Isotope Shifts in Some Lines of Nitrogen

Abstract

The line spectrum of a mixture of approximately 70 atomic percent ${\mathrm{N}}^{15}$ to 30 atomic percent ${\mathrm{N}}^{14}$ was photographed with a Fabry-Perot interferometer. The source used was a liquid-air cooled quartz tube with external electrodes excited by a 12-meter oscillator. The tube was charged with helium at a few mm pressure containing about 0.03 mm of the before-mentioned nitrogen isotope mixture. The lines from this discharge were so sharp that it was possible to resolve the two components due to ${\mathrm{N}}^{14}$ and ${\mathrm{N}}^{15}$ in nine infra-red lines. No nitrogen lines were found in this type of discharge in the visible or ultraviolet. The oscillator excitation produced a remarkably high intensity in the lines observed. The lines arising from ${}_{}{}^4{}_{}{}^{}P\to {}_{}{}^4{}_{}{}^{}P$ and ${}_{}{}^4{}_{}{}^{}S\to {}_{}{}^4{}_{}{}^{}P$ transitions showed a negative isotope shift of the order of 0.06 ${\mathrm{cm}}^{-1\mathrm{}}$. Those from ${}_{}{}^2{}_{}{}^{}P\to {}_{}{}^2{}_{}{}^{}P$ transitions showed a positive shift of the order of 0.07 ${\mathrm{cm}}^{-1\mathrm{}}$. The variations in the isotope shifts within the ${}_{}{}^2{}_{}{}^{}P\to {}_{}{}^2{}_{}{}^{}P$ multiplets indicate some deviation from Russell-Saunders coupling. No hyperfine structure in either component was observed in any of the lines. The directions of the shifts were checked by obtaining the abundance ratio of the isotopes in the discharge from the intensities of the isotopic heads in the second positive ${\mathrm{N}}_2$ bands.