Description and Analysis of the First Spectrum of Iodine

An extensive survey of the spectra of iodine has led to a list of more than 900 lines emitted by neutral atoms in the region from 23070 A in the infrared to 1195 A in the extreme ultraviolet. Wavelengths between 12304 A and 2061 A were derived from measurements of spectrograms obtained with gratings of high dispersion. Wavelengths of lines outside these limits are the computed values for lines observed on photometric tracings of the infrared, inaccessible to photography, and in the ultraviolet with a vacuum-grating spectrograph. For many of the lines Zeeman patterns were obtained in a magnetic field of about 37,000 oersteds. With these data many of the lines have been classified as combinations between odd levels from the electron configurations 5s ² 5p ⁴ np and 5s ² 5p ⁴ nf, and even levels from the configurations 5s ² 5p ⁴ ns and 5s ² 5p ⁴ nd. Among these levels several sets have been recognized as forming Rydberg sequences that are in close agreement in placing the ground state 5p ⁵ $$ of I i at 84,340 cm^-1 below the ground state 5p ^{4 3}P₂ of I ii. This gives 10.45 electron-volts for the ionization potential of the neutral iodine atom. A strong infrared line at 13148.8 A is explained as a magnetic dipole transition between the levels of the ground term 5p ^{5 2}P°.