Triply resonant infrared-infrared-visible sum frequency generation: Three-dimensional vibronic spectroscopy for the investigation of vibrational and vibronic couplings

A theoretical description of the triply resonant (TR) infrared-infrared-visible sum frequency generation (IIV-SFG) is presented, which is an extension of the electronically off-resonant IIV-SFG to the case when the visible field is in resonance with the electronic transition. Since three injected field frequencies can be independently controlled, the measured susceptibility is a three-dimensional spectrum containing information on both the vibrational and vibronic couplings of a molecule in condensed phases. It is shown that the nonlinearities of the dipole moment in the ground electronic state with respect to vibrational coordinates can be separately measured by using the TR-IIV-SFG method. A series of numerical calculations of 2D vibrational and vibronic spectra for a model system are presented to show how the TR-IIV-SFG measurement can be of use in studying specific vibrational and vibronic coupling processes. (C) 2000 American Institute of Physics. [S0021-9606(00)01820-1].