Photon echo, time-integrated

Shortly thereafter came reports of integrated three-pulse photon echoes, especially using the echo peak shift to provide information about spectral diffusion [21, 23]. In one experiment [10, 23] the peak shift shows an intriguing oscillation at short times with a period of about 180 fs, followed by a slower relaxation with a decay time of 1.4 ps. The three-pulse echo amplitude can also be heterodyned, leading to 2DIR experiments [24 26]. The latter experiments provide a wealth of information, and there are several ways to extract the desired spectral diffusion dynamics [149]. 

The infrared echo is also used to measure vibrational dynamics but in the standard implementation involves a further reduction in dimension (35,36,41,42). The excitation interactions I and II are strictly analogous to those in the Raman echo the Raman interaction is simply replaced by a direct absorption (Fig. 3, dashed arrows). However, whereas the Raman echo time resolves the signal during r3, the infrared echo integrates the signal during this time period. In this way, the infrared echo reduces the correlation function to one dimension. The standard, two-pulse photon echo is reduced to one dimension in much the same way. Because the infrared echo derives from the same basic correlation function as the Raman echo,... 

In addition to looking at a signal in the vicinity of zero delay where both excitation pulses overlap in time we also investigated the regime where the excitation pulses were well separated. Now we are in the photon echo regime. For a pulse separation of X 20 nsec the integrated signal intensity behavior is similiar to that shown in fig 2. The 1.9 psec modulation is still... 

Besides various detection mechanisms (e.g. stimulated emission or ionization), there exist moreover numerous possible detection schemes. For example, we may either directly detect the emitted polarization (oc PP, so-called homodyne detection), thus measuring the decay of the electronic coherence via the photon-echo effect, or we may employ a heterodyne detection scheme (oc EP ), thus monitoring the time evolution of the electronic populations In the ground and excited electronic states via resonance Raman and stimulated emission processes. Furthermore, one may use polarization-sensitive detection techniques (transient birefringence and dichroism spectroscopy ), employ frequency-integrated (see, e.g. Ref. 53) or dispersed (see, e.g. Ref. 54) detection of the emission, and use laser fields with definite phase relation. On top of that, there are modern coherent multi-pulse techniques, which combine several of the above mentioned options. For example, phase-locked heterodyne-detected four-pulse photon-echo experiments make it possible to monitor all three time evolutions inherent to the third-order polarization, namely, the electronic coherence decay induced by the pump field, the djmamics of the system occurring after the preparation by the pump, and the electronic coherence decay induced by the probe field. For a theoretical survey of the various spectroscopic detection schemes, see Ref. 10. 

Fig. 11.8 Pulse sequence and field-matter interactions in a three-pulse photon-echo experiment. (A) Pulses 1, 2 and 3, traveling from right to left in this diagram, reach the sample with different wavevectors ki, k2 and kf). Pulses 1 and 2 are separated by time interval t pulses 2 and 3, by interval T. Photon echos with wavevectors kj, (Joi—kf) are measured. (B) Interactions of the sample with the electromagnetic fields wavy arrows) occur at times t-t2,-t2 t (sometime during pulse 1), t-t2,-t2 (during pulse 2), and t-t (during pulse 3), and the echo (P ) is emitted at time t. The signal is integrated from t3 = 0 to oo. Figure 11.13 shows how movable mirrors can be used to control the pulse timing in such experiments...

Fig. 11.11 Dependence of integrated three-pulse photon-echo signals on t (the delay between pulses 1 and 2), as calculated in the impulsive limit with Eq. (11.43) for ti < 0 and with Eq. (11.44) for t > 0. The delay between pulses 2 and 3 12) was 0, 10 or 100, as indicated. The units of time are arbitrary. All calculations used the Kubo relaxation function (Eq. 10.69) with -r = 40 time units and

---