Nuclear hyperfine structure in the X3Σ + state of 91ZrC

Abstract

Electronic band systems of zirconium monocarbide, ZrC, in the 16 000–19 000 cm −1 region have been observed following the reaction of laser-ablated Zr atoms with methane under supersonic free-jet conditions. Rotational analyses of high-resolution spectra have shown that the ground state of ZrC is a 3Σ state, with r 0=1.8066 Å and an unexpectedly small spin–spin parameter, λ=0.5139 cm −1. The spectra are dense because of the five naturally occurring isotopes of Zr. Four of these, with mass numbers 90, 92, 94, and 96, have I=0, but the fifth, 91Zr, present in 11.22% abundance, has I=5/2. Lines of 91ZrC can be assigned in some of the strongest bands, and are found to display sizeable hyperfine splittings, with widths of up to 0.2 cm −1. Analysis shows that the largest hyperfine effects are in the ground state, where b=−0.03133±0.00015 cm −1 and c=−0.00123±0.00037 cm −1 (3 σ error limits). The large Fermi contact parameter, b, indicates that an unpaired Zr 5 sσ electron is present, which, taken together with the small value of λ, means that the ground state must be a 3Σ + state, from the electron configuration (Zr 5 sσ) 1 (C 2 pσ) 1. Internal hyperfine perturbations occur between the F 1 and F 3 electron spin components of the ground state in the range N=2–4, producing extra lines in some of the branches; the perturbations are of the type Δ N=0, Δ J=±2, and are a second-order effect arising because the F 1 ( J= N+1) and F 3 ( J= N−1) spin components both interact with the F 2 ( J= N) component through Δ N=0, Δ J=±1 matrix elements of the Fermi contact operator. Second-order perturbations of this type can only occur in states that are very close to case ( b) coupling.