Bryan Changala

Buckyballs Play by Quantum Rules

Steven Burrows — Fri, 22 Feb 2019 19:56:53 +0000

Buckyballs Play by Quantum Rules Steven Burrows Fri, 02/22/2019 - 12:56

Catherine Klauss / JILA Science Communicator

When the Ye group measured the total quantum state of buckyballs, we learned that this large molecule can play by full quantum rules. Specifically, this measurement resolved the rotational states of the buckyball, making it the largest and most complex molecule to be understood at this level. Image credit: Steven Burrows / JILA

A water molecule has three atoms—two hydrogens and one oxygen. But stack three water molecules side by side and you’ve got the width of a buckyball, a complex molecule of 60 carbon atoms. Medium in size and large in atom count, the buckyball has long challenged the idea that only small molecules can play by quantum rules.

And last month we learned that the buckyball plays by full quantum rules when the Ye group measured its total quantum state. Specifically, this measurement resolved the rotational states of the buckyball, making it the largest and most complex molecule to be understood at this level.

Big Molecules, Big Results

The structure of the buckyball, or formally buckminsterfullerene, is elegant in its complexity. A round molecule of 60 carbon atoms, it mixes hexagons and pentagons like a soccer ball. According to Dr. Marissa Weichman, a JILA postdoc and coauthor of the recent buckyball publication, this is the most symmetric shape a molecule can take.

Given its 60-atom count, the buckyball is large for the quantum world. But while sometimes difficult, large molecules are worth the effort to study, said Bryan Changala, JILA graduate student and lead author of the recent publication. According to Changala, understanding large molecules could potentially further our understanding of all complex systems.

“A lot of AMO [atomic, molecular, and optical] experiments are focused on creating, controlling and manipulating quantum many-body systems in complex states,” said Changala. “But a molecule is nature’s own quantum many-body system.”

The Ye group has been pursuing cooling large molecules and performing high resolution spectroscopy for a number of years, starting when Changala was a first-year graduate student. The first demonstration was achieved in 2016 for molecules such as adamantine (26 atoms).

While the Ye group are not the first to attempt to study buckyballs with spectroscopy, they are the first to attempt to understand it at such a fine quantum level. “Previous structural measurements have been done with X-ray diffraction and electron diffraction,” said Changala, “but these are either not done in the gas phase, or they are done warm,” both of which directly limit the resolution.

Cold and Combed

To achieve high-resolution measurements, the Ye group both chilled and combed their buckybulls. The former quieted vibrations, and the latter parsed through fine quantum structure.

“Our frequency comb was the reason we were able to measure this and no one else had before,” said Weichman.

The cavity-enhanced frequency comb, developed by the Ye lab, is a laser that is simultaneously narrow and broad. Like all frequency combs, this laser has a broad spectrum of precise frequency peaks that can quickly comb through molecular transitions. But it is the cavity enhancement of this particular laser that enables the necessary high sensitivity.

But while both of these factors—the cold gas state and the cavity-enhanced frequency comb—are necessary to probe the buckyballs at a high-resolution (rotational-state) level, they alone are not enough to tackle a molecule with so many atoms.

“In a normal molecule, there are can be hundreds to thousands of rotational states,” said Weichman. “But bigger molecules mean denser rotational states.” For a molecule with as many atoms as the buckyball, Weichman said there can be more than a million rotational states in the ground vibrational state alone.

From Grass to Trees

Even with the cavity-enhanced frequency comb, a million rotational states are too dense to resolve. Weichman likens the signal to an overgrown field of grass, where it is hard to differentiate a single blade from another.

“It’s all moving towards a classical structure,” said Weichman. “where the individual states become so dense that they blur into a continuum.”

But the buckyball is no ordinary large molecule. “It’s perfectly symmetric,” Weichman reminded, “and it is this symmetry of buckyballs that allows us to use small-molecule tools.”

When the research team combed through the buckyball’s spectroscopic signal, they saw not a grass field of fuzzy states, but clear, specific states, “like a forest of trees that were pruned in a very specific way” said Weichman.

