The Band Spectra of the Hydrides of Lithium Part I:Li(7)D

The ${}_{}{}^1{}_{}{}^{}\Sigma \to {}_{}{}^1{}_{}{}^{}\Sigma$ spectrum of LiD has been photographed in absorption in the second order of a 21-foot grating giving a dispersion of 1A/mm. The spectrum is a headless concourse of lines stretching from $\lambda 3200$ to $\lambda 4300A$. About 80 percent of the lines (over 1400 in all) have been measured and assigned to 35 two-branch bands. The initial and final double differences, ${\Delta }_2{F_v}^{\prime }$ and ${\Delta }_2{F_v}^{\prime \prime }$, for ${\mathrm{Li}}^{\mathrm{}\left(7\right)\mathrm{}}$D have been represented as functions of $J$ by a semigraphical method; these represent the observed data on the average to within ±0.023 ${\mathrm{cm}}^{-1\mathrm{}}$ for the upper state and ±0.019 ${\mathrm{cm}}^{-1\mathrm{}}$ for the ground state. This gives ${B_e}^{\prime }=1.6060\mathrm{}$, ${D_e}^{\prime }=4.8\mathrm{}\times {10}^{-4\mathrm{}}$, ${B_e}^{\prime \prime }=4.2338\mathrm{}$ and ${D_e}^{\prime \prime }=2.756\mathrm{}\times {10}^{-4\mathrm{}}$. By using the rotational constants so determined the origins of the bands involved were computed and the values of $\Delta G^{\prime }(v+\frac{1}{2})$ and $\Delta G^{\prime \prime }(v+\frac{1}{2})$ represented by least-square expressions in ($v+\frac{1}{2}$). The $\Delta G^{\prime }$-values require a fifth degree polynomial while the $\Delta G^{\prime \prime }$ values are represented closely by a cubic. The corresponding expression for the origins of the whole system represents the data with an average deviation of ±0.04 ${\mathrm{cm}}^{-1\mathrm{}}$. Here ${{\omega }_e}^{\prime }=183.12\mathrm{}$, ${w_e}^{\prime }{x_e}^{\prime }=-12.741\mathrm{}$, ${{\omega }_e}^{\prime \prime }=1055.12\mathrm{}$, and ${x_e}^{\prime \prime }{{\omega }_e}^{\prime \prime }=13.228\mathrm{}$, with the electronic origin at ${\nu }_e=26,512.05\mathrm{}$. The corresponding vibrational constants for ${\mathrm{Li}}^{\mathrm{}\left(7\right)\mathrm{}}$H are computed from the simple isotope theory and although they differ considerably from the results of Nakamura, represent the isotope shifts within the rather large uncertainty involved. The question of possible 1-uncoupling is examined and no conclusive evidence is found for an appreciable l-uncoupling in either state of ${\mathrm{Li}}^{\mathrm{}\left(7\right)\mathrm{}}D$. This is contrary to the accepted interpretation of the anomalies of the spectrum.