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

We report on the observation 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. When the laser beam excites the 5s5p ${}^1$ $P_1$ resonance level, we show 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 5s10s 7thinsp${;}^1$ $S_0$ 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.