Double pulse laser induced breakdown spectroscopy of a solid in water: Effect of hydrostatic pressure on laser induced plasma, cavitation bubble and emission spectra

There is a growing interest in the development of technologies of augmented information of use in exploration of the deep ocean. Of special interest are techniques for the chemical analysis of submerged solids, which show promise for subsea mining applications where a rapid sorting of materials found in the sea bottom is required. Laser-induced breakdown spectroscopy has demonstrated potential for this application by its unique capability of providing the atomic composition of underwater solids. Here we present a study on the parameters that configure the spectral response of metallic targets in an oceanic pressure environment. Following laser excitation of the solid, the plasma persistence and the cavitation bubble size are considerably reduced as the hydrostatic pressure increases. These effects are of particular concern in dual pulse excitation as reported here, where a careful choice of the interpulse timing is required. Shadowgraphic images of the plasmas demonstrate that cavitation bubbles are formed early after the plasma onset and that the effect of hydrostatic pressure is negligible during the early stage of plasma expansion. Contrarily to the effect observed at atmospheric pressure, emission spectra observed at high-pressures are characterized by self-absorbed atomic lines on continuum radiation resulting from strong radiative recombination in the electron-rich confined environment. This effect is much less evident in ionic lines due to the much larger energy of the levels involved and to the lower extent of absorption effects occurring in the inner part of the plasma, where the ionized states are more abundant. As a result of the smaller shorter-lived cavitation bubble, the LIBS intensity enhancement factor resulting from dual pulse excitation is decreased by increasing the applied pressure.