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Differential cross section and kinetic energy distributions of Hand NH; ions produced by dissociative attachment around 5.6 eV in ammonia have been obtained. Both Hand NH; cross sections show a series of overlapping peaks starting at 4.94 eV with a mean spacing of 115 meV, attributed to the out-of-plane vibrational quanta nv, of a NH; resonance state. A predissociation mechanism similar to the predissociation of the lowest excited singlet state 'A; of neutral ammonia, parent of the resonance state, and a charge transfer between two A resonance states leading to the NH,+ Hand NH; + H products at infinite separation, in a distorted dissociation mechanism (CJ, account for the experimental observations.

We studied the electronic structure of the Pb (2+)-4H 2O system. Analysis of the complex orbital evidenced no mixing between the 6s lone pair orbital of the lead and the 6p orbital components. Moreover, we found that the HOMO is widely described by the mixture of the 6p components with the 7s valence orbital of the lead. This orbital shows an important elliptical electron charge density around the lead ion and opposite the direction of the short lead-water bonds. From these results, we demonstrated that the hemidirected conformation of the Pb (2+)-4H 2O system could be easily explained by the shape of the electron charge density distribution of the HOMO rather than by the stereochemically active character of the 6s (2) lone pair of lead electrons.

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Penning ionization of the H a molecule by the He'(2'S) metastable atom is studied. The energy shift is measured for the three observed X, A and B eiectronic bands of the residual HzO' ion. The absence of the vibrational structures is discussed in light of the known interaction potential surface of the He'-t120 complex. More altention is given to the X electronic band characterized by interference structures at high electron energy and an important angular dependence intensity.