A Simplified Formulation of Linear and Nonlinear Spectroscopy of Ordered Molecular Systems

A simplified formulation for treating the linear and nonlinear spectroscopy of ordered molecular systems is presented，in order to help experimentalists to have an explicit physical picture and quantitative tool on using linear and nonlinear spectroscopy to study molecules in ordered molecular systems. This formulation is expended from our recent quantitative orientational and polarization treatment on second-order nonlinear spectroscopic techniques in interface studies，namely，the Second Harmonic Generation（SHG）and Sum Frequency Generation-Vibrational Polarization Spectroscopy（SFG-VPS）. The key to this formulation is to simplify the effective linear or nonlinear molecular susceptibility and construct the general orientational functional with a clear approach to calculate the orientational and intensity parameters from the experimental parameters，which determines the orientational and polarization behavior of the general orientational functional in a particular experimental configuration. Also discussed are the advantages of coherent spectroscopic techniques over incoherent ones for the accurate measurement of orientation and ordering of ordered molecular system.