Photon stimulated desorption of neutral hydrogen atoms from condensed water and ammonia by resonant O1s and N1s excitation: Search for the signature of ultrafast bond breaking

A highly sensitive detector for neutral particles has been designed and applied to photon stimulated desorption from condensed NH₃, and adsorbed and condensed H₂O. Results on repulsive N1s and O1s core-to-bound excitations are reported, which for the isolated molecules are expected to stimulate ultrafast bond breaking and predominant emission of neutral H atoms. For NH₃ and H₂O molecules on surfaces, strong H-atom signals are obtained for the entire O1s and N1s excitation energy range, but no selective enhancement due to ultrafast dissociation, although the previously observed enhancement of H⁺ emission is well reproduced. Possible matrix effects are discussed as an explanation for this contrasting behavior of isolated and adsorbed/condensed molecules.