ORTHOPARA CONVERSION OF HYDROGEN MOLECULES PHYSISORBED ON SURFACES

Recent EELS experiments on hydrogen molecules adsorbed, at low temperature, on noble metals have indicated that the o-p conversion process on non-magnetic metal catalysts might be very efficient. The Wigner magnetic dipolar mechanism, used since 1933 to interpret experimental data, is thus clearly insufficient to account for these new features. A general and comprehensive survey of the experimental and theoretical work is proposed, with special emphasis on recent developments. The o-p hydrogen conversion is specifically described as an hyperfine tool for surface studies and whenever possible the displayed catalytic rates are related to the surface specifics. Although this review intends by no means to be exhaustive, the selected topics cover a large background : clean surfaces as well as powders, metals and insulators, magnetic and non-magnetic catalysts. In particular, magnetic powders consisting of oxides of the transition metals and non-magnetic single-crystals of noble metals are more extensively discussed. From a theoretical point of view, first-principle calculations are summarized whenever they bring some new insight in the conversion mechanism. A large variety of channels is displayed and classified in terms of the number of necessary steps involved in the process. Each catalytic channel is examined according to its ability to : (i) satisfy the o-p selection rules (ii) transfer the angular momentum (iii) provide the energy required in the o-p transition. Numerous experiments are suggested and various unsolved questions are listed.