We have investigated the water adsorption Cu(100) using metastable-atom, deexcitation spectroscopy (MDS). A new feature was found that indicates the presence of the partially filled electronic states at around the Fermi level. The feature has been assigned to the antibonding states that result from the combination of the water lone pair orbitals (1b(1), 3a(1)) and electronic states of the metal substrate. (C) 2003 Elsevier B.V. All rights reserved.