Millimeter/sub-mm spectroscopy of the CrBr radical in the high spin X6Σ+ state.

Abstract

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 γs 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 re = 2.337 282 (30) Å and a vibrational constant of ωe ≅ 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.