Combinations of Correlation Techniques

Tf = 5 ns, and a detection system of 5% effieieney the eount rate would be lO s. This count rate is unrealistically high. Practically achieved count rates are between a few 10 s and about 10 s Higher excitation power yields higher count rates, but increases the excited volume by saturation [101]. Moreover, photobleaching within the diffusion time results in an apparent reduction of the correlation time [49, 140, 539]. For comparable emission rates photobleaching is faster for two-photon-excitation than for one-photon excitation [140]. 

Interference of the Laser Repetition Frequency with the Eecording Clock 

The minimal time at which correlation data can be obtained with a single TCSPC module is the dead time. Currently fast TCSPC modules have dead times of 100 to 125 ns. A faster macro time clock yields more points on the auto- and crosscorrelation curves, but no correlation data below the dead time. 

Correlation down to 100 ns is usually enough to resolve diffusion times and intersystem crossing. Nevertheless, cross-correlation data at a shorter time-scale can be obtained by using two TCSPC modules with synchronised macrotime clocks (see Fig. 5.120). Synchronisation can be achieved by using the Sync signal, i.e. the laser pulse repetition frequency, as a macro time clock for both modules. This synchronisation works up to about 100 MHz, so that times down to 10 ns can be correlated. 

The correlation techniques described above use different approaches for photon correlation on the picosecond scale and FCS experiments. Although the same 

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