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Why did Ahmed Zewail's femtosecond studies advance the study of chemical reactions?

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A graph showing a potential energy curve for a chemical reaction, with the y-axis labeled potential energy and the x-axis labeled progress of reaction. The curve rises from the initial state (reactants) to a peak labeled transition state (activated complex), indicating activation energy, then drops to the final state (products). — chemical kinetics In the Arrhenius equation, the activation energy (E) represents the minimum amount of energy required to transform reactants into products in a chemical reaction. On a potential energy curve, the value of the activation energy is equivalent to the difference in potential energy between particles in an intermediate configuration (known as the activated complex, or transition state) and particles of reactants in their initial state. The activation energy thus can be visualized as a barrier that must be overcome by reactants before products can be formed.

Ahmed Zewail’s development of femtosecond spectroscopy revolutionized chemical kinetics by enabling scientists to observe the motion of atoms and molecules during chemical reactions in real time.1

Prior to this breakthrough, many believed it was impossible to study these events because they occur on an incredibly fast timescale of 10 to 100 femtoseconds (1 femtosecond = $10^{- 15}$ seconds). Zewail overcame this by using ultrafast laser technology to produce light flashes of just tens of femtoseconds. In his "pulse and probe" method, the first laser pulse initiates the reaction, while the second pulse examines the ongoing action.12

This technique allows researchers to observe the "slow motion" of atoms as they cross the reaction barrier, providing insights into structural changes and energy redistribution that were previously inaccessible. By capturing these fleeting moments, Zewail’s work—which earned him the 1999 Nobel Prize for Chemistry—has allowed scientists to gain greater control over reaction outcomes and has broad applications ranging from catalyst function to the design of molecular electronic components and the development of future medicines.12