A Study of Ablation, Spatial, and Temporal Characteristics of Laser-Induced Plasmas Generated by Multiple Collinear Pulses

Abstract

Multi-pulse laser-induced breakdown spectroscopy (LIBS) in the collinear pulse configuration with time-integrating detection was performed on metallic samples in ambient air in an effort to clarify the contributing processes responsible for the signal enhancement observed in comparison with single-pulse excitation. Complementary experiments were also carried out on another LIBS setup using detection by an imaging spectrograph with high time resolution. The effects of laser bursts consisting of up to seven ns-range pulses from Nd-doped solid-state lasers operating at their fundamental wavelength and separated by 8.5-50 micros time gaps was studied. The ablation and emission characteristics of the generated plasmas were investigated using light profilometry, microscopy, plasma imaging, emission distribution mapping, time-resolved line emission monitoring, and plasma temperature calculations. The experimental data suggest that the two contributing processes mainly responsible for the signal enhancement effect are the plume reheating caused by the sequential laser pulses and, more dominantly, the increased material ablation attributed to the lower breakdown threshold for the preheated (molten) sample surface and/or the reduced background gas pressure behind the shockwave of preceding pulses.

Effect of atmosphere on collinear double-pulse laser-induced breakdown spectroscopy

“…The complexity of DP-LIBS and variations of LIBS results with gas composition and pressure may partly due to the experimental configurations and parameters [44,43].…”

“…The second laser pulse in DP-LIBS interacts with the heated environment of the first LIP and sample created by the first laser pulse. The preheated target surface produced by the first laser pulse and LIP reduce the breakdown threshold as well as to reduce the background gas density behind the shock wave of first LIP [43], which reduces the laser shielding for the second laser pulse [26]. The rarefied gas and higher temperature produced by the first LIP creates an environment that leads to a faster and greater expansion of the plume for DP-LIBS compared to SP-LIBS [40,36].…”

“…However, with longer delays, the sample has time to equilibrate with the environment produced by the first laser pulse and there is less increase in mass ablation. Galbacs et al [43] have suggested that this mechanism may last through inter-pulse delays as long as 10 μs. Another plasma condition that varies with inter-pulse delay is the plasma temperature, which is higher with DP-LIBS and leads to longer lifetimes; however, the effect gets averaged out with integration over the whole plasma volume [40] and with longer inter-pulse delays [38].…”

Analitikai módszerek fejlesztése folyadék és szilárd minták LIBS vizsgálatára

“…A plazma hőmérséklete 1000-1500 K-nel magasabb az egyimpulzusos módszerhez képest, amit Stratis és munkatársainak kísérletei, illetve saját kutatócsoportunk tapasztalatai is alátámasztanak [57,58]. Ezen kívül hatással van a két impulzus alkalmazása a plazma méretére és térfogatára, a csóva tágulási sebességére, a plazma élettartamára, a generált részecskék mennyiségére, az ablált tömeg mennyiségére, a keletkezett kráter alakjára és méretére.…”

“…Az adatok időben és térben integrált felvétele nem csak az érzékenységet növelte, hanem javította az ismételhetőséget is. Az alkalmazott rendszer maximum hat lövést tett lehetővé egy sorozatban, melyek között a késleltetési idő a 20-40 µs közötti tartományba esett [58,66,67,69].…”