Atom-Selective Bond Breaking in a Chemisorbed Homonuclear Molecule Induced by Core Excitation: N₂/Ru(001)

We observe photochemical selectivity for N1s to π^* excitations of chemisorbed N₂. By narrow bandwidth synchrotron radiation we selectively excite one of the two atoms of the molecule. Photon stimulated desorption of neutral N atoms predominates for excitations of the N atom close to the surface, whereas excitation of the outer atom ejects predominantly N₂⁰ and small amounts of N⁺, demonstrating the predominant breaking of the inner or outer bond, respectively, of the N₂ adsorbate. Analysis on the basis of previously obtained decay electron spectra after atom-selective excitation can explain the mechanism of localized bond breaking.