Comparison of single and double-pulse excitation during the earliest stage of laser induced plasma

This paper deals with comparison of single-pulse (SP) and double-pulse (DP) excited plasmas during their earliest phase of life, from 0 to 500 ns. The samples are Si and Al and the irradiance per pulse is approximately 20 GW cm(-2) under ambient conditions. It was found that at the beginning of a plasma lifespan, Si III and Al III ions were excited even under the SP excitation conditions and became much more abundant under DP excitation. The lifetime of doubly-ionized species was found to be very short, and after a delay of 200-300 ns they were not detected. The DP plasma differs from the SP plasma by the intensity of continuum radiation and by the width of emission lines, which are substantially narrower in the DP plasma. The intensity of light emitted from singly and, especially, doubly-ionized species is higher compared with neutral atoms; whereas the decay time of both neutral and ionized species is longer. An excitation mechanism for the DP plasma is proposed enabling qualitative and a quantitative explanation of the experimental data.