This first strong pulse is called the pump pulse and starts a chemical reaction or leads to an increased population for energy levels other than the ground state within a sample of atoms or molecules.
[1] Typically the absorption of light by the sample is recorded within short time intervals (by a so-called test or probe pulses) to monitor relaxation or reaction processes initiated by the pump pulse.
Flash photolysis was developed shortly after World War II as an outgrowth of attempts by military scientists to build cameras fast enough to photograph missiles in flight.
[2] Over the next 40 years the technique became more powerful and sophisticated due to developments in optics and lasers.
Today, flash photolysis facilities are extensively used by researchers to study light-induced processes in organic molecules, polymers, nanoparticles, semiconductors, photosynthesis in plants, signaling, and light-induced conformational changes in biological systems.