Laser-induced breakdown spectroscopic studies of calcite (CaCO3) marble using the fundamental (1064 nm) and second (532 nm) harmonic of a Nd:YAG laser

Abstract

Calcium is the fifth most abundant element in the Earth’s crust and finds many applications in different fields of science and technology. Laser-induced breakdown spectroscopy (LIBS) was used for the analysis of naturally occurring calcite (CaCO3) marble at atmospheric pressure using the fundamental (1064 nm) and second (532 nm) harmonic of a Q-switched Nd:YAG laser. The emission spectra consist of neutral and singly ionized spectral lines of Ca and Mg, while traces of Al, Sr, and Na were also detected. The Ca lines were observed to dominate the shorter as well as longer wavelength regions and the 4s4d 3D2 → 4s4p 3P2 transition of Ca (I) at 445.74 nm possessed higher intensity than any other spectral line in the observed spectra. An increase in the signal intensity of the spectral lines was noted with the increase in laser pulse energy. The relative abundance of the pertinent elements was determined by integrated intensities of the strongest lines, and integrated intensities of all lines from each element and the result was 79.74% Ca and 15.49% Mg, revealing them as major constituting elements. The results of the LIBS technique agreed with the outcomes of the same sample studied for micro-chemical analysis using an electron probe micro analyzer coupled with energy dispersive x-ray spectroscopy. Self-absorption was noted for a doublet of singly ionized Ca (II) line at 397.1 nm. The experimentally observed integrated line intensities of six Ca (I) lines at 364.53, 443.80, 458.86, 526.18, 616.83, and 650.04 nm were used to extract the electron temperature (Te) using the Boltzmann plot method. The electron number density (Ne) was determined from the Stark broadening profile of a neutral Ca (I) line at 504.37 nm. The values of Te and Ne at a 0.05 mm distance from the target surface with a pulse energy of 40 mJ were found to be 8500 K and 3.53 × 1016 cm−3 for the 1064 nm laser. The same parameters were observed to be 6800 K and 3.87 × 1016 cm−3 for the 532 nm laser. The relationship between electron temperature and number density was found to be directly related to the laser irradiance, while inverse proportionality was observed for the distance (0.05–2.0 mm) from the target surface.