Lights, camera . . . collide!

Scientists attempt to create a 'movie' of chemical reactions

University physical chemist Richard A. Loomis is combining powerful lasers with clever timing schemes to characterize how chemical reactions occur with very precise atomic and time resolution.

Understanding the mechanisms and physics of a chemical reaction at the most fundamental level could provide valuable insights into new directions for the field of chemistry.

Richard Loomis and graduate student Dave Boucher work with a laser system
Richard Loomis, Ph.D. (left), assistant professor of chemistry in Arts & Sciences, and chemistry graduate student Dave Boucher examine a $600,000 high-powered, femtosecond (one quadrillionth of a second) laser system in Loomis’ laboratory. Loomis and his group are attempting to become the first research group to capture a ‘movie’ of two molecules colliding to better understand the dynamics and apply results to new directions in chemistry. — Photo by David Kilper

Loomis, Ph.D., assistant professor of chemistry in Arts & Sciences, is building on the femtochemistry advances of Ahmed H. Zewail — a 1999 Nobel Prize-winner who observed, in real time, chemical bonds breaking as a molecule falls apart.

Loomis’ research group is tackling one of the next major hurdles in chemistry: observing in real time how two molecules collide and form reaction products.

These novel efforts are driven by the hopes of understanding how, as Yeats chronicled in the last century, “Things fall apart,” and as Loomis now emphasizes, “Things are made.”

Using lasers with extremely short pulse durations and very specific colors, Loomis makes real-time “movies” of molecules forming and then breaking.

“What we’re trying to do is find how molecules prefer to come together to form new compounds, and what forces and geometries encourage the breaking of bonds,” Loomis said.

“This is a complicated business. We’re trying to not only learn the road map — the hills and valleys and winding curves — that molecules follow during a reaction, but also watch these reaction events happen in real time.”

As a physical chemist, Loomis’ research interests are centered on probing and controlling reaction dynamics with atomic resolution — the most fundamental level.

The experiments in Loomis’ laboratory uniquely blend a combination of established molecular beam techniques that allow them to cool reactants to the lowest possible temperatures, about minus-273 degrees Celsius, with sophisticated laser technology. That in turn enables them to initiate the reactions with specific energies and preferred orientations at well-defined times.

Simply irresistible

At the low temperatures achieved in the experiments, two molecules find each other irresistible and are drawn together by weak non-chemical forces. However, as they approach each other, they don’t have enough energy to react.

“They end up hanging out near each other, forming a small cluster solely comprised of the two reactants,” Loomis said. “We trap them in a cluster prior to reaction.

“This cluster serves as a launching pad from which a laser can be used to excite the molecules at a well-defined time to specific energies and geometries and thus turn the reaction on.”

By using multiple lasers, Loomis and his group can not only precisely start the reactions but also monitor the decay of the reactants or the formation of the products using a second laser set to appropriate spectroscopic transitions.

At a given delay in time between the first and second laser, a snapshot of the populations of the reactants and products, as well as the relative orientations between the atoms involved in the reaction, can be recorded at that instant along the reaction pathway.

By recording numerous snapshots at incrementally increasing delay times between the lasers, a movie of the reaction at the atomic level is generated with sufficient time resolution, less than 0.0000000000001 seconds, to see geometries changing, bonds breaking and new bonds forming.

Loomis is also using sophisticated laser pulse-shaping methods and implementing quantum mechanics to control the fate of reactions.

Starting with a single ultrashort laser pulse, a computational genetic learning algorithm is used to generate a very complicated pulse sequence that focuses the molecules at desired orientations and energies at a specific time. Such an algorithm derives its behavior from a metaphor of evolution processes in nature.

The learning algorithm can be told to enhance the yield of a chemical reaction or to enhance one reaction product over other, undesired reaction products.

“Imagine hitting a key on your computer keyboard and getting one reaction product,” Loomis said. “Then hit a different key and get a different product without changing anything else.”

He and graduate student David Boucher are already able to control the vibrational motion within small molecules, and they have begun controlling the dissociation of specific bonds within molecules.

“Several research groups have recently gotten to this ability level, but now begins the fun stuff — controlling reactions between molecules,” Loomis said. “These experiments are the ones that will enable us to learn about chemistry and bimolecular reaction pathways.”

The use of lasers to dictate chemistry could actually create entirely new possibilities in chemistry.

For instance, it may be possible in the future to simply shine a powerful light with the right properties at just the right time on a bulk mixture of reactants to increase the efficiency of expensive reaction schemes. This could be especially important for industrial chemical production, where an increase in a reaction yield of just a few percent could mean millions of dollars in profit.

Lofty goals, such as improving air quality by blocking the formation of halogen waste products that are formed in combustion and industrial processes, also may be in reach.

Loomis’ research directions are not limited to molecular reactions. In collaboration with the research group of William E. Buhro, Ph.D., professor of chemistry, Loomis plans to use his ultrafast pump-probe techniques to look at the motions of electrons along semiconductor nanowires and nanostructures.

“There is a large investment from the federal government and industry to develop semiconductor nanotechnololgy so that devices with faster and more efficient electronic properties can be developed,” Loomis said. “However, right now scientists are still in the early stages of understanding how positive and negative charges behave in these small systems, where quantum mechanics dictates the energetics and relaxation processes of the charges.”

Loomis plans on using single molecule spectroscopy to image a single semiconductor nanostructure, synthesized in Buhro’s laboratory. By using two different ultrashort lasers that can be delayed in time from each other, they will excite an electron in the semiconductor and then watch it propagate along the structure using the second laser to image different spatial regions.

Again, Loomis will make real-time movies of how the charges, and thus current, travel in the semiconductor device.

“It is our goal to test and characterize the applicability of these unique semiconductor structures for use in electronics,” he said.

Another exciting impact area in which Loomis is striving to make grand contributions is quantum computing.

Here, Loomis wants to use the learning algorithm and the carefully tailored laser pulse sequences to quantum-mechanically encode information into molecules and materials. He would use the second laser to extract the encoded information from the system at a later time.

This aspect of Loomis’ research may make significant impacts on the future of computer design as well as the teleportation or encoded communication of information through space.