Inverse melting or disordering, in which the disordered phase forms upon cooling, is known for a few cases in bulk systems under high pressure. We show that inverse disordering also occurs in two dimensions: For a monolayer of 1,4,5,8-naphthalene-tetracarboxylic dianhydride on Ag(111), a completely reversible order-disorder transition appears upon cooling. The transition is driven by strongly anisotropic interactions within the layer versus with the metal substrate. Spectroscopic data reveal changes in the electronic structure of the system corresponding to a strengthening of the interface bonding at low temperatures. We demonstrate that the delicate, temperature-dependent balance between the vertical and lateral forces is the key to understanding this unconventional phase transition.

Using high-resolution photoemission spectroscopy we demonstrate that the electronic structure of several organic monolayer systems, in particular 1,4,5,8-naphthalene tetracarboxylic dianhydride and Copper-phtalocyanine on Ag(111), is characterized by a peculiar excitation feature right at the Fermi level. This feature displays a strong temperature dependence and is immediatly connected to the binding energy of the molecular states, determined by the coupling between the molecule and the substrate. At low temperatures, the line-width of this feature, appearing on top of the partly occupied lowest unoccupied molecular orbital of the free molecule, amounts to only ≈ 25 meV, representing an unusually small energy scale for electronic excitations in these systems. We discuss possible origins, related e.g. to many-body excitations in the organic-metal adsorbate system, in particular a generalized Kondo scenario based on the single impurity Anderson model. For more than twenty years there has been a thorough investigation on π-conjugated organic molecules, which have shown to be suitable for the application in organic electronic devices [1][2][3]. These molecules often form long-range ordered films on single-crystalline metal substrates, allowing a systematic and fundamental study by various surface sensitive techniques. In particular from electron spectroscopy methods, as photoemission spectroscopy (PES), inverse photoemission (IPES), and x-ray absorption (XAS), deep insight into many important features of the electronic properties of condensed films and their interfaces has been achieved [4][5][6][7][8]. The latter are of particular importance since they crucially determine the properties of possible devices. Additional microscopic and spectroscopic information has been obtained by use of scanning tunneling microscopy (STM), leading to complementary information about the relation between geometrical and electronic structure [9][10][11][12].Usually, even the spectra of highly ordered films show features with a line-width of several hundreds of meV, mostly determined by vibronic excitations within the adsorbed molecule [7,13,14]. Therefore, even experiments with modern high-resolution photoemission spectrometers for VUV photoemission (UPS) display only features which are about two orders of magnitude larger than the most narrow peaks in other solid state or surface systems [15]. Local spectroscopic measurements by STM, on the other hand, show narrow features in the tunneling conductivity measurements through a single molecule [10,[16][17][18][19], which have been attributed to a possible Kondo like process within the charge transport through * present address: SOLEIL L'Orme des Merisiers, Saint-Aubin -BP 48, 91192 GIF-sur-YVETTE CEDEX FRANCE SOLEIL, Paris, France the adsorbed molecule. However, neither a direct evidence of a local magnetic moment, necessary for the Kondo effect, nor an immediate experimental observation of the spectral function does exist yet.Here we report about a high-resolution photoemission study o...