While the bonding of molecular adsorbates to graphene has so far been characterized as physisorption, our study of adsorbed ammonia and water using near-edge X-ray absorption spectroscopy provides unambiguous evidence for a chemical contribution to the adsorption bond. We use the situation, unique to graphene, to characterize the unoccupied valence band states of the partners in the bond on the basis of the complementary adsorbate and substrate X-ray absorption K edges. New adsorbate-induced features on the substrate (carbon) K edge are interpreted as hybrid states in terms of a simple model of chemical interaction.
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