Sodium absorption spectroscopy in liquid water using laser-induced breakdown as an in situ broadband light source

Nanosecond laser pulses were focused in NaCl solutions to create plasmas where the sodium mass concentration ranged from 1.88 to 9.11 wt% in deionized water. Elongated plasmas approximately 1 cm long appeared near the focal point in the liquid medium, generating multiple bubbles that quickly merged into a large bubble accommodating the plasma. Intense broadband emission was observed early in the plasma evolution and became stronger with increasing sodium concentration. Prominent absorption lines of Na I (sodium neutral) were detected between 20 and 120 ns after laser firing, emerging on the broadband emission spectrum. The absorption occurred mostly near the plasma, as sodium neutrals absorbing photons were present only in the plasma vicinity. The absorption strength of Na I was found to be a reliable indicator of sodium concentration, confirming the feasibility of utilizing the plasma as an in situ broadband light source for laser-induced breakdown absorption spectroscopy.