INTRODUCTION

The difficulties in performing Laser-Induced Breakdown Spectroscopy (LIBS) on the surface or in the bulk of a liquid sample are well-known.  Single-pulse “panoramic” (broadband) spectroscopy on the surface of water for a variety of pulse energies, delay times, and experimental configurations has been studied previously.1,2 

In the absence of intentionally-added calibration elements, the dominant atomic spectral features at short delay times are the hydrogen H-alpha and H-beta emission lines at 656 and 486 nm, respectively.  During LIBS in the presence of atmospheric gas, however, the strength of these atomic emissions decreases rapidly with time and by approximately 8 ms after the ablation pulse the emission from molecular species created during and after the ablation process dominates the spectrum.

Dominant spectral features are:

        A²S⁺-X²P OH molecular band system

                0-0 band: R and Q branches at 306-308 nm

                1-0 band: R and Q branches at 281 nm

        A³P-X³S^- NH molecular band system

                  0-0 band: R and Q branches at 330 and 336 nm

These molecular emissions are readily apparent at delay times approaching 40 microseconds, long after any atomic emission (from hydrogen or atmospheric atomic nitrogen and oxygen) has disappeared.   Using a high-resolution Echelle spectrometer, individual rotational transitions are easily identifiable in the vibration bands, suggesting possible applications for molecular spectroscopy.

 

 

1 O. Samek, “Application of laser-induced breakdown spectroscopy to in situ analysis of liquid samples,” Opt. Eng. 39 (8), pp.2248-2262 (2000)

2 B. Charfi, “Panoramic laser-induced breakdown spectrometry of water,” Spect. Acta B 57, pp1141-1153 (2002)

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LINES AND BANDS USED IN WATER / AIR ANALYSIS

 

 

ANALYSIS

 

Region of empty spectrum near emission line averaged to measure background

Standard deviation of this empty region used to calculate “noise”

Area under the background-subtracted curve = intensity (numerical integration performed due to inability to perform fitting on hundreds of rotational peaks in molecular spectra). 

Signal-to-background (SBR) and signal-to-noise (SNR) calculated.

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Temperatures calculated from H-beta / H-gamma intensity ratio using Boltzmann equation:

 

FWHM of Stark-broadened H-beta and H-gamma lines

used to calculate electron density, Ne:

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