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 ${\Delta }_2{F}_v\mathrm{}\left(J\right)\mathrm{}$ values have been represented as polynomials of the usual form in $J(J+1\mathrm{})\mathrm{}$, where the ground state data are represented to within ±0.018 and the upper state data to within ±0.021 ${\mathrm{cm}}^{-1\mathrm{}}$ (i.e., to within roughly ±0.0024A for the whole spectrum). Constants. Important constants for the ground state are ${B_e}^{\prime \prime }=7.5131\mathrm{}$, ${D_e}^{\prime \prime }=-8.617\mathrm{}\times {10}^{-4\mathrm{}}$, ${H_e}^{\prime \prime }=11.4\mathrm{}\times {10}^{-8\mathrm{}}$, ${{\omega }_e}^{\prime \prime }=1405.65\mathrm{}$, ${x_e}^{\prime \prime }{{\omega }_e}^{\prime \prime }=+23.20\mathrm{}$ and ${y_e}^{\prime \prime }{{\omega }_e}^{\prime \prime }=+0.1633\mathrm{}$; 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×${10}^{-4\mathrm{}}$, 100(±30)×${10}^{-8\mathrm{}}$, 234.41, -28.95 and -4.185. The constants for the ground state show a small discrepancy between $\rho$ and $\frac{{{\omega }_e}^i}{{\omega }_e}$, and ${\rho }^2$ and $\frac{{B_e}^i}{B_e}$. In fact from these two ratios we find $\Delta \rho =+0.000143\mathrm{} (\pm 0.00003)$ and $\Delta \rho =+0.00020\mathrm{} (\pm 0.00006)$, where the plus sign indicates the fact that the observed $\rho \text{'}\mathrm{s}$ are these amounts greater than the atomic mass $\rho$ 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 $\rho$ 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 ${B_v}^{\prime }$ and $\Delta G^{\prime }\mathrm{}(v+1\mathrm{})\mathrm{}$ 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 ${}_{}{}^1{}_{}{}^{}\Sigma \to {}_{}{}^1{}_{}{}^{}\Sigma$ system of LiH is 0.0(±0.5) ${\mathrm{cm}}^{-1\mathrm{}}$.