Concurrent polarization IR analysis to determine the 3D angles and the order parameter for molecular orientation imaging
Young J. Lee
A non-tomographic approach based on polarization-dependent infrared (IR) spectroscopy is proposed for a simultaneous measurement of the 3D angles and the orientational order parameter of a molecular orientation distribution. Unlike conventional spectroscopic approaches based on separate analysis of 2D-projected polarization signals, this new approach concurrently analyzes polarization profiles of two non-parallel IR modes to determine the our-of-plane component of a molecular orientation. Using the phase angle difference and the maximum-minimum ratios of the two polarization profiles, this method determines not only the 3D orientation angles but also the tilt-free second order orientation order parameter without iteration or fitting. The use of the dimensionless observables makes the analysis results unaffected by variations in the sample concentration, sample thickness, absorption coefficient, and other instrumental conditions. A simple error propagation analysis with two simulation data examples indicates that the uncertainty in 3D angles are comparable to the uncertainties of the input phase angles and the uncertainty in the second order parameter is as low as 0.01. Based on the properties, including non-tomographic data measureable by a single polarization scan; concentration-independent dimensionless observables; and direct and iteration-free computation, model-free computation, this method can be used for hyper-structural imaging applications that can provide a full orientational characterization with a diffraction-limited spatial resolution.