UV-photodissocn. of NO₂ adsorbed on top of a satd. layer of NO chemisorbed on Pd(111) yields fast desorption of NO fragments at 6.4 eV photon energy. The wavelength dependence and the quantum state distributions demonstrate the nonthermal origin of the ejected NO fragments. A novel approach utilizing polarized light provides clear evidence that the primary process consists of an electronic excitation of the metallic substrate rather than direct absorption by the adsorbate.