Optical Cross Section of Resonance Lines Emitted by Flames under Conditions of Partial Thermal Ionization

TLDR: A method for determining optical cross sections, based on finding the point of intersection of the low and high-density asymptotes of an intensity-density curve, is extended to elements of low ionization potentials as discussed by the authors.

Abstract: A method for determining optical cross sections, based on finding the point of intersection of the low- and high-density asymptotes of an intensity-density curve, is extended to elements of low ionization potentials. These elements suffer partial thermal ionization in the flame gases at the relatively high temperatures of 2500° and 2760°K which are used as spectroscopic light sources. Since the degree of ionization of the carrier atoms depends on their partial pressure, the intensity-density curves may be distorted. This difficulty is overcome by increasing the concentration of electrons in the flame gases and thereby shifting the ionization equilibrium to the side of neutral atoms. For the alkaline earth elements the existence of equilibrium ionization is tested for by direct, independent measurements of the partial pressures Avhich enter into the Saha equilibrium quotient. Optical cross sections are measured for the resonance lines of Li, Na, K, Rb, Cs, Ca, Sr, and Ba. The values for the alkalis exhibit a moderate increase with atomic weight which is in accordance with the Weisskopf-Lindholm theory of impact broadening.