Hyperfine Structure and Intensities of the Forbidden Lines of Pb I

Abstract

The hyperfine structure of the forbidden lines of lead is studied in order to establish the intensity rules in the hyperfine structure of such lines. Those here investigated arise from transitions between the levels of the $6p^2$ configuration, and show hyperfine structure because of the presence of a nuclear spin and magnetic moment in the isotope 207. They are excited very intensely by 3-meter standing waves in helium at about 5 mm pressure, containing saturated Pb vapor at 800°C. The type of discharge and its dependence on several factors are studied. Intensities are quantitatively determined from calibrated plates. For the quadrupole line $\lambda 5313$, the intensity ratio (3: 2), and the ratio of the distances from the center of gravity, are found to agree with the Rubinowicz formulas transcribed for hyperfine structure by Opechowski (selection rule $\Delta F=0\mathrm{}$, ±1, ±2). For the magnetic dipole line $\lambda 4618$, these ratios agree with the well-known rules for electric dipole transitions (2: 1). In the case of $\lambda 7330$, which is of mixed type, the intensities are approximately those for ordinary electric dipole transitions, showing that this line is mostly magnetic dipole radiation and that for such radiation the selection rule $\Delta F=0\mathrm{}$, ±1 holds. An upper limit of the admixture of electric quadrupole can be evaluated from these measurements. The measurements also show that the total transition probabilities for electric quadrupole and magnetic dipole are independent of the existence of a magnetic moment of the nucleus since they are the same for the various isotopes. Finally, the relative intensities of these three lines and of two others, $\lambda 4659$ and a newly found line at $\lambda 9250({}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}-{}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{})$, have been measured for comparison with theory. The results are $I_{4618}:I_{5313}=5.0\mathrm{}\pm 0.3$ and $I_{4659}:I_{7330}:I_{9250}=0.023\mathrm{}\pm 0.006:1\mathrm{}:0.84\mathrm{}\pm 0.07$. These ratios are independent of the furnace temperature. The ratio $I_{5313}:I_{4659}$ varies somewhat with temperature, changing from about 15 to 12 in going from the lower to the higher temperatures.