The plasma phenomena in three-photon ionization of Ca

The investigation of thresholds for the occurrence of the collective effects (plasma effects) in the two-photon resonant, 4s 2 1 S 0 to 4s6s 1 S 0 , three-photon ionization of calcium atom is presented. Our experiments were carried out using an excimer/dye laser system scanned through the resonance two-photon transition. Ions and electrons were produced in an interaction region inside the vacuum chamber where the laser beam and atomic Ca beam intersect. The plasma effects (collective effects), i.e. the effects caused by the action of ions/electrons created in the interaction region on charges leaving that zone are highlighted and analysed. The yields and time-of-flight (TOF) distributions for ions produced in such three-photon ionization of atomic calcium have been investigated as a function of laser intensity and laser detuning. We present the results of simple theoretical models for the plasma effects such as 40 Ca + and 44 Ca + time separation in TOF distribution as well as a model based on atomic physics for the time shift and asymmetry of 40 Ca + due to an increase of the laser intensity. These results are compared with experimental data which are obtained first time for such a heavy element in the periodic table.