The Spectrum of Barium Hydride

A BaH band system in the region 6925A to 6380A has been photographed with a 21-foot grating (dispersion=2.04A per mm), using a metallic barium arc in a hydrogen atmosphere at reduced pressure as a source. The spectrum consists of the (0, 0), (1, 1) and (2, 2) bands only, and the analysis of the (0, 0) band shows the system to be the expected ${}_{}{}^2{}_{}{}^{}\Pi \to {}_{}{}^2{}_{}{}^{}\Sigma$ case $a$ type with twelve branches in each band. Assignments of the frequencies to these branches are presented, together with the principal combination differences. The $\Lambda$-type doubling is large in the ${}_{}{}^2{}_{}{}^{}\Pi _{\frac{1}{2}}^{}{}_{}{}^{}$ state, but small and of opposite sign in the ${}_{}{}^2{}_{}{}^{}\Pi _{1\frac{1}{2}}^{}{}_{}{}^{}$ state, giving $p_0=+0.85\mathrm{}$ and $q_0=+0.0067\mathrm{}$. The spin doubling in the ${}_{}{}^2{}_{}{}^{}\Sigma$ state is regular, with ${\gamma }_0=+0.186\mathrm{}$. Other constants are ${B_0}^{\prime \prime }=3.404\mathrm{}$, ${D_0}^{\prime \prime }=-9.61\mathrm{}\times {10}^{-5\mathrm{}}$, ${r_0}^{\prime \prime }=2.22\mathrm{}\times {10}^{-8\mathrm{}}$ cm, ${B_{0,-\frac{1}{2}}}^{\prime }=3.4468\mathrm{}$, ${D_{0,-\frac{1}{2}}}^{\prime }=-1.13\mathrm{}\times {10}^{-4\mathrm{}}$, ${B_{0,+\frac{1}{2}}}^{\prime }=3.5156\mathrm{}$, and ${D_{0,+\frac{1}{2}}}^{\prime }=-1.20\mathrm{}\times {10}^{-4\mathrm{}}$. The separation of the two ${}_{}{}^2{}_{}{}^{}\Pi$ sublevels yields $A=462\mathrm{}$. The smallness of this value ($A=832\mathrm{}$ for lowest ${}_{}{}^3{}_{}{}^{}P$ levels of Ba) indicates that this state is possibly formed from a combination of the lowest ${}_{}{}^3{}_{}{}^{}P$ and ${}_{}{}^3{}_{}{}^{}D$ states of the Ba atom. A large perturbation in one of the ${}_{}{}^2{}_{}{}^{}\Pi _{1\frac{1}{2d}}^{}{}_{}{}^{}$ levels, and the sign and magnitude of the $p_0$ and $q_0$, indicate another lower-lying excited level.