EXCITATION AND IONIZATION OF LASER-PUMPED Ba VAPOUR

L'excitation et l'ionisation de vapeur dense de Baryum (1019 à 2021 m-3) par irradiation de lumière laser résonnante (λ = 553.5 nm) est décrite. On discute les processus qui transfèrent à l'ionisation l'énergie d'excitation des atomes. On trouve que l'efficacité de l'ionisation dépend de la densité et on en tire la conclusion que des collisions sont le mécanisme prépondérant. Il a été établi que des électrons séminaux sont chauffés dans des collisions super-élastiques avec les atomes excités par laser et que subséquemment l'excitation et l'ionisation - ainsi que la photoionisation des niveaux élevés - mène à la création d'un nombre croissant d'électrons. Le transfert d'énergie aux électrons par collisions super-élastiques prend place à partir non seulement du niveau de résonance pompé par laser, mais aussi des niveaux métastables de Ba qui se trouvent en dessous. Ces niveaux étaient peuplés immédiatement via le niveau de résonance.We describe the excitation and ionization of dense Ba vapour (1019 to 2021 m-3) by resonant (λ = 553.5 nm) laser radiation and discuss the processes responsible for the transfer of energy from the laser-excited atoms into ionization. Ionization was found to be density-dependent and this pointed to collision-dominated ionization mechanisms. It has been established that seed electrons were heated in superelastic collisions with laser-excited atoms, and that subsequent electron-impact excitation and ionization - as well as photoionization of high-lying levels - lead to the creation of more electrons. The observed transfer of the excitation energy to the electrons by superelastic collisions requires contributions not only from the laser-pumped resonance level, but also from the lower-lying metastable Ba levels. These were, in fact, found to be very efficiently populated via the resonance level

Giant impact: accretion and evolution of the Moon

Our planetary system has not always been as serene as it appears to us today. Exploration of the Moon has shown that disastrous collisions and violent epochs have occurred in the early part of its history. Indeed, a collision of the Earth with another planet – the Giant Impact – is the most widely accepted theory for the origin of the Moon. Several hundred million years later, Moon and Earth received a Late Heavy Bombardment that created the large basins on the Moon and must have devastated the atmosphere and hydrosphere of the Earth