Article pubs.acs.org/JPCC
Inhomogeneous and Homogeneous Line Broadening of Optical Spectra of PTCDA Molecules Adsorbed at Step Edges of Alkali Halide Surfaces A. Paulheim,† C. Marquardt,† H. Aldahhak,‡ E. Rauls,‡ W. G. Schmidt,‡ and M. Sokolowski*,† †
Institut für Physikalische und Theoretische Chemie, Universität Bonn, Wegelerstraße 12, 53115 Bonn, Germany Lehrstuhl für Theoretische Physik, Universität Paderborn, Warburger Straße 100, 33098 Paderborn, Germany
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ABSTRACT: 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-3,4,9,10tetracarboxylic 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.
1. INTRODUCTION There exists a high interest in obtaining highly resolved fluorescence (FL) and absorption spectra of large organic molecules, for instance, because a detailed analysis of the vibronic modes yields information about the configuration and structures of the molecules.1−5 A traditional way for obtaining such spectra ̀ ̆ matrices.6,7 Another more recent approach is the use of Shpolskii is to attach the molecules to or into He nanodroplets.4,5,8 Typical line widths of about 10 cm−1 can be achieved for molecules in ̀ ̆ matrices at low temperatures.9−11 For He nanodroplets Shpolskii much narrower lines have been reported, e.g., of the order of 0.7 cm−1.5 ̀ ̆ The line width of optical transitions of molecules in Shpolskii matrices is typically attributed to inhomogeneous line broadening in combination with homogeneous line broadening related to thermal induced dephasing.12,13 However, it is difficult to obtain structural information about the local environment of the respective molecules in bulk matrices. This limits further interpretations of the detailed mechanisms responsible for the line width. In this work we demonstrate that highly resolved optical spectra with very narrow lines, comparable with those ̀ ̆ matrices,9−11 can also be obtained, if obtained for Shpolskii molecules are deposited on a well-defined, i.e., chemically clean and structurally ordered, dielectric surface. Besides giving access to optical spectra of high quality, such experiments also allow one to investigate the structural details of the molecular sites by other experimental methods in parallel, e.g., scanning tunneling microscopy (STM). This can support the interpretation of line broadening mechanisms. © XXXX American Chemical Society
FL spectroscopy of molecules adsorbed on surfaces requires the use of wide-band-gap materials (insulators), since on metal or semiconductor surfaces, the fluorescence is rapidly quenched and no signal can be detected.14 Notably, the adsorption and structure formation of organic molecules on insulator surfaces has gained increasing interest over the past years.15−19 In this work we will consider the prototype π-conjugated organic molecule PTCDA. Important in the context of the present work is the following. On ionic surfaces, e.g., the KCl(100),20 NaCl(100),21 and KBr(100) surfaces,22 it was found that Coulomb interactions between the negative partial charges on the anhydride groups of PTCDA and surface cations lead to adsorption at specific and well-defined sites, similar to the situation that is found for PTCDA on suitable metal surfaces.23 This situation is advantageous, since the population of defined adsorption sites facilitates the discussion of the corresponding optical spectra, in particular the variations in the adsorption sites. However, so far no detailed experiments on the line shape and possible line broadening mechanisms of the optical transitions of fluorescent molecules adsorbed on such surfaces have been reported. This will be the topic of the present work. Importantly, we will consider the limit of very low molecular surface concentrations, where intermolecular interactions are negligible. This situation has to be discerned from that of a completed monolayer of molecules on a surface where intermolecular Received: February 25, 2016 Revised: April 25, 2016
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DOI: 10.1021/acs.jpcc.6b01956 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. Top: schematic hard sphere models of the different adsorption sites of PTCDA on KCl(100). The sites are labeled by X1 to X12. We note that structural relaxations of the molecules or the substrate atoms are not included. The corresponding adsorption energies for the molecule at the specific sites (not including the energy for creation of the sites itself) are given. These energies were calculated earlier by DFT.40,42,43 Bottom: graphical representation of adsorption energies of the molecule at the specific sites for the ground state (S0) taken from the DFT40,42,43 and the adsorption energies of the molecule in the excited S1 state calculated by eq 2. The differences of the two adsorption energies correspond to the optical transition energies, i.e., the energetic position of the 0−0 transition in the optical spectra. Equation 2 was calibrated at the experimentally determined values for the transition energies of the X1 (t site) and the X11 (deep vacancy site) sites. These are given by black numbers. All other transition energies were calculated from eq 2 and are given relative to the transition energy of the site X11 (blue numbers). The experimentally observed transition energies of the earlier reported metastable states, A, B, and C,39 are also included for comparison (black numbers). These values are also given relative to the transition energy of the site X11.
after deposition of PTCDA onto the cold samples (
