Why are stopped-flow methods not suitable for reactions with extremely short half-lives?

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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.

Stopped-flow methods are not suitable for reactions with extremely short half-lives because they are limited by the time required to mix the reactants. In this technique, reactants are forced into a reaction chamber and the flow is suddenly stopped to measure the amounts of substances present.1

Because the mixing process itself takes a finite amount of time, these methods cannot accurately measure reactions with a half-life of less than approximately one-hundredth of a second. When a reaction occurs faster than the time it takes to complete the mixing, the initial state of the reaction cannot be determined, rendering the measurement unreliable. For processes occurring on faster timescales, researchers instead utilize pulse and probe methods, such as flash-photolysis or relaxation methods, which bypass the physical limitations of mechanical mixing.1