Optical investigations on PTCDA on KCl(100)

Abstract

In the present work, condensed and diluted phases of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) were investigated on thin KCl(100) films on the Ag(100) surface. The samples were structurally characterized by spot profile analysis low energy electron diffraction (SPA-LEED), and bright field microscopy. The optical properties of the samples were investigated by fluorescence spectroscopy (FL), fluorescence excitation spectroscopy (FLE), and fluorescence microscopy. The work tackled three advanced questions of the so far already quite well understood sample system. <br /> I The optical and structural properties of the condensed PTCDA/KCl(100) brickwall phase were reinvestigated for improved preparation conditions and an extended temperature range. Here a focus lied on the enhancement of the pure electronic transition with respect to the vibronic modes, called superradiance. The temperature dependence of the superradiance showed an atypical initial rise in the low temperature regime and could be explained by a theory based on finite size effects. It could be shown that the atypical low temperature behavior is the result of node planes in the wavefunctions of the excitonic ground states of brickwall-type aggregates. The node planes are induced by weak repulsive next-nearest-neighbor interactions and become dominant in the case of highly asymmetric aggregates. The experimental temperature dependence could be simulated by weighting the calculated temperature dependencies of individual aggregates with a domain size distribution extracted from a detailed SPA-LEED analysis. <br /> II The homogeneous line profiles of single PTCDA molecules located at KCl-step edge sites were investigated by high resolution single molecule FLE spectroscopy. Here first a splitting of the inhomogeneous broadening into two main species could be observed. On top of the inhomogeneous broadening a statistical fine structure consisting of individual homogeneously broadened single molecule profiles was observed. Far off the center of the inhomogeneous profile, also spectrally isolated single molecule profiles were measured. The FLE measurements of these single molecules adsorbed on a surface revealed a molecular behavior qualitatively equivalent to that observed in earlier single molecule studies on molecules in solid hosts. This includes aspects as: the width and the form of the absorption profiles, the saturation behavior with the intensity of the excitation light, an observed broadening with temperature, and a tendency to undergo laser induced and independent spectral jumps. <br /> III An UHV compatible light microscope for the brightfield and fluorescence application was developed and used to study the macroscopic surface morphology and the fluorescence signal from molecules on the sample surface. The microscope was developed for the investigation of samples under ultra high vacuum conditions and images the surface through a glass window. <br /> Bright field images of the sample revealed a characteristic mesoscopic corrugation and a variety of local defects. In the fluorescence images, a mostly homogeneous coverage of the sample could be proven. However, also exceptions from this homogeneity were observed and could be correlated with the sample morphology. These included shading effects of mesoscopic surface protrusions related to an inclined angle of the molecular beam, occasional intensity variations presumably due to a local inhomogeneity of the KCl film thickness, and very local and bright fluorescence signals at some special surface defects.