Metastable Excited He Assisted Optical H and D Emission in Laser Induced Plasma Spectroscopy
8th International Symposium on Modern Optics and Its Applications
Bandung – Indonesia, 4 – 7 July 2011
A.M. Marpaung, Z.S. Lie, H. Niki, K. Kagawa, K. Fukumoto, M. Ramli, S.N. Abdulmadjid, N. Idris, R. Hedwig, M.O. Tjia, M. Pardede, E. Jobiliong, M.M. Suliyanti, K.H. Kurniawan
The laser induced breakdown spectroscopy (LIBS) is one of the most widely adopted modern methods for practical and rapid spectrochemical analysis in wide ranging fields of scientific and industrial investigation. Despite its well earned reputation in those applications, its performance in the analysis of light elements, particularly hydrogen and deuterium, has long been known to suffer from undesirable effects of intensity diminution and line broadening. These problems were shown to have their separate origins in the time mismatch between the rapid passage of the light atoms and the formation of the laser generated plasma shock wave, and the presence of charge particles (electrons and ions) responsible for the Stark effect in the early stage of the plasma formation. It was shown that replacing the ambient air in the standard operation of LIBS with He ambient gas has succeeded to improve to a large extent the spectral quality of the H and D emission.
It has been argued that the improvement was related to the existence of He in its metastable excited state in the plasma, which served as the temporary micro energy reservoir for the delayed excitation of the analyte atom at the moment when the plasma was relatively free of the charge particles, while at the same time replacing the shock wave induced thermal excitation process with the He-assisted excitation process. For the unusually large energy transfer process, was proposed to be mediated by Penning like ionization involving electron exchange between the helium and hydrogen atoms, followed by the radiative energy transfer for the excitation of H atom. A crucial test of this unusual role of He ambient gas in the laser induced plasma spectroscopy (LIPS) has been successfully demonstrated in the complete resolution of the optical emission lines of H I 656.2 nm and D I 656.1 which are of hair splitting spectral separation of 0.09 nm at different gating delay of (a) 500 ns, (b) 2 µs and (c) 3.5 µs.