Small Molecules in Hole-Burning Systems: A Local Antenna for Infrared-Induced Spectral Diffusion Processes

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Abstract
Light-induced spectral diffusion in photochemical hole-burning systems is investigated with respect to its dependence on the frequency of the irradiated infrared light in the wavelength range between 2 μm and 11 μm. The investigated system is polymethylmethacrylate (PMMA) containing up to one volume percent water molecules and doped with free-base phthalocyanine (H2Pc). Different resonant contributions to the infrared-induced spectral diffusion show that the microscopic origin of the observed diffusion process is a reorientation of H2O molecules. A simple kinetic model, based on the idea of two possible sites for each H2O molecule, allows to describe the dependence of the hole-broadening on infrared intensity and illumination time in a very good quantitative fashion.