Measuring picosecond excited-state lifetimes at synchrotron sources

A new analysis method for the short excited-state lifetime measurement of photosensitive species in crystals is described. Based on photocrystallographic techniques, this method is an alternative to spectroscopic methods and is also valid for non-luminescent excited species. Two different approaches are described depending on the magnitude of the lifetime tau. For very short lifetimes below the width of the synchrotron pulse, an estimated tau can be obtained from the occurrence of the maximal system response as a function of the pump-probe delay time Delta t. More precise estimates for both short and longer lifetimes can be achieved by a refinement of a model of the response as a function of the pump-probe delay time. The method also offers the possibility of the structure determination of excited species with lifetimes in the 40-100 ps range.