Investigation of Level Kinetics and Reorientation by Means of Double Pulse Excited Fluorescence

Fluorescence spectroscopy is advantageously applied in order to measure excited state lifetime and reorientation of the transition moment after excitation (see, e. g., [1], [2]). Applying ultrashort light pulses the time resolution is limited in most cases by the signal detection system. MAHR et al. [3] proposed a method in which an excite-and-probe scheme is combined with fluorescence observation. The dependence of the integrated fluorescence signal on the relative delay between two successive exciting light pulses is measured. That dependence arises under the condition that the second pulse views a ground state population depleted by the first pulse. The time resolution is only limited by the pulse duration and hence values in the sub-psrange can be achieved. The method proposed in [3] represents a modification of that given in [4] where the fluorescence from a higher singlet level has been measured after a two step excitation S0 → S1 → Sx, in which one of the exciting pulse trains has been middulated.