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Ionization of laser-excited strontium vapor: The dynamics of superelastic collisions with metastable populations.

The observations and absolute measurements of the population dynamics for several Sr I and Sr II levels when the vapor is illuminated by a short laser pulse are reported. When the laser beam excites the 5s5p 1P1 resonance level, it is shown that a pool of metastable population is created, allowing an efficient heating of the medium by superelastic collisions with seed electrons, and a strong final ionization is achieved. When the laser source excites the 5s 10s 1S0 Rydberg level, superelastic collisions do not occur and a drastic change in the population behavior is observed. In both cases the dynamics of the energy transfer towards the plasma is detailed. 16 references.