Abstract
In this work, laser-induced breakdown spectroscopy (LIBS) of gaseous ammonia (NH3) molecules on- and off-resonant vibrational excitation was studied in open air. A wavelength-tunable, continuous wave (CW), carbon dioxide (CO2) laser tuned at a resonant absorption peak (9.219 µm) within the infrared radiation (IR) range was used to resonantly excite the vibration of the N-H wagging mode of ammonia molecules. A pulsed Nd:YAG laser (1064 nm, 15 ns) was used to break down the ammonia gas for plasma imaging and spectral measurements. In this study, plasmas generated with the ammonia molecules without additional CO2 laser beam irradiation and with additional CO2 laser beam irradiation with the wavelengths on- and off-resonant vibrational excitation of ammonia molecules were investigated and referred as LIBS, LIBS-RE-ON and LIBS-RE-OFF, respectively. The experimental results showed that the temporal and spatial evolution as well as electron temperature and density of plasmas induced with LIBS and LIBS-RE-OFF were consistent but differed from LIBS-RE-ON. Compared with LIBS and LIBS-RE-OFF, plasmas in LIBS-RE-ON showed larger spatial expansion and enhanced emission after a delay time of 1 µs in this study, as well as significantly enhanced electron temperature by ∼ 64%. Time-resolved electron temperatures and densities showed that the emission signal enhancement in LIBS-RE-ON can be primarily attributed to the electron temperature enhancement. Signal enhancement in LIBS indicated improved detection sensitivity. This study could inspire future works on LIBS for gas detection with improved sensitivity and selectivity probably by using ultrafast/intense laser-induced molecular breakdown/ionization with resonant vibrational excitation of molecules.
Highlights
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Spectra and images of ammonia plasmas Optical emission spectra and images of the laser-induced gaseous ammonia plasma at different delay times were measured by the spectrometer and intensified charge-coupled device (ICCD) camera, respectively, to investigate the spectral and spatial evolution of the plasmas
The spectral and spatial evolution of plasmas generated in LIBS, LIBS-RE-ON, and LIBS-RE-OFF were investigated by spectral analyses and plasma imaging
Summary
A commercial Nd:YAG laser (Quantel DRL, 15 ns, 1064 nm) operated with a pulse energy of 630 mJ and a repetition rate of 5 Hz was horizontally focused by a plano-convex lens (diameter = 50.8 mm, focal length = 75 mm) to a spot size of 0.3 mm in diameter at a location 1.5 mm below the nozzle to induce breakdown of the gaseous ammonia. An ICCD camera, comprised of a Nikon micro lens (105 mm, f/2.8 D) and an attached ICCD detector (Andor Tech., DH-734), was fixed in the horizontal plane with an angle of 135 degrees clockwise along the CO2 laser beam direction and used for fast imaging.
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