Molecular orientation and angular distribution probed by angle-resolved absorbance and second harmonic generation

Second harmonic generation (SHG) and angle-resolved absorbance with photoacoustic detection were combined to evaluate both the mean orientation angle and the width of the angular distribution for two surface-bound molecular systems. Assuming a Gaussian distribution function, a physisorbed stilbene dye on fused silica exhibited a narrow distribution centered at 73 degrees with a root-mean-square (rms) width of less than approximately 8 degrees. In contrast, a covalently bound azo dye resulted in a broad orientation distribution (rms width of approximately 30 degrees) centered at 60 degrees. It was also demonstrated that the combination of nonlinear (SHG) and linear (absorbance) spectroscopic techniques provides valuable insight into molecular orientation that a combination of two linear techniques (such as fluorescence and absorbance) is unable to provide.