Inhomogeneous and Homogeneous Line Broadening of Optical Spectra of PTCDA Molecules Adsorbed at Step Edges of Alkali Halide Surfaces

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• 作者：A. Paulheim ; C. Marquardt ; H. Aldahhak ; E. Rauls ; W. G. Schmidt ; M. Sokolowski
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：June 9, 2016
• 年：2016
• 卷：120
• 期：22
• 页码：11926-11937
• 全文大小：614K
• 年卷期：0
• ISSN：1932-7455

文摘

We report a detailed analysis of the line shapes of the 0–0 transitions in the fluorescence (FL) and fluorescence excitation (FLE) spectra of perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) molecules adsorbed at terrace and step edge sites of (100)-oriented alkali halide films (KCl and NaCl). At low temperatures (6–20 K), we find very narrow FLE lines with a fwhm of 4.5 cm^–1 (3.0 cm^–1) on KCl (NaCl). These line shapes are dominated by inhomogeneous broadening related to the structural variation of the environment of the PTCDA molecules. We explain this site broadening on the basis of structural models for the adsorption sites at the step edges from earlier scanning microscopy data and density functional theory calculations. With increasing temperatures, the 0–0 lines in the FL and FLE spectra broaden; e.g., in the FL, the fwhm increases to 26 cm^–1 (18 cm^–1) at 100 K on the KCl (NaCl) surface. This temperature induced broadening is of Lorentzian shape and can be described by the theory of Hsu and Skinner, based on dephasing by coupling to acoustic phonons of the substrate. Discrepancies remain for experimentally observed small line shifts. We discuss how surfaces can be used and optimized as sample systems for a highly resolved optical spectroscopy of molecules.