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

Abstract

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 J/cm 2), 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.