The Band Spectra of the Hydrides of Lithium

Abstract

About 1000 lines, comprising 26 bands, of ${\mathrm{Li}}^{7}$H have been remeasured and given quantum assignments. The experimental arrangements are similar to those given in Part I on ${\mathrm{Li}}^{7}$D. The corresponding ${\ensuremath{\Delta}}_{2}{F}_{v}(J)$ values have been represented as polynomials of the usual form in $J(J+1)$, where the ground state data are represented to within \ifmmode\pm\else\textpm\fi{}0.018 and the upper state data to within \ifmmode\pm\else\textpm\fi{}0.021 ${\mathrm{cm}}^{\ensuremath{-}1}$ (i.e., to within roughly \ifmmode\pm\else\textpm\fi{}0.0024A for the whole spectrum). Constants. Important constants for the ground state are ${{B}_{e}}^{\ensuremath{'}\ensuremath{'}}=7.5131$, ${{D}_{e}}^{\ensuremath{'}\ensuremath{'}}=\ensuremath{-}8.617\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, ${{H}_{e}}^{\ensuremath{'}\ensuremath{'}}=11.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$, ${{\ensuremath{\omega}}_{e}}^{\ensuremath{'}\ensuremath{'}}=1405.65$, ${{x}_{e}}^{\ensuremath{'}\ensuremath{'}}{{\ensuremath{\omega}}_{e}}^{\ensuremath{'}\ensuremath{'}}=+23.20$ and ${{y}_{e}}^{\ensuremath{'}\ensuremath{'}}{{\ensuremath{\omega}}_{e}}^{\ensuremath{'}\ensuremath{'}}=+0.1633$; corresponding constants for the upper state although less certain because of the errors of extrapolation (which are greater than for LiD) are in order 2.8186, 15.8\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$, 100(\ifmmode\pm\else\textpm\fi{}30)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$, 234.41, -28.95 and -4.185. The constants for the ground state show a small discrepancy between $\ensuremath{\rho}$ and $\frac{{{\ensuremath{\omega}}_{e}}^{i}}{{\ensuremath{\omega}}_{e}}$, and ${\ensuremath{\rho}}^{2}$ and $\frac{{{B}_{e}}^{i}}{{B}_{e}}$. In fact from these two ratios we find $\ensuremath{\Delta}\ensuremath{\rho}=+0.000143 (\ifmmode\pm\else\textpm\fi{}0.00003)$ and $\ensuremath{\Delta}\ensuremath{\rho}=+0.00020 (\ifmmode\pm\else\textpm\fi{}0.00006)$, where the plus sign indicates the fact that the observed $\ensuremath{\rho}'\mathrm{s}$ are these amounts greater than the atomic mass $\ensuremath{\rho}$ values. Consideration of Dunham's higher order calculation of the interaction of vibration and rotation accounts for about 1/3 of the discrepancy in the $\ensuremath{\rho}$ from the ratio of the ${B}_{e}\mathrm{s}$. The vibrational numbering adopted here agrees with Nakamura's and is such that the maxima in the ${{B}_{v}}^{\ensuremath{'}}$ and $\ensuremath{\Delta}{G}^{\ensuremath{'}}(v+1)$ curves occur at energies which are approximately the same for the LiH and the LiD molecule. We find no evidence of $l$-uncoupling and feel that the anomaly of the upper state is to be ascribed to an anomalous potential curve. The electronic origin isotope shift of this $^{1}\ensuremath{\Sigma}\ensuremath{\rightarrow}^{1}\ensuremath{\Sigma}$ system of LiH is 0.0(\ifmmode\pm\else\textpm\fi{}0.5) ${\mathrm{cm}}^{\ensuremath{-}1}$.