What is the meaning of lifetime measurement?
  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? 
Christophe Jouvet (PPM), Gilles Gr\'egoire (LPL), Claude Dedonder   Lardeux (PPM)