And according to Weichman, this pruning is due to the buckyball having a perfect icosahedral structure. “The atoms are all exactly spaced. It’s not approximate, it’s exact.”

Because of this exact spacing, the atoms are indistinguishable, much like how one hexagon corner on a perfect soccer ball looks just like any other hexagon corner. And when the atoms are indistinguishable, quantum statistics declares many rotation states are forbidden, thereby pruning the forest. In the end, only one for every 60 states remain, said Changala, or a little less than 2%.

In future experiments, the Ye group hopes to observe the spectrum of imperfect buckyballs, in which a single carbon-13 atom replaces a typical carbon-12 atom.

“The indistinguishability would completely disappear, because all of the atoms will now be distinguishable based on their distance and location relative to the impurity. So you would see the spectroscopy signal change from individual trees back to grass,” said Weichman.

This research was published in the January 4th 2019 issue of Science.

The research work was conducted by graduate student Bryan Changala, postdoc Marissa Weichman, and Fellow Dr. Jun Ye. They acknowledge the support of co-authors Kevin Lee and Martin Fermann, of IMRA American Inc., who built the infrared laser necessary for this experiment.

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The Energetic Adolescence of Carbon Dioxide

Steven Burrows — Fri, 12 Jan 2018 18:15:52 +0000

The Energetic Adolescence of Carbon Dioxide Steven Burrows Fri, 01/12/2018 - 11:15

Catherine Klauss / JILA Science Communicator

The DOCO molecule interconverts between isomers trans-DOCO (left) and cis-DOCO (right) through rotation of the single bond (center). The multiple colors of the frequency comb enabled Bui et. al to not just identify cis-DOCO, but all of the molecules in the reaction simultaneously. Image credit: Steven Burrows / JILA

The reaction, at first glance, seems simple. Combustion engines, such as those in your car, form carbon monoxide (CO). Sunlight converts atmospheric water into a highly reactive hydroxyl radical (OH). And when CO and OH meet, one byproduct is carbon dioxide (CO₂) – a main contributor to air pollution and climate change.

But it’s more complicated than that. Before CO₂ is formed, a short-lived, intermediate molecule, called the hydrocarboxyl radical (HOCO), is formed. The existence of HOCO was first proposed over 40 years ago, but the unstable nature of the molecule made it difficult, nearly impossible, to observe. The Ye group of JILA, however, has been closing in on the impossible.

The deuterated version of HOCO, called DOCO, was observed by Bryce J. Bjork, Thinh Q. Bui, Jun Ye, and their collaborators in 2016. Deuteration, or substituting a heavier version of the hydrogen atom, called deuterium, reduced signal contamination from the atmospheric water in their system.

Yet their understanding of the reaction was incomplete. The version of DOCO that the Ye group observed did not dissociate into carbon dioxide. “There were a lot of missing details in the reaction pathway” said Thinh Bui, a postdoctoral researcher in the Ye group.

In their new paper, published in Science Advances, every step of the reaction, starting from the reactants, OD and CO, all the way to the final product of CO₂, and every intermediate in between, is finally accounted for.

Specifically, the group detected the two variations of DOCO, trans-DOCO and cis-DOCO. The cis- and trans- prefixes denote different geometric isomers, or arrangements of the atoms within the molecule. The two isomers differ by only a single bond rotation, with cis-DOCO resembling a boat, and trans-DOCO resembling a chair.

Cis-DOCO is the more elusive of the two isomers because it is more energetic. Like a child running around, the high energy cis-DOCO rotates, vibrates, and generally evades detection. Remove this high energy with a molecular collision and the cis-DOCO calms down. Let cis-DOCO run around however, and something more interesting happens: cis-DOCO will dissociate into deuterium and CO₂, a trick that the calmer trans-DOCO cannot do.

The original work by the Ye group identified only the trans-DOCO variation. To identify cis-DOCO, the group had to wade through heaps of overlapping molecule signals.

