Secondary plasma formation after single pulse laser ablation underwater and its advantages for laser induced breakdown spectroscopy (LIBS)

Abstract

In this work we present studies of spatial and temporal plasma evolution after single pulse ablation of an aluminium target in water. The laser ablation was performed using 20 ns long pulses emitted at 1064 nm. The plasma characterization was performed by fast photography, the Schlieren technique, shadowgraphy and optical emission spectroscopy. The experimental results indicate the existence of two distinct plasma stages: the first stage has a duration of approximately 500 ns from the laser pulse, and is followed by a new plasma growth starting from the crater center. The secondary plasma slowly evolves inside the growing vapor bubble, and its optical emission lasts over several tens of microseconds. Later, the hot glowing particles, trapped inside the vapor cavity, were detected during the whole cycle of the bubble, where the first collapse occurs after 475 μs from the laser pulse. Differences in the plasma properties during the two evolution phases are discussed, with an accent on the optical emission since its detection is of primary importance for LIBS. Here we demonstrate that the LIBS signal quality in single pulse excitation underwater can be greatly enhanced by detecting only the secondary plasma emission, and also by applying long acquisition gates (in the order of 10-100 μs). The presented results are of great importance for LIBS measurements inside a liquid environment, since they prove that a good analytical signal can be obtained by using nanosecond pulses from a single commercial laser source and by employing cost effective, not gated detectors.

Laser-Induced Breakdown Spectroscopy (LIBS): Fast, Effective, and Agile Leading Edge Analytical Technology

“…Formation of this secondary plasma has been explained by complex interaction of the hot plasma front with the surroundings, which leads to the propagation of a high temperature and high pressure region back to the target. 16 This second plasma is characterized by a lower electron number density and a much lower continuum emission, and is consequently useful for analytical purposes. The sequence of events has been documented in a recent report 17 and is presented in Figure 3.…”

Early appearance of crystalline nanoparticles in pulsed laser ablation in liquids dynamics

“…The characterization of the process poses considerable obstacles to access the dynamics on a nanosecond to microsecond time scale as well as quantifying ejected clusters and nanoparticles in the hierarchical structure formation process. [15][16][17][18] While metal excitation and mass expulsion develop on a picosecond to nanosecond time scale, the formation of long-lived plasmas (for nanosecond lasers) 15,[19][20][21] and the formation of vapour/cavitation bubbles span a submillisecond time scale and macroscopic sizes. 22 Optical methods are either limited to the macroscale dynamics or to detection of selected species, such as in laser-induced breakdown.…”

CaOH Molecular Emissions in Underwater Laser-Induced Breakdown Spectroscopy: Spatial–Temporal Characteristics and Analytical Performances

“…What we can summarize is that, in most studies of underwater LIBS, the used analytes in the spectrum are either atomic lines or ionic lines, while very few works care about the molecular emissions from the plasma in water. , Indeed, molecular features have gained increasing interest in the diagnostics of laser-induced plasma and the related LIBS applications in gas environment during recent years. For example, laser ablation molecular isotopic spectrometry (LAMIS) was introduced by Russo and co-workers , to perform isotopic analysis that takes advantage of the significant isotopic shift of molecules when compared to that of atomic species.…”