Determination of initial expansion energy with shadowgraphy in laser-induced breakdown spectroscopy

Highlights: • Observation of shockwave for four different samples in wide time interval: 100 ns – 10 μs. • Expansion of the shockwave at different energy fluences of the ablation laser. • Use of Sedov – Taylor model to determine the energy efficiency of the ablation. • Computation of initial energy of the shockwave by Sedov-Taylor and Jones model. Laser-induced plasma (LIP) is dependent on many experimental conditions, as well as on the material and type of the ablated sample. Typically, the spectral data from a LIP are used as an analytical tool. However, the problem is that the data suffer from instability of the plasma, matrix effect, shot-to-shot fluctuation, etc. One of the possibilities how to improve the performance of LIBS is a deep understanding of behaviour of the plasma at various times during its evolution. Therefore, we used shadowgraphy setup to observe the shockwave created by the explosion during ablation of the sample. Our goal was to observe differences in shadowgraphy data for four different samples, each of them with unique matrix. Then, we aimed to use the shadowgraphy data to calculate the initial energy of the expansion. For each sample, we used various laser energies between 7 and 50 mJ. Sedov-Taylor model was applied as it fit the measured data. The model accurately fitted the experimental data up to 1.3 μs from the ablation of the sample. For further time delays, a slightly different approach was proposed, using model's equation with a changed power exponent. Later, Sedov-Taylor and Jones models were employed to calculate the initial energy of the shockwave. The calculation revealed a similarity in the amount of energy that was needed to ablate steel and glass sample. As for the ablation of bronze and soft tissues, approximately half of that energy was needed. These results were then compared with thermal properties of the samples, where similar results can be seen.