Fine Structure of the Beryllium Fluoride Bands

Wave-length measurements of BeF bands.---New wave-length measurements have been made of the rotational lines of the BeF bands in the regions λλ3009.5-3026.3, 2908.9-2927.8, and 3126.1-3140.8, which include all the stronger lines of the (0, 0), (1, 0) and (0, 1) bands. The source was a cored carbon arc run at atmospheric pressure. The Harvard 21-foot grating was used in the second order, giving a linear dispersion of 0.97 A per mm and an effective resolving power of nearly 100,000. The wave-numbers of the lines in the six branches of each band are tabulated for rotational quantum numbers up to about $K=50$ in the R branches, 45 in the Q branches, and 30 in the P branches. Visual intensity estimates are also included.Term analysis of BeF bands.---The rotational structure is found to be that characteristic of a ²Π→²Σ electronic transition, with a relatively small value of ΔEB, +15.667, in the Π state. The combination relations among the lines of the (1, 0), (0, 0) and (0, 1) bands are satisfied within the experimental error in every case where exact agreement is to be expected. The R-Q, Q-P relations show a small combination defect, which is referred to Λ-type doubling in the initial state. This doubling is slightly greater for the levels involved in the low-frequency sub-band, and therefore it is assumed that the ²Π state is normal. Rotational terms having the same K in the initial state draw rapidly together as the rotation increases, due to the loose coupling of the spin with the molecular axis. Quantitative agreement is obtained between the observed term-differences, ΔF, and their values calculated by the theoretical energy formulas derived by Hill and Van Vleck for the transition from case a to case b type of spin coupling. Equations capable of representing all the lines in a given band are deduced.Constants of BeF molecule.---The following values for the more important molecular constants are obtained: B^(0)'=1.41060 cm^-1, B^(0)''=1.47928, α^'=0.01610 cm^-1, α^''=0.01685, D^(0)'=-8.301×10^-6 cm^-1, D^(0)''=-8.209×10^-6, β^'=1.060×10^-8 cm^-1, β^''=2.903×10^-8, Iₑ^_^'=(19.50±0.03)10^-40 g cm², Iₑ^_^''=(18.59±0.03)10^-40, rₑ^_^'=1.390×10^-8 cm, rₑ^_^''=1.357×10^-8, ωₑ^_^'=1172.56 cm^-1, ωₑ^_^''=1265.62, xₑ^_^'=0.007488, xₑ^_^''=0.007206, νₑ=33,233.61 cm^-1, ΔE=22.10±0.1 cm^-1.