Two-Photon Ionization of Cesium and Sodium Vapors

The breakdown and the growth of plasma of cesium and sodium vapors by an intense laser beam are investigated. The minimum intensity which is observable by a double probe is found to be inversely proportional to the root of the neutral atom density for either vapor. This result confirms that the two-photon ionization is a dominant process in the stage of breakdown. Further increase of the intensity of laser beam leads to a steep rise of the plasma density proportional to the square of the intensity followed by a slow rise and then the second steep rise. The first steep rise is explained by the two-photon ionization process too. The electron temperatures are estimated to be about 10 4 °K under the condition of the present work, by the probe and optical methods.