Measurement of Excited Argon Atom in Glow Discharge Plasma by Diode Laser Coherent Forward Scattering Spectrometry (DL-CFS)

Abstract

When non-metallic atoms are excited in a low-pressure glow discharge plasma, absorption transitions can be observed in the 640–930 nm wavelength range where laser diodes are commercially available. These excited atoms can be sensitively probed by diode laser atomic absorption spectrometry (DL-AAS). Because an atomic absorption transition can also be detected by coherent forward scattering (CFS) spectrometry and CFS spectrometry with a diode laser has more attractive features than DL-AAS, diode laser coherent forward scattering spectrometry (DL-CFS) was employed to investigated the absorption transition of excited argon atom at 842.46 nm. Ar (I) 842.46 nm line was adopted due to the tunable wavelength range of available diode laser in this experiment. Excited argon atoms were produced in a glow discharge plasma. CFS signal intensity at 842.46 nm attained maximum at discharge current of 20 mA in 3 Torr (399 Pa) of argon and at magnetic field of 160 mT. Calibration curve of argon was prepared to mix a small amount of argon into 6 Torr (798 Pa) of helium. The signal intensity depended on the 4.4th power of the number density of argon. The 4.4th power dependence is too large to be explained by the theoretically predicted quadratic dependence. When a small amount of molecular nitrogen was mixed into argon plasma, strong suppression of CFS intensity was observed. Molecular gases such as air were found to be unsuitable for the plasma gas to excite the target atoms.