The generation of a plasma in cesium vapor via a resonance process is described. The maximum irradiance used was 1.5 W/cm<sup>2</sup>, which is several orders of magnitude lower than the irradiances conventionally used to form plasmas. Plasma emission consisted of radiative decay from excited-state Cs atoms. Optical emission from the plasma was observed from laser powers of 200 μW with a nonintensified charge-coupled device (CCD). The ionization efficiency of the plasma was approximately 0.001, with the majority of the atoms in the plasma ionized. The plasma was found to behave linearly with respect to laser power; however nonlinear behavior was observed as the number density was altered. A collisional mechanism is proposed for the formation of the plasma.
Dimitri Pappas, Benjamin W. Smith, Nicolo Omenettó, and James D. Winefordner, "Formation of a Cesium Plasma by Continuous-Wave Resonance Excitation," Appl. Spectrosc. 54, 1245-1249 (2000)