Electrostatic expansion of a tenuous plasma produced by excimer laser-irradiation of MgO

We describe the expansion dynamics of the tenuous plasmas formed by irradiating single crystal MgO with a KrF excimer laser 248 nm.in vacuum. At low fluences 0.03–0.5 Jrcm2., the positive ion kinetic energy distribution peaks at about 11 eV and is relatively narrow "2 eV.. As the fluence is increased, the energy distribution broadens dramatically. Also, we find that a portion of the positive ions have been turned around – i.e., significant ion intensities can be detected behind the sample. We argue that these effects result from the electrostatic expansion of a net positive, tenuous plasma, where the ions outnumber the electrons by approximately 0.02% of the total charged particle density. A one dimensional model which simulates this expansion mechanism is presented. q1998 Elsevier Science B.V.