Surface induced vibrational modes in the fluorescence spectra of PTCDA adsorbed on the KCl(100) and NaCl(100) surfaces

Abstract

We report a combined experiment-theory study on low energy vibrational modes in fluorescence spectra of perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) molecules. Using very low coverages, isolated molecules were adsorbed on terrace sites or at sites located at residual steps on (100) oriented alkali halide films (KCl and NaCl). The low energy modes couple to the optical transition only because the PTCDA molecule is geometrically distorted (C) upon adsorption on the surface; they would be absent for the parent planar (D) PTCDA molecule. The modes differ in number and energy for molecules adsorbed on regular terrace sites and molecules adsorbed at step edge sites. Modes appearing for step edge sites have the character of frustrated rotations. Their coupling to the optical transition is a consequence of the further reduced symmetry of the step edge sites. We find a larger number of vibrational modes on NaCl than on KCl. We explain this by the stronger electrostatic bonding of the PTCDA on NaCl compared to KCl. It causes the optical transition to induce stronger changes in the molecular coordinates, thus leading to larger Franck-Condon factors and thus stronger coupling. Our results demonstrate how optical spectroscopy can be used to gain information on adsorption sites of molecules at low surface concentrations.

Catalytic Enhancement of Inductively Heated Fe3O4 Nanoparticles by Removal of Surface Ligands

“…The simulated vibrational modes are indexed to the bulk OA and compared to the bonded OA on the NP surface. Although the molecular vibrational modes shift upon adsorption and interparticle interaction, [35] a qualitative identification could be obtained. The analysis of the simulated spectra focused on the terminal hydroxy group to differentiate between OA and OL binding.…”

Long Vertical Distance Bonding of the Hexagonal Boron Nitride Monolayer on the Cu(111) Surface

“…Molecules adsorbed on hexagonal boron nitride constants of the substrates. If the molecules are located at step sites on the surface, the interaction of the molecule with the step edge lifts up one anhydride group of the molecule, making the distortion asymmetric [114].…”

“…The high energy mode has been observed in thin films [220,221], on Ag (111) (at 1,385 cm −1 ) [219], and on the Cu( 111) surface (at 1,386.0 cm −1 ). The lower energy mode, on (111), on Cu (111), on Ag (111) [219], in thin films on Au(111) [220], in thin films on H-passivated Si(111) [221], in single crystals [223], and on KCl/Ag(100) [114]. The data for hBN/Cu (111), Cu (111), and KCl/Ag (100) were measured in FL experiments.…”

The influence of an interfacial hBN layer on the fluorescence of an organic molecule

“…The error margins given are the standard deviation. The values for PTCDA/Cu(111) refer to coverages of 1.20 ML for T S = 20 K and 1.00 ML for T S = 300 K. For comparison, the Raman shifts of 1 ML PTCDA/Ag(111) measured by Schneider et al [53], the vibrational energies of PTCDA/KCl/Ag(100) measured by Paulheim et al in FL experiments [54], and the Raman shifts of the PTCDA single crystal measured by Tenne et al [55] [33], nor with the bonding distance, or the amount of molecular distortion [32,35,56,57]. However, in some way it reflects the change in the distortion motif of the PTCDA molecule.…”

“…Those modes are listed as "n.o.". for PTCDA on different substrates, going from KCl films on Ag(100) (1,288 cm −1 , derived from FL spectra)[54], to Ag(111) (1,298 cm −1 )[53], hBN/Cu(111) (1,301 cm −1 , derived from FL spectra), and Cu(111) (1,313 cm −1 , derived from FL spectra), contrary to the other modes. This trend neither conforms with the strength of the (chemisorptive) bond to the substrate surface…”