The lifetime measurement of molecular excited state has been the subject of
many papers and experiments. Very often the experimental data are fitted by
single or bi exponential decays which in many case is the best fit that can be
done owing the signal to noise ratio. The times constants obtained from these
fit are often discussed in term of one species associated with one lifetime:
depending on the studied system, the species can be one type of molecule, one
isomer from a given molecule or local environment. How justified is this
assumption