The reaction, at first glance, seems simple. Combustion engines, such as those in your car, form carbon monoxide (CO). Sunlight converts atmospheric water into a highly reactive hydroxyl radical (OH). And when CO and OH meet, one byproduct is carbon dioxide (CO2) – a main contributor to air pollution and climate change.

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The Radical Comb-Over

Steven Burrows — Thu, 27 Oct 2016 18:32:34 +0000

The Radical Comb-Over Steven Burrows Thu, 10/27/2016 - 12:32

Julie Phillips / Science Communicator

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: Steven Burrows / JILA

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.

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Stalking the Wild Molecules

Steven Burrows — Wed, 04 May 2016 18:56:02 +0000

Stalking the Wild Molecules Steven Burrows Wed, 05/04/2016 - 12:56

Julie Phillips / Science Communicator

Infrared-laser comb spectroscopy can now identify large, complex molecules if the molecules are first cooled to ultralow temperatures with a buffer-gas cooling technique developed at Harvard. Image credit: Steven Burrows / JILA

The Ye group just solved a major problem for using molecular fingerprinting techniques to identify large, complex molecules: The researchers used an infrared (IR) frequency comb laser to identify four different large or complicated molecules. The IR laser-light absorption technique worked well for the first time with these larger molecules because the group combined it with buffer gas cooling, which precooled their samples to just a few degrees above absolute zero. This innovative combination is expected to provide insights into the physics of molecular structure and behavior. It will also improve trace gas detection techniques used in breath analysis, hazardous gas identification, atmospheric chemical analysis, and climate science studies.

The reason even medium-sized molecules previously posed a problem in molecular fingerprinting is that all the atoms in molecules rotate and vibrate in complex patterns. And the more atoms there are moving wildly and communicating among themselves, the more spectral lines crowd into one spectrum. The spectrum becomes so congested it cannot be analyzed. This congestion occurs because even medium-sized organic molecules (containing carbon, hydrogen, and oxygen) typically have millions of rotational-vibrational states at room temperature. These states all show up in a single IR spectrum. And, the problem with congested spectra gets even worse as molecules get bigger and bigger.

The researchers who solved the problem of analyzing large, complex wild molecules include research associates Ben Spaun and Oliver Heckl, graduate students Bryan Changala and Bryce Bjork, Fellow Jun Ye and their colleagues Dave Patterson and John Doyle from Harvard University.

Spaun and his team were well positioned to make this breakthrough. The Ye group figured prominently in the development of the original laser frequency comb, or ruler of light; the group also developed the first mid-IR laser frequency comb. The mid-IR comb consists of thousands of evenly spaced spectral lines that can be used to detect, measure, and identify unknown substances with exquisite accuracy––as long as the spectrum isn’t too congested.

In this breakthrough experiment, researchers first cooled molecules of nitromethane (7 atoms), naphthalene (18 atoms), adamantine (26 atoms), or hexamethylenetetramine (22 atoms). Then the researchers placed the molecules in a hollow chamber with comb light bouncing back and forth through the molecules. The molecules absorbed the comb light at the specific frequencies at which the molecules rotate and vibrate. These absorption patterns, which are unique for each different molecule, are the molecular fingerprints that made their identification possible.

Identifying these large, complex molecules also required the use of an innovative cooling technique known as buffer-gas cooling, which was developed by Patterson and Doyle. This technique used a small commercial liquid helium refrigerator cooled to 4 K (just 4 degrees above absolute zero). First, the researchers flowed helium atoms into the refrigerator. These atoms collided with the walls of the refrigerator, which cooled them to 4 K.

Second, the researchers introduced some their “hot” test molecules to the refrigerator. The test molecules collided with the helium atoms, which transferred heat from the test molecules into the walls of the refrigerator. Once the test molecules got very cold, their vibrations and rotations calmed way down. The molecules now moved very slowly through the hollow chamber where the comb light was bouncing back and forth between two mirrors.

The wild molecules were tamed, and the molecular fingerprinting process worked like a charm!

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