Doubly resonant sum-frequency generation from molecules with general harmonic potential surfaces

A many-body approach is used to analyse the doubly resonant infrared-visible sum-frequency generation from molecules with general harmonic potential surfaces. This multi-mode model includes arbitrary linear and quadratic electron-phonon coupling coefficients, and is valid at any temperature. Numerical simulations have been performed on a simple two-mode model with equilibrium position displacements, frequency shifts and Duschinsky rotation under electronic excitation. Our study leads us to propose these types of experiments to discriminate between the effects of linear and quadratic electron-phonon coupling terms.