Theory of quantum beats in optical transmission-correlation and pump-probe measurements

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Abstract
A full density-matrix theory for quantum beats in optical transmission-correlation or pump-probe type of experiments is developed. The model used is a three-level system with split excited-state levels. It is demonstrated on the basis of perturbation theory that quantum beats should be observable in such experiments. Theoretical and numerical results are compared with the complex details, including damped sinusoidal decays, observed in recent femtosecond relaxation studies of large organic dye molecules. It is also shown that there is no separately identifiable decay component corresponding to any homogeneous or inhomogeneous optical dephasing time in such experiments. Similar beating should also be observable in ultrafast spectroscopy using incoherent light and in Raman-type energy-level configurations where the ground state contains split levels.