Atom-Selective Bond Breaking in a Chemisorbed Homonuclear Molecule Induced by Core Excitation: $N_2/\mathrm{Ru}\left(001\right)$

We observe photochemical selectivity for $\mathrm{N}1s$ to ${\pi }^{*}$ excitations of chemisorbed ${\mathrm{N}}_2$. 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 ${\mathrm{N}}_2^0$ and small amounts of ${\mathrm{N}}^{+}$, demonstrating the predominant breaking of the inner or outer bond, respectively, of the ${\mathrm{N}}_2$ adsorbate. Analysis on the basis of previously obtained decay electron spectra after atom-selective excitation can explain the mechanism of localized bond breaking.