Chemistry
Using Flash Photolysis to Measure Fast Radical Reaction Kinetics
Quick fact
Flash photolysis, developed in 1949 by Ronald Norrish and George Porter, was the first technique to directly observe radicals and other reaction intermediates on sub-millisecond timescales, a breakthrough that earned them the Nobel Prize in Chemistry in 1967.
Why this is interesting
How do chemists measure processes that last only a billionth of a second, faster than the blink of an eye?
Read the full explanation
Understanding Using Flash Photolysis to Measure Fast Radical Reaction Kinetics
Imagine trying to photograph a hummingbird's wing in motion – you need a fast flash. Flash photolysis works on the same principle. You take a chemical system that is stable in the dark, hit it with a powerful flash of light (the 'pump' pulse), and that pulse breaks chemical bonds, creating a burst of reactive species like radicals. Then, you need to see how fast they react. To do that, you shine a second, weaker light beam through the sample (the 'probe' beam). As radicals are created and then decay, they absorb specific wavelengths of this probe beam differently. By measuring how the absorption of the probe light changes over time – using fast electronics that sample every microsecond or nanosecond – you can trace the concentration of the radical as it reacts away. What makes this so powerful is that the pump pulse is over in a flash, essentially creating a synchronized starting gun, and the probe beam tracks the ensuing chemical race.
A deeper explanation
At its heart, flash photolysis uses a pump-probe strategy. The pump is a short, intense pulse of visible or UV light (often from a laser). This pulse excites molecules into a higher electronic state or directly causes photodissociation, generating radicals. A second, delayed, and much weaker probe beam passes through the sample. This 'probe' is chosen at a wavelength where the intermediate species absorb, called their absorption spectrum. A monochromator filters the probe light to a selected wavelength, and a photomultiplier tube (PMT) converts the transmitted light intensity into an electronic signal. By recording the PMT signal as a function of time after the pump pulse, an oscilloscope builds up a trace of absorbance versus time. Because of Beer-Lambert Law, the absorbance is proportional to the concentration of the absorbing species, so you're directly measuring the concentration of the radical as it undergoes reactions. The decay of this concentration vs. time curve is fitted to kinetic models, such as first-order or second-order kinetics, to extract rate constants. For first-order decays, the half-life is directly read from the trace; for second-order, the reciprocal concentration gives a straight line. Crucially, the time resolution of the experiment is fundamentally limited by the duration of the pump pulse. If you want to see even faster processes, you use a femtosecond laser to create pulses that last only femtoseconds, extending the method to the true 'birth' of chemical reactions. Thus, flash photolysis is a powerful way to quantitatively map how radicals react, which is essential for building predictive models of combustion, ozone degradation in the atmosphere, and even biological oxidation damage.