On The Band Spectrum of Calcium Hydride

An arc of calcium burning in hydrogen at low pressure emits numerous bands in the region 6000-7000A. This spectrum was photographed at high dispersion and two groups of bands, shading toward the violet may be distinguished: the A groups with heads at λλ7035, 7028, 6921, 6903 and the B groups with heads at λλ6389, 6382. In addition to these, the arc emits an isolated C group---a single band in the ultra-violet at λ3533.6. This group is identical with a band of calcium hydride recently studied by R. S. Mulliken. The structures of A, B and C are very different. The A group forms a doublet system (A₁, A₂) of P-Q-R branches. The bands of the B group have a similar structure to that of the violet cyanogen bands, signified by doublet P₁, P₂ and R₁, R₂ branches. The C group consists of a single band having P-R branches. In all bands the series deviate largely from polynomials of second degree. Thus, in B and C there is a remarkable "red-shift" of high numbered lines, accompanied by a sharp cut-off in their intensity. From combinations found between the P-R branches, conclusions are reached regarding the spectral terms in CaH. The A, B and C groups have a common final (N) electronic term with a rotational doubling (ε₂=±1/2, σ₂=0). The initial state of A forms an electronic doublet (A₁, A₂) with the emission electron in a σ-orbit (ε₁=0, σ₁>0), thus explaining the appearance of Q branches in A. In B (initial) there is again a rotational doubling (ε₁=±1/2, σ=0). In C (initial) only one ε component is present (ε₁=-1/2, σ₁=0). The departure from half-integral quantum numbers in C is avoided by accepting a large Kratzer's linear term 2δj. The nuclear spacings in the CaH molecule are not in correlation with their vibration frequencies, violating a rule by Birge and Mecke. A comparison of the A group with the spectra of ZnH, CdH and HgH shows several interesting parallels, confirming the theory of Mulliken regarding these spectra.