| Molecular orientation is often the defining parameter in characterizing thin surface films. However, orientation measurements can be influenced by a wide variety of effects, many of which are difficult to correct for quantitatively.; One effect that had not been previously addressed quantitatively is that of surface roughness. In this work, both theory and experimental methodology were developed to combine “macroscopic” orientation measurements with roughness measurements to evaluate the local chromophore orientation. This approach was developed and evaluated for both linear (absorbance) and non-linear (second harmonic generation, or SHG) spectroscopic techniques.; One of the most significant findings of this work was that a “magic angle” exists for SHG. Sufficiently broad orientation distributions were demonstrated to yield an apparent orientation angle of 39.2° by SHG when a narrow distribution is erroneously assumed. This discovery indicates that the width of the orientation distribution can be as important as the mean in interpreting spectroscopic orientation measurements.; In other investigations, SHG and angle-resolved absorbance with photoacoustic detection (ARAPD) measurements were combined to evaluate both the mean and width of orientation distributions for surface-bound chromophores.; Another effect which can lead to complications in interpreting spectroscopic measurements is that of coverage-dependent changes in orientation. The necessary theory was developed to obtain orientation insensitive coverage measurements. The theory was tested for both adsorption isotherm and kinetics measurements by SHG. Errors in the experimental constants of up to nearly 100% were observed when changes in molecular orientation were neglected. |
