Laser fluence effects on ion emission from a laser-generated Cu plasma

Abstract

We present details of an experimental facility developed for the diagnostics of highly charged ions produced during pulsed laser ablation of solid targets. A range of laser fluences (2–10 J cm −2 ) from a Q-switched Nd : YAG laser (wavelength = 1064 nm, pulse duration ∼10 ns) was used to generate a copper plasma. The ion diagnostics were based on the time-of-flight (TOF) methods; an ion collector and a 45° parallel plate electrostatic ion energy analyser were used. A channel electron multiplier located 1.31 m away from the Cu target was used to record the energy-resolved TOF ion spectrum. The effect of laser fluence on the total ion charge, average ion energy and charge state distribution was investigated. The estimated threshold fluence for the onset of the plasma was 2.5 J cm −2 . About four times increase in both average ion energy and total ion charge was observed in the investigated laser fluence range. The maximum attainable charge state of the Cu ions increased from 1+ to 7+ with the increase in laser fluence. The correlation between relative abundance of the various ion charge states indicated that the formation of Cu n + occurred through ionization from Cu ( n −1)+ by the impact of fast electrons or by multiphoton interactions.