The ground state and zero-field splitting of the GdO molecule

The X-band ESR spectrum of GdO was obtained in an argon matrix at 4 °K as a series of 16 fine-structure lines between 0 and 12 000 G. Analysis proved that most of these lines were due to off-principal-axis transitions and established the ground state as 9Σ. An accurate fit to the data required the inclusion of a small b40 term (quartic in Sz) besides the usual b20 = D term in the spin Hamiltonian. The derived parameters in an argon matrix are g∥ = 1.990(9), g⊥ = 1.986(5), ‖ b20 ‖ = ‖ D ‖ = 0.2078(3) cm−1, and ‖ b40 ‖ = 0.000 40(5) cm−1. The two strongest lines were also observed in a neon matrix.