Enhanced Pathlength Absorption Measurements Using Optical Waveguide Structures
Stephens, Dennis Antony
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https://hdl.handle.net/2142/70433
Description
Title
Enhanced Pathlength Absorption Measurements Using Optical Waveguide Structures
Author(s)
Stephens, Dennis Antony
Issue Date
1988
Doctoral Committee Chair(s)
Bohn, Paul W.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Analytical
Abstract
The central focus of this research was to devise methods to utilize the slab optical waveguide as a sampling geometry for quantitative enhanced pathlength absorption measurements. Studies were conducted both for analytes doped into polymer thin film waveguides and monolayers on top of the waveguide.
Initial experiments explored the possibility of controlling the coupling efficiency of the prism coupler by making the gap between the coupling prism and waveguide surface reproducible. The gap thickness was monitored using a reflectance technique. It was found that small variations in the gap thickness ($\sim$8%) resulted in large changes in the coupling efficiencies ($\sim$50%). Spatial studies revealed that the polymer film was undergoing mechanical deformation as a result of pressure that was required to hold the prism in contact with the sample.
A multiple wavelength approach was found to be an effective method for separating the absorptive contribution from the experimental total attenuation. An absorption enhancement of 1.3 $\times$ 10$\sp4$ was observed for Pr$\sp{3+}$ that was doped into a poly(vinylpyrrolidone) (PVP) waveguide.
A multiple mode approach was investigated for its ability to separate the surface scatter contribution from the bulk attenuation. Again, the Pr$\sp{3+}$/PVP film system was utilized. Correlation lengths of the surface roughness and refractive index inhomogeneities were quantitated using out-of-plane elastic scatter measurements. Data suggest that the correlation length is shorter than the distance between total internal reflections, proving that independent sampling of the surface occurs. Excellent agreement between experiment and theory was obtained for nonabsorbing films, while agreement for the absorbing films was not as good.
Enhancements in the measured monolayer absorption for the monolayer samples were on the order of 10$\sp3$. The thickness of a metal free phthalocyanine film was found to be 14 A, which was in excellent agreement with conventional absorbance measurement, recorded at the peak of the Q band, which yielded 17 A. Linear dichroism experiments indicate that the macrocycle is lying flat on the waveguide surface. p-Nitrobenzoic acid chemisorbed monolayer on a glass waveguide was also studied. A surface density of 1.5 $\times$ 10$\sp $ cm$\sp{-2}$ was measured, while linear dichroism experiments also indicated that the electron transition dipole moment had an average orientation of 62$\sp\circ$ $\pm$ 6$\sp\circ$.
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