Fourier-transform-spectroscopy measurements in the spectra of neutral lithium, ${}_{}{}^6{}_{}{}^{}\mathrm{I}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{I}$ (Li I)

The most recent measurements leading to improved Li I energy levels appeared in the late 1950s. Since then, many high- and ultrahigh-precision spectroscopy techniques have been used to measure the fine and hyperfine structures and isotope shifts of this atom. Most previous measurements were made on ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$. Using high- and low-current hollow cathode sources and Fourier-transform spectrometry, we have made measurements in both ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ including nine previously unmeasured infrared lines. Wave-number measurements were obtained on 34 lines from 1829 to 30 925 ${\mathrm{cm}}^{\mathrm{-}1}$. The strongest lines have been determined to ±0.0010 ${\mathrm{cm}}^{\mathrm{-}1}$. We also measured 22 new isotope shifts, and added specific mass shifts for five levels. The resonance lines at 6707 Å in both isotopes, emitted by low-current, commercial lithium hollow cathode sources, were measured to ±0.0005 ${\mathrm{cm}}^{\mathrm{-}1}$. The energy levels have been recalculated using the new data.