Intensity correlation method, lifetime measurement

Luminescence lifetime spectroscopy. In addition to the nanosecond lifetime measurements that are now rather routine, lifetime measurements on a femtosecond time scale are being attained with the intensity correlation method (124), which is an indirect technique for investigating the dynamics of excited states in the time frame of the laser pulse itself. The sample is excited with two laser pulse trains of equal amplitude and frequencies nl and n2 and the time-integrated luminescence at the difference frequency (nl - n2 ) is measured as a function of the relative pulse delay. Hochstrasser (125) has measured inertial motions of rotating molecules in condensed phases on time scales shorter than the collision time, allowing insight into relaxation processes following molecular collisions. 

Since few room temperature dual emitters were known prior to the hetero-cyclic-substituted platinum-1,2-enedithiolates, methods, other than those described in Eqs. 7-9, have been developed to circumvent problems with absolute luminescence intensity measurements. These methods generally require a measurement that is correlated to emitter lifetime. The ratio of the lifetime in the absence of the quencher,To, and at some quencher concentration, x is defined in Eq. 10, where kq is the oxygen-quenching rate constant. Plotting xQ/x — 1 versus [02] allows kqxQ (K(), as defined in Eq. 4) to be determined from the slope (Eq. 11). 

Table 2.5 yields the different values of the mean fluorescence lifetime and of intensity of fluorescein in presence of increased concentrations of KI. Lifetimes were measured with both frequency domain and Time correlated single photon counting methods. Figure 2.20 displays the normalized values at different Kl concentrations. 

---