The plasma phenomena in three-photon ionization of Ca

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