Biomedical subjects

KL Rowlen

Publications and source records attributed to KL Rowlen.

4 recordsLinked to original sources

PubMed2000

Orientation-insensitive methodology for second harmonic generation. 1. Theory

The use of second harmonic generation as a means to probe either adsorption isotherms or the kinetics of adsorption/desorption is limited by the fact that the detected signal is dependent upon both surface coverage and molecular orientation. Thus, the second harmonic intensity must be tediously corrected if either the mean orientation angle or the width of the orientation distribution changes as a function of surface coverage. In this study, a new mathematical view of the second harmonic intensity as a function of excitation polarization is developed. This new approach predicts an experimental geometry that is relatively insensitive to molecular orientation, such that appropriate choice of the excitation polarization rotation angle allows for direct measurement of surface coverage. The theory is presented for the three common dominant hyperpolarizability tensor elements, beta(z'z'z'), beta(x'x'x'), and beta(x'x'z').

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PubMed2000

Orientation-insensitive methodology for second harmonic generation. 2. Application to adsorption isotherm and kinetics measurements

In this work, the theory presented in part 1 for an experimental configuration that provides a means to isolate surface coverage measurements by SHG from coverage-dependent molecular orientation changes was tested. The adsorption isotherm for disperse red 1 on glass was found to be quite sensitive to the choice of excitation polarization rotation angle due to a coverage-dependent change in molecular orientation. Appropriate selection of the polarization rotation angle, as dictated by the theory presented in part 1, yielded reliable adsorption isotherm results that were essentially independent of changes in molecular orientation. Use of a standard experimental approach (e.g., probing the intensity of p-polarized second harmonic for p-polarized fundamental), resulted in large error (nearly 100%) in the calculated equilibrium constant. The adsorption isotherm for rhodamine 6G on glass was found to be relatively insensitive to the choice of polarization rotation angle since its apparent orientation angle did not change significantly as a function of coverage.

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PubMed2000

Adhesion forces measured by atomic force microscopy in humid air

Adhesive forces measured with an atomic force microscope under ambient conditions are generally regarded to be dominated by non-surface-specific capillary force. In this study, the nature of the "pull-off" force on a variety of surfaces was investigated as a function of relative humidity. The results indicate that even under the condition where capillary condensation occurs there is chemical specificity in the measured pull-off force. Issues such as tip-surface contact time and surface roughness were ruled out as possible artifacts. A mathematical model of pull-off force as a function of relative humidity is proposed in which the chemical specificity is explained.

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PubMed2000

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.

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