Interaction strength and molecular orientation of a single layer of pentacene in organic-metal interface and organic-organic heterostructures

A single layer (SL) of pentacene molecules deposited on the Cu(119) surface and on an organic selfassembled monolayer (SAM) has been investigated by near-edge x-ray absorption fine structure (NEXAFS) at the C K edge, and by photoemission at the C 1s core level. The lowest unoccupied molecular orbitals (LUMOs) of the pentacene SL on the SAM are basically unaffected with respect to that of pentacene in the gas phase, indicating a weak interaction of pentacene with the SAM, while a strong redistribution of the LUMO-related final states is observed when the molecules are deposited on the Cu(119) substrate, sign of an electronic mixing between molecular and metal electronic states, in agreement with recent theoretical predictions. The strong dichroic response of the NEXAFS signal indicates upstanding pentacene molecules oriented 16° off normal for the organic-organic heterostructure. The rehybridization of pentacene orbitals with Cu states at the pentacene/Cu interface hinders an accurate determination of the molecule orientation, which is, however, compatible with molecules lying flat on the Cu(119) terraces.