Spectroscopy of isolated PTCDA molecules on the KCl(100) surface: Vibrational spectra and azimuthal orientation

Abstract

Small amounts of the model molecule perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) were vacuum deposited on epitaxial KCl films on Ag(100). The use of a low substrate temperature (20 K) during deposition hampered molecular diffusion resulting in isolated monomers on the surface. Fluorescence and fluorescence excitation spectroscopy performed on these monomers yielded highly resolved spectra with narrow lines corresponding to individual vibronic modes. This high resolution in our spectra is caused by a very small inhomogeneous broadening due to well-defined adsorption sites of the molecule on the substrate. Indeed, by polarization dependent fluorescence spectroscopy we show that the flat-lying molecules exhibit a preferred azimuthal orientation on the surface, the long molecular axis being oriented along the [011] or the equivalent [011] direction of the substrate. Furthermore, the high resolution in the spectra allowed a detailed analysis of the vibronic modes. The vibrational modes of the adsorbed molecule are very similar to those of the free PTCDA molecule, but due to the presence of the substrate additional low energy modes which are relevant for the full understanding of the spectra couple to the transition.

Finite size line broadening and superradiance of optical transitions in two dimensional long-range ordered molecular aggregates

“…The spectra of both phases show well resolved molecular vibronic modes, which are mainly related to in-plane modes of the flat lying molecule. 13 For the spectra of the d phase, consisting of isolated molecules, most of the vibronic modes are in excellent agreement with those of isolated, non-interacting PTCDA molecules (monomers) studied in He nano-droplets. 18 This reveals that the distortion of the potential energy surface due to the adsorption on the surface is small.…”

Probing weak dipole-dipole interaction using phase-modulated nonlinear spectroscopy

“…For such molecules, the electronic transition dipoles are typically in the order of several Debye. Dimers with specific interaction strength could be formed for example by linking the organic molecules by spacers [21,22], by selfassembly in helium-nanodroplets [11,23] or by depositing the molecules on a surface with a small coverage [24,25] (using single molecule spectroscopy techniques one then can select close-by molecular pairs). The distances between the molecules can range from a fewÅngström to several nanometer.…”

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

“…For a detailed interpretation of these modes we refer to Scholz et al 13 We note that the vibrational energies for the KCl t sites in Table 1 are based on more recent and better resolved spectra and thus partially differ (r19 cm À1 ) from those that were reported earlier in ref. 19. The vibrational energies of the NaCl t sites are tabulated here for the first time on the basis of recent spectra.…”

“…The optical properties of isolated PTCDA molecules adsorbed on KCl(100) films 19,28 and NaCl(100) films 2,29 have been reported already. Important in the context of the present work is the aspect that one can distinguish the population of different adsorption sites from in the optical spectra.…”

“…The appearance of SIM on KCl was briefly reported by us in ref. 19. In the present work, we give a more detailed analysis on the basis of spectra measured with higher resolution.…”