How does flash photolysis create a nonequilibrium population of excited atoms or molecules?

Flash photolysis creates a nonequilibrium population of excited atoms or molecules by subjecting a chemical system to an intense, short-duration burst of light.123

When molecules absorb this photon energy, electrons are promoted to previously unoccupied orbitals, resulting in the formation of excited states such as singlet or triplet states. Because this absorption occurs almost instantaneously while the nuclei initially remain in their ground-state positions, the system is driven out of its original equilibrium state. In some cases, this intense radiation also causes photodissociation, breaking molecules down into short-lived, highly reactive fragments.12

By creating this transient, nonequilibrium population, researchers can use a second, weaker probe pulse to monitor the evolution of these species via spectroscopy as they undergo relaxation or chemical change. This allows for the study of fast kinetic processes that occur on timescales ranging from milliseconds down to a few femtoseconds.13