Fritz W. Hofmann and Hedwig Kohn, "Optical Cross Section of Resonance Lines Emitted by Flames under Conditions of Partial Thermal Ionization*†," J. Opt. Soc. Am. 51, 512-521 (1961)
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.
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At 2500°K: p(Na) = p(Natotal) = 8.70 × 10−5 atm at a concentration of 1 mole per liter of the sprayed NaCl solution.
Value in the limit of low densities.
Corrected for the decrease of p(Cstotal)/p(Cs) with concentration.
At 2760°K: p(Na) = p(Natotal) = 5.73 × 10−5 atm at a concentration of 1 mole per liter of the sprayed NaCl solution.
Table III
Ionization of the alkaline earth elements.
Molar concentration of calcium solution in powers of 2
H. H. Landolt and Börnstein, Zahlenwerte und Funktionen (Springer-Verlag, Berlin, 1955), 2nd ed., Vol. I/1, p. 264.
E. Trefftz and L. Biermann, Z. Astrophys. 30, 275 (1952).
C. W. Allen, Astrophysical Quantities (Athlone Press, London, 1955), p. 77.
H. Kopfermann and G. Wessel, Z. Physik 126, 440 (1949); 130, 100 (1951), footnote on p. 106. A further correction has to be made which lowers the value f = 1.8 to 1.6 (private communication).
Table IV
Ratios k of total absorptions of the alkali resonance doublet components in an acetylene-air flame of 2500°K.
At 2500°K: p(Na) = p(Natotal) = 8.70 × 10−5 atm at a concentration of 1 mole per liter of the sprayed NaCl solution.
Value in the limit of low densities.
Corrected for the decrease of p(Cstotal)/p(Cs) with concentration.
At 2760°K: p(Na) = p(Natotal) = 5.73 × 10−5 atm at a concentration of 1 mole per liter of the sprayed NaCl solution.
Table III
Ionization of the alkaline earth elements.
Molar concentration of calcium solution in powers of 2
H. H. Landolt and Börnstein, Zahlenwerte und Funktionen (Springer-Verlag, Berlin, 1955), 2nd ed., Vol. I/1, p. 264.
E. Trefftz and L. Biermann, Z. Astrophys. 30, 275 (1952).
C. W. Allen, Astrophysical Quantities (Athlone Press, London, 1955), p. 77.
H. Kopfermann and G. Wessel, Z. Physik 126, 440 (1949); 130, 100 (1951), footnote on p. 106. A further correction has to be made which lowers the value f = 1.8 to 1.6 (private communication).
Table IV
Ratios k of total absorptions of the alkali resonance doublet components in an acetylene-air flame of 2500°K.
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