Electronic transitions of cobalt carbide, CoC, near 750 nm: A good example of case (<i>b</i>β<i>S</i>) hyperfine coupling

TLDR

CoC’s X²Σ⁺ ground state exemplifies Hund’s case (bβS) coupling, with Co’s nuclear spin (I = 7/2) producing rotational hyperfine splittings exceeding 0.5 cm⁻¹ and the state gradually uncoupling toward case (bβJ) at higher rotation. The study employed laser excitation of the 2Π₃/₂–2Δ₅/₂ transition, monitored via strong 2Π₃/₂–X²Σ⁺ emission, to characterize the low‑lying 2Δ₅/₂ state at high resolution. High‑resolution laser‑induced fluorescence of jet‑cooled CoC near 750 nm revealed 879 assigned rotational‑hyperfine transitions among 2Π₁/₂, 2Π₃/₂, 2Δ₅/₂, and 2Σ⁺ states, identified a low‑lying 2Δᵢ state at 221 and 1173 cm⁻¹, and provided matrix elements for the 2Σ⁺ state in case (bβS) coupling.

Abstract

The laser induced fluorescence spectrum of jet-cooled CoC near 750 nm has been measured at high resolution following the reaction of laser-ablated cobalt atoms with methane. The X2Σ+ ground state of CoC is an unusually good example of Hund’s case (bβS) coupling. Since Co has a nuclear spin I=7/2, each rotational level is split by the Fermi contact interaction into G=3 and G=4 components, where G=I+S; the splitting for N=0 is more than 0.5 cm−1. The X2Σ+ state begins to uncouple toward case (bβJ) with increasing rotation. Transitions to various 2Π excited states occur in the region 13 000–14 500 cm−1; the most prominent of these (for which high resolution spectra have been recorded) lie at 13 079 cm−1 (2Π3/2) and 13 343 cm−1 (2Π1/2). The (bβS) coupling in the ground state produces some unexpected hyperfine intensity patterns, which have been studied in detail. A very low-lying 2Δi state, whose Ω=5/2 and Ω=3/2 components lie at 221 and 1173 cm−1, has been identified. Laser excitation of the 2Π3/2–2Δ5/2 transition has been observed by monitoring the strong 2Π3/2–X2Σ+ emission, which has allowed the 2Δ5/2 state to be characterized at high resolution. A total of 879 rotational-hyperfine transitions between the various 2Π1/2, 2Π3/2, 2Δ5/2, and 2Σ+ states have been assigned and fitted. Matrix elements for a 2Σ+ state in case (bβS) coupling are listed.