Exploring factors conditioning laser-induced acoustics and its potential as a LIBS-hyphenated
technique


Markéta Bosáková^a,b, Pablo Purohit ^a, César Álvarez-Llamas ^a, Javier Moros^a, Karel Novotný^b
and Javier Laserna ^a


a UMALASERLAB, Departamento de Química Analítica, Universidad de Málaga, C/Jiménez Fraud 4, Malaga
29010, Spain.

b Department of Chemistry, Masaryk University, Kamenice 5, Brno 625 00, Czech Republic

bosakova@uma.es

Keywords: Laser-Induced Acoustics, compositional contribution to laser-induced shockwaves,
time-domain interferences, hyphenated techniques.

Despite the interest shown previously in employing the laser-induced acoustic shockwave generated
alongside the plasma as a mean to expand the information delivered by LIBS inspection of solids^1,
the multiple sources of uncertainty linked to sound waves have hindered their potential application
in in-lab and off-lab sensors. Still, the recent integration of a microphone synchronized to the
LIBS laser in the Perseverance rover deployed as part of the NASA mission Mars 2020 has re-kindle
the motivation towards comprehending the different phenomena taking place during the lifetime of
the shockwave in order to extract from it the contribution of the inspected sample.

In this work, sets of morphologically-controlled samples coming from chemically identical materials
are probed under similar experimental conditions to systematically test the impact of
sample-related traits as well as environmentally-induced factors upon the collected audio signal.
Geometrical factors such as diameter and thickness as well as sampling position are explored using
aluminum samples. Sample absorption of the impinging laser light was also tested in Al and plastic
samples as it conditions the laser-sample coupling characteristics of the produced plasma. These
studied verified the relationship between the sample’s extinction coefficient and the total
acoustic response. Moreover, 3D-printed probes were used to evaluate how material porosity modified
the resulting acoustics.

Environmental effect were taken into account. The use of diverse flooring in the sample-microphone
path revealed changes in the acoustic spectra after the first positive maximum, indicating the
formation of terrain-dependent interference patterns. Lastly, we observed that parameters such as
height of the microphone or the microphone-to-sample distance play a key role in acoustic
experiments. Based on these results, final experimental conditions should be well-defined and
chosen in order to avoid non-chemical environmental and sample contributions altering laser-induced
acoustics.

[1] Santiago Palanco and Javier Laserna, "Spectral analysis of the acoustic emission of
laser-produced plasmas," Appl. Opt. 42, 6078-6084 (2003). DOI: 10.1364/AO.42.006078