Highly sensitive absorption measurements in organic thin films and optical media

Abstract

An important signature of material properties is the absorption spectrum. However, for weak absorption in thin films or for subtle spectral features, standard measurement techniques are inadequate. Ideally suited for this regime is photothermal deflection spectroscopy (PDS) which can give several orders of magnitude greater sensitivity than transmission techniques. With the sensitivity of PDS, it is possible to detect features that cannot be observed by other means, such as singlet-triplet transitions, weak charge-transfer bands, and weak photo-chemical changes at the surface. The films studied were those commonly used as charge transport materials in organic photoconductors, and are comprised of a small organic molecule and a polymer binder. The PDS spectra indicate the formation of charge transfer complexes between the two components, in the solid state. The strength depends on the various combinations and can be correlated to relative energy level offsets between the HOMO of the small molecule and the LUMO of the polymer binder. Reversible photochemistry resulting from UV exposure can also be detected at the surface. In addition, using PDS, it has been possible to obtain, for the first time, the vibrational overtone spectra in the range from the near IR to the visible (0.4 - 2 eV) for polymeric thin films. The absorption spectra of polycarbonate, for instance, exhibits C-H stretch modes for Δn > 1 where n is the vibrational quantum level. Previous measurements were constrained to either long fibers, which pose the problem of scattering, or to a restricted wavelength region in the visible accessible by dye laser sources. The unparalleled sensitivity of PDS allows precise determination of frequency, lineshape, and intensity of the various modes, even for films just a few microns thick over a very broad energy range. The overtone spectra can be used as a probe of various basic molecular properties such as bond energy, anharmonicity, and vibrational energy localization. In a manner similar to NMR spectroscopy, it is possible to study specific atomic bonds.