Quantitative Investigation of the Radiation of the Negative Iodine Ion

A pure iodine plasma is generated by a cylindric arc at 1 atm and 6 to 30 amps within a quartz tube of 8 mm diameter. The radiation emitted along the arc axis was measured absolutely in the wavelength range from 0.2 —\.9 fx. In the uv-region it consists mainly of the affinity continuum due to transitions of free electrons to the ground level of the negative iodine ions. The first threshold of this continuum at 4048 A yields an electron affinity of the iodine atom of 3.062 eV. The resulting photodetachment cross-section of the I~-ion exhibits a broad peak at 2400 A, which can be attributed to a I_-compound-state. The continuous background radiation in the visible and infrared regions shows, after subtraction of the electron-ion continuum, two long-wavelength thresholds near 1.25 [X and 0.64 [A. The absolute intensity of this radiation, its wavelength and temperature dependence and its threshold energy and threshold structure suggest the contribution of two dominant radiation processes: a second affinity continuum due to an excited state of the I~-ion about 2 eV above the ground level and an association continuum of I2+-molecules. Towards longer wavelengths 2 > 1 . 5 p electron-atom Bremsstrahlung becomes more important.