Spectroscopic Evidence of the B₂ Molecule and Determination of Its Structure

In a discharge in helium with a trace of boron trichloride a new band system is found. The observation of isotope bands and in particular of an intensity alternation in the fine structure shows that the system is due to the diatomic boron molecule, B 2 . The B 2 11 band heads are represented by the formula[Formula: see text]The bands consist of single R and P branches. The fine structure analysis yields the following rotational constants: B e ′ = 1.160, B e ″ = 1.212, α′ = 0.011 and α″ = 0.014 cm. −1 . From these the internuclear distances r e ′ = 1.625 and r e ″ = 1.590Å are obtained. It is shown that in all probability the system represents a 3 Σ u − − 3 Σ 0 − transition, the lower state being the ground state.From the intensity alternation in the B 2 11 bands, the spin of the B 11 nuclei is found to be 5/2. This value is in disagreement with recent theoretical predictions based on the Hartree as well as on the α-particle model of the nucleus.Finally, a comparison with the other homonuclear diatomic molecules of the second period of the periodic system is made as well as a comparison of B 2 and C 2 with B 2 H 6 and C 2 H 6 .