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The aim of the present study was to investigate the reaction between ammonia and pre-adsorbed oxygen on Ni at 600 K using metastable impact electron spectroscopy (MIES) and UPS techniques. For a given oxygen-pretreated nickel surface, the changes in the MIES spectra along with ammonia adsorption can be decomposed into three different steps: a first domain without significant changes, a second domain corresponding to drastic changes in the intensity of a signal at 15.8 eV; and a third domain where no more change is detected. The first domain may correspond to the ammonia chemisorption on the surface, whereas the changes in the second domain would be attributed to surface dehydroxylation. The existence of NHx groups on the surface has been detected by UPS; they result from ammonia chemisorption. The changes in the MIES spectra have also been studied versus the initial oxygen uptake on Ni. It emerged that the first domain is larger with increasing initial oxygen exposure: this may be explained by the poorer surface reactivity of NiO, contrary to the oxygen-adsorbed phase. Copyright  2002 John Wiley & Sons, Ltd.

The present study was aimed at investigating the reduction by hydrogen of an oxygen-predosed nickel surface at high temperature (530 K) using metastable impact electron spectroscopy (MIES) and UPS (with He(I)) experimental techniques. Both techniques have allowed us to follow the evolution of the electronic structure of the valence band during H 2 deposition. The work function variation seems to be related directly to the oxygen coverage of the surface and exhibits two stages: an induction period during which no reduction reaction occurs; a rapid reaction period corresponding to the reduction of oxygen. The reaction mechanism entails the formation of hydroxyl compounds but, owing to their thermal desorption at lower temperature in the form of water, no evident OH features have been observed in the MIES and UPS spectra. The 3d band intensity, measured at 0.5 eV under the Fermi level using UPS, increased linearly during H 2 deposition. This evolution could correspond to that of bare Ni atoms in the three or four surface layers. On the other hand, evolution of the MIES signal taken at 5.7 eV under the Fermi level provided us with more accurate information about the evolution of the outermost layer of the predosed nickel surface during H 2 exposure.

The evolution of a polycrystalline magnesium surface during oxidation at room temperature has been studied by Metastable Impact Electron Spectroscopy (MIES). This technique allowed us to follow the metal-to-insulator transformation of the top layer of the surface. An electronic signal corresponding to a metallic behavior of the surface evidences the presence of under-stoichiometric MgO species on the surface. The total covering by oxygen of the Mg surface uppermost layer, obtained at around 10 L of oxygen deposition, does not correspond to a fully insulating surface. An insulating surface is obtained after 30 L of oxygen deposition. Depositions of CO2 on a clean and a preoxidized polycrystalline Mg surface have been analyzed to give information about the composition of the surface and its evolution. CO2 adsorption in the form of CO32− compounds on preoxidized Mg is more efficient than on clean Mg. Oxygen species, corresponding to chemisorbed oxygen less bounded than oxygen in the MgO lattice, allows the formation of CO32−. Therefore, it is concluded that during oxygen deposition at room temperature, MgO islands and chemisorbed oxygen species coexist on the surface. Moreover, the larger the oxygen predeposition is, the less CO32− compounds are formed, meaning a decrease of available chemisorbed oxygen sites. From oxidation measurements at high temperature (420 K), we show that MgO islands and uncovered Mg domain coexist. Further, no under-stoichiometric compound features have been observed. The high temperature allows the direct formation of oxide MgO species in islands.

Penning ionization of the CO2 molecule by Ne* (3 3P2,0) metastable atoms is studied by electron spectroscopy. The recorded electron energy spectrum is dominated by an important vibrational progression that is decomposed over the nν1 and nν1+2ν3 vibrational series up to n=5. In addition, we observe a low-energy broad hump extending from 0.5 to 1.5 eV. The presence of this new feature is attributed to a non-van der Waals interaction between the colliding particles. Making the hypothesis that an ion–pair state channel opens during the collision, we are in a position to interpret the entire set of experimental results. We believe that the opening of this channel is responsible for the production of the CO2 molecule in an autoionized Tanaka–Ogawa Rydberg state.

Nous avons étudié l'ionisation Penning de H2O par des métastables 21S et 23S de l'hélium, ainsi que 3P0 et 3P2 du néon. Nous avons analysé l'énergie cinétique des ions créés au cours de la collision (parents et fragments). Afin d'interpréter certaines particularités expérimentales, l'hypothèse de transferts d'énergie de vibration en énergie cinétique est proposées. Par ailleurs, les caractéristiques des distributions en énergie des ions fragments sont expliquées par la prédissociation de l'état 2B2 de H2O+.

We have measured the kinetic energy of the ions H 2 + , D 2 + , H 3 + produced by the collision of helium on metastable neon, on molecular hydrogen or on deuterium: from measurements on H 2 + and D 2 + it has been possible to confirm the existence of a two-step Penning collision, the second step being a transfer from vibrational to kinetic energy. The measurement of the kinetic energy of H 3 + and D 3 + leads us to conjecture that a certain amount of (H 2 ) 2 and (D 2 ) 2 dimers is present in the target Pour les ions H 2 + et D 2 + , confirmation d'une ionisation de Penning en deux etapes, avec un transfert d'energie vibrationnelle en energie cinetique dans la 2eme etape. Hypothese de la formation de dimeres (H 2 ) 2 et (Δ 2 ) 2 dans les conditions de l'experience pour expliquer la formation de H 3 + et D 3 +

Experimental results for the energy distributions of CO + ions produced by chemi-ionisation of carbon monoxide are fitted using a simple mathematical model. The impact of helium and neon metastables is considered. In both cases the experimental distributions can be reproduced only if a two-step collision process is assumed. The second step involves non-radiative de-excitation of CO + vibrational levels, the vibrational energy being transformed into kinetic energy. For the purpose of comparison, results are also given for chemi-ionisation of argon.

In this paper we try to elucidate the origin of the electron continuum background observed in Penning ionization of molecular hydrogen by helium metastables states 21S and 23S. Using mass-spectrometry studies we verify that this anomaly is not due to the presence of water in our apparatus. However, we found an H+3 ion which we believe is produced by the reaction: H ∗ 2 + H 2 → H + 3 + H + e leading to electronic emission.

Angular and energy dependencies of generalized oscillator strengths have been determined for the 11S → 21S transition in helium in the range 35 – 150 eV for scattering angles between 0° and 20°. The results are compared with the theoretical oscillator strengths obtained by Kim and Inokuti.