By fluorescence decay measurements

physicists

chemists and biologists can study the behaviour of numerous excited species whose lifetimes range typically from 10

-6

to 10

-9

second.With the related technique of time-resolved emission spectroscopy

an emission spectrum can be recorded in a few nanoseconds after excitation and observed the changes in the spectra with time.These measurements have been carried out from microsecond to picosecond time scale by the use of the single photon counting technique.