A physical chemist at Washington University in St. Louis 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 A. Loomis, Ph.D., assistant professor of chemistry, is a physical chemist building on the femtochemistry advances of Nobel Prize Winner (1999) Ahmed H. Zewail of Cal Tech who observed chemical bonds breaking as a molecule falls apart in real-time. Loomis' research group is tackling one of the next major hurdles in chemistry, observing two molecules collide and form reaction products in real-time. 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."
Loomis discussed his work Oct. 29, 2002, at the 40th New Horizons in Science Briefing, sponsored by the Council for the Advancement of Science Writing, held Oct. 27-30, at Washington University in St. Louis.
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 learn the road map -- the hills and valleys and winding curves -- that molecules follow during a reaction."
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 the Loomis laboratory uniquely blend a combination of established molecular beam techniques that allow them to cool reactants to the lowest possible temperatures, about -273 degrees Celsius, with sophisticated laser technology which in turn enables them to initiate the reactions with specific energies and preferred orientations at well-defined times.
Simply irresistible, but no energy
At the low temperatures achieved in the experiments, two molecules find each other irresistible and are drawn together. However, they don't have enough energy to react. "They end up hanging out near each other," Loomis explained. "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.
As if watching and characterizing chemical reactions isn't enough, 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. Loomis emphasizes the utility of this chemistry tool.
"Imagine hitting a key on your computer keyboard and getting one reaction product. Then hit a different key and get a different product without changing anything else," he said.
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 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.
Another exciting impact area in which Loomis is striving to make keg contributions is in 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.
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