Vibrational properties of the compressed and the relaxed 1,4,5,8-naphthalene-tetracarboxylic dianhydride monolayer on Ag(111)

Layers of 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA) grown on Ag(111) at about 80 K have been investigated using Fourier transform infrared spectroscopy, spot profile analysis low-energy electron diffraction, and temperature programmed desorption. Specifically, the compressed and the relaxed monolayer, as well as the transformation between the two ordered phases have been analyzed in detail. It is found that the two monolayer phases display distinctly different vibrational spectra and can thus be discriminated with high accuracy and sensitivity. For the NTCDA/Ag(111) monolayer strong in-plane vibrational modes point at a marked dynamic charge transfer between molecule and metal substrate and provide clear evidence for an efficient electronic coupling to the Ag(111) surface in conjunction with a partially filled electronic level at the Fermi energy. The bilayer, on the other hand, is largely electronically decoupled from the substrate and, according to the vanishing infrared-active in-plane vibrational modes, is oriented parallel to the surface. On the basis of spectroscopic data the metastable nature of the bilayer phase is identified as such, leading to an improved understanding of processes encountered in the course of layer preparation and resolving inconsistencies reported in the literature. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3699030]