Light source for producing self-reversed spectral lines

Abstract

In atomic emission spectra, the presence of self- reversed spectral lines provides valuable information for the analysis of spectral structure. Because such lines generally arise from transitions to the ground state or to low-lying states of the atom, they may be used to deduce this important part of the energy level structure. In searching for regularities in a line-list containing tens of thousands of entries, as do those of most rare-earth elements, it is a great simplification to recognize a small group of lines the presence in some spectral regions of absorption bands due to water molecules, the superposition of metallic spark lines in emission, and the total absence of absorption for certain metals, presumably because of instant oxidation. (3) A d-c arc discharge with a current of several amperes nearly always produces self-reversal of the very strong lines in the spectrum of the electrode material by absorption in the sheath of cool vapor surrounding the discharge. The degree of self- reversal increases exponentially with the product of the intensity of the radiation and the number of absorbing atoms through which it passes (3). The spectra of rare-earth elements are charac- teristically lacking in lines of outstanding high intensity. Therefore, they rarely contain self- reversed lines in the low-current arc. It has been found that by raising the current in the arc, one usually enhances self-reversal because of the increased rate of vapor production. This increases the density of absorbing atoms surrounding the discharge. A substantial rise in current, however, will generally melt the electrodes unless adequate cooling is provided. A convenient method for obtaining a cool, high-current arc is described in this paper.