Reduced intermolecular interaction in organic ultrathin films

Submonolayer sensitivity was achieved using in situ ellipsometry to monitor the evolution of the dielectric function of tris(8-hydroxyquinoline)-aluminum(III) (Alq(3)) layers from submonolayer coverage to thick bulklike layers. The Alq(3) layers were deposited under ultrahigh vacuum conditions onto hydrogen passivated silicon. The characteristic vacuum-ultraviolet (VUV) absorption lines of Alq(3) were detected using synchrotron radiation as light source. In such ultrathin films the absorption lines corresponding to molecular transitions of the Alq(3) are found to be spectrally blueshifted with respect to bulklike layers. We attribute the shift to the effect of reduced intermolecular interaction in the submonolayer regime. (c) 2006 American Institute of Physics.