MOLECULAR-ORIENTATION AT DIELECTRIC SURFACES BY ANGLE-RESOLVED PHOTOACOUSTIC-SPECTROSCOPY

Angle-resolved photoacoustic spectroscopy (ARPAS) has been developed as a sensitive (sub-monolayer detection) and nearly universal method to study molecular orientation on dielectric surfaces or in thin films. ARPAS is based on the photoacoustic signal generated as the angle between incident pulsed, plane-polarized light and the sample is varied. The theory required to describe the angular dependence of the acoustic signal, experimental details, and two comparative molecular orientation studies are presented herein. Specifically, the orientation of 1,4-bis(2-methylstyryl)benzene partitioned into a chromatographic allkyl phase was investigated as a function of solvent overlayer. ARPAS yielded approximately the same average angles of orientation as previously determined by fluorescence anisotropy. The orientation of dimethylphenylsilane covalently bound to quartz was also investigated. A value of 71 degrees +/- 3 degrees with respect to the surface normal was obtained for the transition centered near 260 nm. This value provides supporting evidence that the phenyl ring is tilted similar to 70 degrees with respect to the surface normal.