Fourier-transform VUV spectroscopy of $^{14,15}$N and $^{12,13}$C

Accurate Fourier-transform spectroscopic absorption measurements of vacuum ultraviolet transitions in atomic nitrogen and carbon were performed at the Soleil synchrotron. For $^{14}$N transitions from the $2s^22p^3\,^4$S$_{3/2}$ ground state and from the $2s^22p^3\,^2$P and $^2$D metastable states were determined in the $95 - 124$ nm range at an accuracy of $0.025\,\mathrm{cm}^{-1}$. Combination of these results with data from previous precision laser experiments in the vacuum ultraviolet range reveal an overall and consistent offset of -0.04 \wn\ from values reported in the NIST database. %The splitting of the $2s^22p^3\,^4$S$_{3/2}$ -- %$2s2p^4\,^4$P$_{5/2,3/2,1/2}$ The splittings of the $2s^22p^3\,^4$S$_{3/2}$ -- $2s2p^4\,^4$P$_{J}$ transitions are well-resolved for $^{14}$N and $^{15}$N and isotope shifts determined. While excitation of a $2p$ valence electron yields very small isotope shifts, excitation of a $2s$ core electron results in large isotope shifts, in agreement with theoretical predictions. For carbon six transitions from the ground $2s^22p^2\,^3$P$_{J}$ and $2s^22p3s\, ^3$P$_{J}$ excited states at $165$ nm are measured for both $^{12}$C and $^{13}$C isotopes.