Surface Science 410 (1998) 21–38 A tight-binding Green’s function approach to adsorbate electronic ground and excited states and their lifetimes Tillmann Klamroth, Peter Saalfrank * Institut fu ¨r Physikalische und Theoretische Chemie, Freie Universita ¨t Berlin, Takustrasse 3, D-14195 Berlin, Germany Accepted for publication 26 March 1998 Abstract A tight-binding Green’s function approach is used to describe the bonding of a diatomic molecule to a metallic substrate. The molecule is idealized as a two-level system with an occupied donor orbital and an empty low-lying acceptor level, and the metal as a single half-filled band. The formation of a metastable metal-to-ligand charge transfer state or negative ion resonance after electronic excitation is demonstrated. The lifetime of the negative ion resonance is determined and effective atomic charges and bond orders are computed for both the ground and the excited states. Potential energy curves and the coordinate dependence of the lifetimes are computed by varying the adsorbate–surface distance, and hence the coupling strength between the molecule and the substrate. Based on a truncated Born expansion of the perturbed Green’s matrix, an approximate and physically intuitive expression for the negative ion resonance width is derived and numerically justified. Variation of tight-binding parameters other than the molecule–surface coupling allows for the simulation of a broader class of substrates and adsorbates. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Chemisorption; Green’s function methods; Low index single crystal surfaces; Metallic surfaces; Photochemistry; Semi- empirical models and model calculations; Single crystal surfaces 1. Introduction the (ultrafast) nuclear dynamics during, say, pho- todesorption of interest [3–14], but, as a prere- In recent years, photochemical [1] or scanning quisite to dynamical studies, reliable electronic tunneling microscope (STM ) [2] induced chemical ground and excited states and the (non-adiabatic) transformations of molecules adsorbed on solid couplings between them are also of interest. While surfaces (i.e. their desorption, dissociation or reac- good progress has been made in the electronic tion) have received considerable attention, for both structure theory of adsorbates in their ground state fundamental and applied reasons. A common char- [15–17], much less is known for the electronic acteristic of these processes is that they typically excited states [1]. Here, the main problem is that proceed via electronically excited states, desorption many coupled low-lying electronic states of various induced by electronic transitions (DIET ) [1,3] characters (substrate excitations, adsorbate exit- being one popular example. ations, metal-to-ligand charge transfer (MLCT ) From the theorist’s point of view, not only are states, etc.) exist for a typical adsorbate–substrate complex. This is different from gas-phase chemistry involving only small molecules, and is a conse- * Corresponding author. Fax: ( +49 ) 30 8384792; e-mail: saalfrank@chemie.fu-berlin.de quence of the extended nature of the solid surface. 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0039-6028(98)00288-X