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 τ. For very short lifetimes below the width of the synchrotron pulse, an estimated τ can be obtained from the occurrence of the maximal system response as a function of the pump-probe delay time Δt. 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.