Alternating Intensities and Isotope Effect in the Blue-Green Absorption Bands ofLi2

Term analysis. The analysis of the blue-green ${\mathrm{Li}}_2$ bands previously given by Wurm is extended by new data obtained with second order plates from the 15-ft. concave grating. The band heads are given by ${\nu }_h=20,398.40+267.3\mathrm{}{v}^{\prime }-3.1\mathrm{}{v}^{\prime 2}-347.4\mathrm{}{v}^{\prime \prime }+2.5\mathrm{}{v}^{\prime \prime 2}$. Wave-numbers of the rotational structure lines of the (0, 1) band are tabulated, and the evaluation of the rotational term-differences in the lower state completely confirms the assignment of this system to the electronic transition ${}_{}{}^1{}_{}{}^{}\Pi \leftarrow {}_{}{}^1{}_{}{}^{}\Sigma$. The molecular constants, as completely re-determined from our data, using a more accurate procedure than that of Wurm, agree closely with his values. The new constants resulting from our work are $F_e^{}{}_{}{}^{\prime }=1.337\mathrm{}\times {10}^{-10\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $F_e^{}{}_{}{}^{\prime \prime }=1.535\mathrm{}\times {10}^{-10\mathrm{}}$, ${\alpha }^{\prime \prime }=0.00744\mathrm{}$, and ${\beta }^{\prime \prime }=0.765\mathrm{}\times {10}^{-8\mathrm{}}$. By means of the final constants and the theoretical equations, an accurate representation of the wave-numbers of the lines of the (0, 0) band is obtained. The origin of this band is found to be 20,395.96 ${\mathrm{cm}}^{-1\mathrm{}}$.