Millimeter/sub-mm spectroscopy of the CrBr radical in the high spin X⁶ς⁺ state

The millimeter/submillimeter spectrum of the CrBr radical has been recorded in the frequency range of 220-300 GHz using direct absorption techniques, utilizing a new instrumental design. This study is the first spectroscopic investigation of this radical species by any method. CrBr was synthesized in a DC discharge by the reaction of chromium vapor, produced in a Broida-type oven, with Br₂CH₂ in argon. Six to nine rotational transitions were measured for four isotopologues of this molecule in their natural abundances, ⁵²Cr⁷⁹Br, ⁵²Cr⁸¹Br, ⁵³Cr⁷⁹Br, and ⁵³Cr⁸¹Br. Each transition was found to consist of six distinct fine structure components, indicating a ⁶ς⁺ ground electronic state, as observed for CrF and CrCl. Lines originating in the v = 1 and 2 vibrational states were recorded for ⁵²Cr⁷⁹Br and ⁵²Cr⁸¹Br as well. The spectra were analyzed using a Hund's case (b) Hamiltonian, and rotational, spin-spin, and spin-rotation parameters were determined. The third-order spin-rotation constant γ_s and the fourth order spin-spin term θ were necessary for the analysis; these parameters are thought to play a role in states with high multiplicities. Equilibrium parameters were also derived for the CrBr; a bond length of r_e = 2.337 282 (30) Å and a vibrational constant of ω_e ≅ 300 cm^-1 were determined. The sign and magnitude of the spin-spin and spin-rotation constants suggest the presence of nearby ⁴Π and ⁶Π excited states in CrBr, lying ∼9000 cm^-1 above the ground state. The new instrument design, employing more compact, free-space optics utilizing an offset ellipsoidal mirror, facilitated these measurements.