Vibronic quantum beat in pump-probe experiments

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Abstract
The temperature dependence of vibronic quantum beats in dye-doped polymer films has been investigated using femtosecond accumulated photon echoes. It has been found that the vibronic quantum beat is not discernible above liquid-nitrogen temperature. It makes a striking contrast with the previous observation of the quantum beat in transient pump-probe experiments performed at room temperature. Based on the density matrix incorporating the fluctuation of the transition frequency, it is pointed out that there is no contribution of sublevel coherence to the accumulated photon echo, and the line broadening of the vibronic state or phonon sideband obscures the quantum beat at higher temperature. On the other hand, in the transient pump-probe experiment, the quantum beat is caused by the sublevel coherence, as was previously pointed out by Mitsunaga and Tang [Phys. Rev. A 35, 1720 (1987)]. Hence it is concluded that the influence of the frequency fluctuation on the sublevel coherence is relatively small despite the serious line broadening of the vibronic state.