Signatures of vibrational interactions in coherent two-dimensional infrared spectroscopy

The influence of three vibrational interaction mechanisms on two-dimensional (2D) spectra derived from infrared (IR) photon echo experiments is investigated. In particular it is shown that the existence of a nonlinear signal requires one of the following vibrational nonlinearities: (1) anharmonicity in the ground state potential, (2) nonlinear dependence of the transition dipole moment on the vibrational coordinates, or (3) level-dependent dephasing dynamics. 2D spectra arising from these interactions are investigated using a model of two coupled cubic anharmonic oscillators. The third-order nonlinear response for this model arises from 20 different vibronic coherence pathways involving six possible vibrational states. It is shown that the three interaction mechanisms are revealed by distinct signatures in the 2D spectrum: the positions, amplitudes, and shape of peaks, respectively. These signatures provide an intuitive framework for the analysis of 2D IR spectra. (C) 2001 Elsevier Science B.V. Ah rights reserved.