Detection of Electronic Coherence via Two-dimensional Electronic Spectroscopy in the Condensed Phase
Laboratory of Soft Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Two dimensional Fourier transform electronic spectroscopy (2DES) is a nonlinear spectroscopic method also known as three photo echo or four wave mixing with high temporal and spectral resolution. Thus it seems to be limited by uncertainty principle and the wave-packet collapse during the coherent detection. The principle of 2DES was reviewed briefly with special emphasis on that how 2DES circumvents the above two restrictions arising from quantum mechanics, leading to the successful detection of the coherence in terms of energy difference between the eigenstates showing as the quantum beats. A more physical description of the density matrix evolution for an ensemble interacting with three sequential pulsed electric fields was given in terms of optical Bloch equation, as well as of Feynman diagrams. The possible mixing among the pure electronic transition, single-mode and multi-mode coupled vibronic transition leading to the observed beating phenomena have also been addressed. Finally, recent advances in experimentally distinguishing between the electronic coherence and the vibrational coherence was briefly discussed. This review provides a bridge between the principles and intuitive physical description of 2DES for tutorial purpose.