Experimental and Theoretical Study of Azimuth Angle and Polarization Dependences of Sum-Frequency-Generation Vibrational Spectral Features of Uniaxially Aligned Cellulose Crystals

Comprehensive interpretation of vibrational sum-frequency-generation (SFG) spectroscopic features of SFG-active nanodomains interspersed in amorphous bulk requires the knowledge of nonlinear susceptibility, chi((2))(ijk), of the SFG-active phase as a function of its spatial arrangement in the bulk as well as the polarizations of the probe lights. This study reports the full analysis of the azimuth angle and polarization dependence of SFG signals from a control sample consisting of uniaxially aligned cellulose I beta crystals. The chi((2))(ijk) terms of cellulose were estimated from quantum mechanics calculations using time-dependent density functional theory (TD-DFT), and a simple structural model was built with truncated glucose dimers. The theoretical azimuth angle and polarization dependences of characteristic CH/CH2 and OH stretch modes of cellulose were compared with the experimentally observed trends. These comparisons revealed that the relative polarity of crystallites within the SFG coherence length, the random quasi phase-matching of polycrystalline domains, and the preferential packing of crystallites in the bulk play important roles governing the spectral features. Compared to that of small molecules, the difference between chiral and achiral responses in SFG spectra is more difficult to observe because of the inhomogeneous distribution of crystallites in the bulk sample.