Quantum beats induced by spectral diffusion between independent two-level systems

We analyze theoretically the time-resolved signal detected in three-pulse, photon echo experiments where several independent two-level systems are involved, in a case when the time delay T between the two last pulses is much longer than delay τ between the two first pulses. A spectral diffusion process on a long time scale is included in the calculation of the τ-dependent response signal. If spectral diffusion occurs between the different systems, the signal exhibits quantum beats instead of the polarization interferences observed without spectral diffusion. This analysis provides different insights on dynamical processes and spectroscopy, as demonstrated in examples of experimental results obtained on vibrational dynamics of molecules embedded in matrices.