Selective photodissociation via specific core-to-bound excitations: a comparison of condensed layers of water, ammonia and methane

Abstract

Prior work has shown that excitation of a core electron into an antibonding molecular orbital of adsorbed and condensed H2O/D2O (O 1s → 4a1) leads to an unusually high efficiency of H+/D+ formation. This effect and the unexpected angular behaviour observed for it have been tentatively explained by comparable time scales of electronic decay and nuclear motion, and by symmetry breaking through the influence of hydrogen bonding and/or the coupled excitation-deexcitation-dissociation sequence. We have now investigated the corresponding behaviour in multilayers of the isoelectronic molecules NH3 (at N 1s) and CH4 (at C 1s), including mixed NH3/CH4 layers. While pure NH3 shows essentially the same behaviour as H2O, CH4 does not; and in mixed CH4/NH3 layers the H+ "threshold peak" is quenched even for NH3 (at N 1s). This suggests that hydrogen-bonding is of importance for this interesting behaviour. The possible physical effects involved are discussed.