ELECTROSTATIC EXPANSION OF A TENUOUS PLASMA PRODUCED BY EXCIMER LASER-IRRADIATION OF MGO

We describe the expansion dynamics of the tenuous plasmas formed by ir radiating single crystal MgO with a KrF excimer laser (248 nm) in vacu um. At low fluences (0.03-0.5 J/cm(-2)), the positive ion kinetic ener gy distribution peaks at about 11 eV and is relatively narrow (+/-2 eV ). As the fluence is increased, the energy distribution broadens drama tically. Also, we find that a portion of the positive ions have been t urned around - i.e., significant ion intensities can be detected behin d the sample. We argue that these effects result from the electrostati c expansion of a net positive, tenuous plasma, where the ions outnumbe r the electrons by approximately 0.02% of the total charged particle d ensity. A one dimensional model which simulates this expansion mechani sm is presented. (C) 1998 Elsevier Science B.V.