A Comparative Study of Pressure-Dependent Emission Characteristics in Different Gas Plasmas Induced by Nanosecond and Picosecond Neodymium-Doped Yttrium Aluminum Garnet (Nd:YAG) Lasers

Abstract

An experimental study has been performed on the pressure-dependent plasma emission intensities in Ar, He, and N2 surrounding gases with the plasma induced by either nanosecond (ns) or picosecond (ps) yttrium aluminum garnet laser. The study focused on emission lines of light elements such as H, C, O, and a moderately heavy element of Ca from an agate target. The result shows widely different pressure effects among the different emission lines, which further vary with the surrounding gases used and also with the different ablation laser employed. It was found that most of the maximum emission intensities can be achieved in Ar gas plasma generated by ps laser at low gas pressure of around 5 Torr. This experimental condition is particularly useful for spectrochemical analysis of light elements such as H, C, and O, which are known to suffer from intensity diminution at higher gas pressures. Further measurements of the spatial distribution and time profiles of the emission intensities of H I 656.2 nm and Ca II 396.8 nm reveal the similar role of shock wave excitation for the emission in both ns and ps laser-induced plasmas, while an additional early spike is observed in the plasma generated by the ps laser. The suggested preference of Ar surrounding gas and ps laser was further demonstrated by outperforming the ns laser in their applications to depth profiling of the H emission intensity and offering the prospect for the development of three-dimensional analysis of a light element such as H and C.

A comparative study of emission efficiencies in low-pressure argon plasmas induced by picosecond and nanosecond Nd:YAG lasers

“…Ar ambient gas is chosen in this study as it was shown to give the maximum emission intensity of the ablated atoms in a previous study. 21) It is found that almost the same plasma size of 20 mm diameter is obtained for both ps and ns laser irradiations using 150 Pa Ar ambient gas.…”

“…The experimental setup used in this study is basically the same as the one described in our previous report. 21) The chamber used is the same special cylindrical chamber of 115 mm inner diameter, which was designed with several entry ports to allow for the control and monitoring of the specific conditions inside the chamber as needed for the experiment. For the current experiment, a ps Nd:YAG laser (Ekspla PL 2143, 1,064 nm, 20 ps, maximum energy of 30 mJ) and a ns Nd:YAG laser (Quanta Ray Lab Series 1,064 nm, 8 ns, maximum energy of 450 mJ) are employed separately, and are operated in the same Q-switched mode at 10 Hz repetition rate and the same fixed laser output energy of 25 mJ for a reason explained later.…”

“…Furthermore, it is worthwhile to recalling the result of a previous study 21) on the pressures dependent plasma emission intensities of a number of elements such as H, C, O, and Ca from an agate sample in different ambient gases (Ar, He, N 2 ) using a ps YAG laser. The result showed widely different pressure effects on the different analyte atom emission lines, which further vary with the ambient gases used.…”