Physical phenomena and analytical applications of helium microwave discharges

Abstract

The intensity of atomic emission from a microwave-induced helium gas discharge as a function of pressure in the range 13–130 mbar is described. Two of the spectra studied were given by excited helium atoms, and one by (a species of) an excited triplet helium molecule (He 2 *). The pressure dependence of the concentration of helium atoms in the triplet metastable state was studied by absorption spectroscopy. After introduction of known quantities of mercury, chlorine, and iodine into the helium plasma, the emission was measured for some intense lines in the visible and near-u.v. Comparison of the data suggests that the atoms of these elements can be excited by helium atoms and molecules to levels at which they emit light in the visible and near-u.v. The use of a helium discharge tube for the detection of single elements in gas chromatographic fractions is described. Selectivity is greatly improved by wavelength modulation. The method allows a highly sensitive and selective determination of nanogram amounts of organic compounds which contain the elements sought, including stable isotopes such as deuterium, carbon-13 and nitrogen-15.