Effect of grounding and isolation of the target on the emissive properties of laser-produced plasma

Abstract

During laser ablation, ions are accelerated by electrons, some of which definitely escape from the plasma by passing through a potential barrier that is created on the outer regions of the plasma produced by the laser. The escaping electrons generate a net positive charge at the target surface. Depending on whether the target is grounded or isolated from the vacuum chamber, either this positive charge is neutralized by the current flowing from the ground or generates a voltage on the isolated target corresponding to its capacitance. Under certain experimental conditions, it can be expected that this charge can retroactively affect not only the expansion velocity of the emitted ions and their number, but also the total number of escaped electrons. In this work, we deal with ion emission and measuring the voltage arisen on an isolated target or the current of electrons flowing between the ground and a grounded target. For the first time, this made it possible to estimate the ratio of the number of electrons released from plasma to the number of electrons stopped by a potential barrier. Irradiating various targets with a 60–110 mJ energy delivered by a 23-ns KrF excimer laser, this portion is 10−3 for grounded targets and 10−5 for targets isolated from the vacuum chamber. The observation of target currents also makes it possible to identify the level of influence of the contamination of the target surface with chemisorbed hydrocarbons on plasma production and ion separation caused by their different atomic weights.