Characterization of ablationcraters associated toLaser Induced Breakdown
Spectroscopy (LIBS)
measurements
The purpose of this study is dual: 1) to characterize the ablation craters
associated to Laser Induced Breakdown Spectroscopy (LIBS) measurements ona fresco sample (Figure 1); and 2), to compare the craters on a 2-cent euro coinassociated to a portable instrument (EasyLIBS) and a laboratory instrument(Figure 2).

PACEA is a research laboratory for
the French National Centre for
Research (CNRS), the University ofBordeaux, and the French Ministryof Culture and Communication. Our
research mostly focuses on Paleolithic
cultures in Europe and Africa andtheir environments, biologicalanthropology, funerary practices, androck art. The authors of this study areLena Bassel and Alain Queffelec, withthe contribution of Maud Mulliez(Archéovision), Bruno Bousquet andJulian Guezenoc (CELIA laboratory),
Rémy Chapoulie (IRAMAT laboratory),and Clément Melkebeke (CELIA andIRAMAT laboratories).

Figure 1. Analyzed fresco sample.
CASE STUDY CHARACTERIZATION OF ABLATION CRATERS ASSOCIATED TO LASERINDUCED BREAKDOWN SPECTROSCOPY (LIBS) MEASUREMENTS1abFigure 2. Overview of the coin a) Zoom-in on the area analyzed using the portable instrumentb) Zoom-in on the area analyzed using the laboratory instrument.

■ Measurements
Part 1: Analyses of ablation craters on a fresco sample
A study on the dimension and depth of the ablation craters associated with
the LIBS measurements was conducted. The craters were analyzed using the3D optical profilometer S neox in Confocal mode. For this purpose, six ablationcraters were formed following 1, 3, 8, 10, 15 and 20 laser shots (Figure 3). Aninitial qualitative observation established that the crater is accompanied bya dark aureole, which demonstrates that the matter was highly heated uponimpact. This is termed the heat-affected zone or HAZ, which is well known inlaser ablation.

Figure 3. Location of the points analyzed at the level of the yellow decor

overlaying the red pigment.
2

CASE STUDY CHARACTERIZATION OF ABLATION CRATERS ASSOCIATED TO LASERINDUCED BREAKDOWN SPECTROSCOPY (LIBS) MEASUREMENTS

The average size of the craters is approximately 800 µm x 400 µm on all of thecraters formed. The crater formed by 20 shots was subject to a more in-depth
study (Figure 4). Indeed, within the crater, a hollower area is observed, withdimensions of approximately 160 µm x 100 µm. The same observation wasmade for each of the ablation craters.
abFigure 4. Crater formed by 20 shots. a) 3D view of the crater formed by 20 shots;b) Dimensions of the crater in µm and observation of a hollower zone (shown in blue).

A profile taken along each of the craters demonstrates that the depth of the craters increases
with the number of shots (Figure 5). The sample was hollowed out over around 100 µm in thedeepest area after 20 shots.

abc

Figure 5. Depth of the craters in µm. a) and b) top-view topographies, c) profile.

Part 2: Comparison of ablation craters associated to two different LIBS
instruments (EasyLIBS and a laboratory Instrument).
In the case of the EasyLIBS intrument, a broad even ring can be observed
demarcating the craters formed (Figure 6). Overall, the craters are very flat andshallow: it appears that a molten mass has been formed that has remained atthe crater site and has not been ejected.
CASE STUDY CHARACTERIZATION OF ABLATION CRATERS ASSOCIATED TO LASERINDUCED BREAKDOWN SPECTROSCOPY (LIBS) MEASUREMENTS

3ab

Figure 6. a) 3D view of the crater obtained after 150 shots with the portable instrument;b) 3D view of the crater obtained after 500 shots with the portable instrument.

For the measurements carried out with the laboratory instrument, matterejections are visible all around the crater (Figure 7). In addition, the craters
formed using the laboratory instrument all have “edges” where the matteraccumulates. The height of these edges in relation to the surface of the coinvaries between 4 µm and 20 µm depending on the number of shots fired.
abFigure 7: Crater obtained after 20 shots with the laboratory instrument, the matter ejections arevisible a) Image taken with the confocal microscope S neox; b) 3D view of the crater.

The comparison of the depth of the craters between the EasyLIBS and theLaboratory Instrument for different amounts of laser shots is shown in Table 1.
In addition, Figure 8 and Figure 9 show 2D pictures of the resulting craters forboth LIBS instruments after 50 and 500 shots, respectively.
Depth of craters
EasyLIBS

Laboratory instrument

10 shots

Non-measurable as merged with theroughness of the coin

~21 μm20 shots

Not measured~35 μm30 shots

Not measured~46 μm40 shots

Not measured~55 μm50 shots

The gradient of the coin is greater thanthe depth of the crater. ~0,5 μm~63 μm

150 shots

Not measured~160 μm but difficult to measure as thebasi is not very well lit

500 shots~8 μmNon-measurable as too deep

Table 1. Comparison of the depth of the craters based on the number of shots

for the portable instrument (EasyLIBS) and the laboratory instrument.

4

CASE STUDY CHARACTERIZATION OF ABLATION CRATERS ASSOCIATED TO LASERINDUCED BREAKDOWN SPECTROSCOPY (LIBS) MEASUREMENTS

ab

Figure 8. a) Crater obtained after 50 shots with the portable instrument (EasyLIBS);b) Crater obtained after 50 shots with the laboratory instrument.
abFigure 9. Shape and size of the craters; a) Crater obtained after 500 shots with the portable

instrument (EasyLIBS); b) Crater obtained after 500 shots with the laboratory instrument.

■ Conclusions
The 3D optical profilometer S neox has been shown to be an accurate, fast andeasy-to-use tool to investigate ablation craters associated to Laser Induced
Breakdown Spectroscopy measurements. The 3D optical profilometry allowed
qualitative and quantitative analysis, in particular, Confocal technology
has proven to be an effective technique to investigate and characterize
the size, diameter and depth of the craters of different LIBS instruments.
This is important to archaeologists, curators, since they need to assess thebenefits/risk before applying micro-destructive methods on patrimonial
artifacts.

CASE STUDY CHARACTERIZATION OF ABLATION CRATERS ASSOCIATED TO LASERINDUCED BREAKDOWN SPECTROSCOPY (LIBS) MEASUREMENTS

5

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