Experimental and ab initio theoretical near-edge x-ray-absorption fine-structure (NEXAFS) spectra of Cz/Agd 10) are presented. The results show that chemisorption can strongly affect the NEXAFS of a molecular species. From examination of the molecular orbitals associated with the resonance structures of gas phase and adsorbed C2 it is further demonstrated that near-edge spectroscopy can be used to gain information about the mechanism of molecule-substrate interaction.PACS numbers: 78.70.DmIn assigning the resonance features in the near-edge xray-absorption fine-structure (NEXAFS) spectra of an adsorbed molecule, it is often assumed that the near-edge structure of the molecule is qualitatively unaltered by the adsorption process. The assignment of adsorbate spectral features is then made by analogy with corresponding features present in the gas-phase spectrum of the molecule, the so-called "decoupled molecule" approach. So many apparently successful assignments have been made using this approach that it is tempting to conclude that the process of chemisorption has little effect on the nearedge spectrum and that chemisorption-induced features are insignificant [1,2].In this Letter, we discuss the near-edge spectra of a diatomic species, C2, adsorbed on Ag(llO), where the ''decoupled molecule" approach clearly breaks down. We shall show that the experimentally determined spectra are inconsistent with that of gas-phase C2, and that there are chemisorption-induced changes in the spectra which reveal a great deal about the nature of the chemisorptive bond. Moreover, we will demonstrate that some of the features in these spectra are due to metal-adsorbate antibonding interactions, whose presence in near-edge spectra have been generally ignored. The effect of reduced symmetry caused by interaction with the surface, as postulated by Somers et at. [3], will be clearly demonstrated for one of these metal-adsorbate resonances.The theoretical results presented were obtained from first principles using a combination of ab initio cluster calculations and Stieltjes moment theory analysis. Generalized valence-bond (GVB) and configuration-interaction calculations were first used to determine the ground-state geometry for C2 adsorbed in the long bridge site on Ag(llO) using a 24-atom cluster model [4]. A six-Ag-atom cluster model with the same molecular geometry was then used for calculating the improved virtual orbitals (IVOs) of the Hartree-Fock ground-state wave function [5]. Cross sections for the bound-state transitions were obtained from the resulting transition energies and oscillator strengths after broadening with Gaussian functions (FWHM=2.00 eV). For the contin-uum cross section, Stieltjes moment theory techniques were used to find quadrature points for the cumulative oscillator strength distribution from which the photoabsorption cross section was then determined [6]. Similar ab initio calculations have been beneficial in interpreting NEXAFS spectra of gas-phase molecules [6]; this is the first application of the method...