frequency comb “watching” the reaction of a molecule of carbon monoxide (CO, red and black) and hydroxyl radical (OD red and yellow) as they form the elusive reaction intermediate DOCO (red/black/red/yellow) before DOCO falls apart. This chemical reaction was seen for the first time under normal atmospheric conditions in the laboratory by the Ye group. Credit: The Ye group and Steve Burrows, JILA
Thinh Bui (l) and Bryce Bjork (r) in the lab using frequency comb spectroscopy to watch a chemical reaction unfold. Credit: Steve Burrows, JILA

Frequency comb spectroscopy is making it possible to watch chemical reactions unfold in real time

Using frequency comb spectroscopy, the Ye group has directly observed transient intermediate steps in a chemical reaction that plays a key role in combustion, atmospheric chemistry, and chemistry in the interstellar medium. The group was able to make this first-ever measurement because frequency combs generate a wide range of laser wavelengths in ultrafast pulses. These pulses made it possible for the researchers to “see” every step in the chemical reaction of OH + CO → HOCO → CO2 + H.

This reaction is an example of the importance of free radicals such as the hydroxyl radical (OH), which has an unpaired electron that makes it highly reactive. Understanding (and one day controlling) the reaction OH + CO → HOCO → CO2 + H will lead to a better understanding of combustion processes as well as atmospheric chemistry and greenhouse gases. In the atmosphere, for example, the reaction of OH with CO adds carbon dioxide (CO2) to the atmosphere when fossil fuels are burned. 

Here on Earth, Bryce Bjork, Thinh Bui, and their colleagues in the Ye group used frequency comb spectroscopy to observe the detailed intermediate steps of the full reaction of OH with CO for the first time. The researchers used one trick to make their job easier. They substituted deuterium (D), or heavy hydrogen, for the H in the OH.

Deuterium is easier to distinguish in the measurement of OD reacting with CO to form DOCO, which is the heavy-hydrogen analog of the short-lived HOCO intermediate. Although long predicted to exist, HOCO wasn’t identified in conditions seen in nature until this year. Like HOCO, DOCO has so much energy that it rapidly shakes apart to form D and CO2. That’s why DOCO (and HOCO) have been so hard to find in the lab. They come and go in the blink of an eye.

With frequency comb spectroscopy, however, the researchers were able to “see” the formation DOCO, how much of it was made, and watch DOCO separate into CO2 and D.  They were also able to take 10-microsecond snapshots of the spectra of the atoms and molecules as they interacted and reacted, thus making a complete record of the chemical reaction. The researchers not only observed the chemical reaction from start to finish, but also made a movie of it!

“What’s nice about frequency combs is that you have a broad array of spectral lines to use to unambiguously identify atoms and molecules,” Bjork explained, “But you also get to use an optical cavity, which drastically increases the sensitivity. Plus, our camera allows us to take pictures of what’s happening––almost in real time. These three components are what enabled us to do this experiment.”

The researchers responsible for putting all this together include graduate students Bryce Bjork and Bryan Changala, research associates Thinh Bui, Oliver Heckl, and Ben Spaun, Fellow Jun Ye, and their colleagues from Crystalline Mirror Solutions, the University of Vienna, and the California Institute of Technology.––Julie Phillips