Rotational spectroscopy of IO X 2Πi

Pure rotational spectra of the IO radical have been observed for vibrational levels up to v=13 in the X1 Π3/22 state and up to v=9 in the X2 Π1/22 state. Isotopically enriched I18O rotational spectra have been observed for vibrational levels up to v=5 in both the X1 and X2 states. These are the first high-resolution spectra of any kind reported for the X2 state and greatly extend the available data for the X1 state. The data for both isotopomers have been fitted simultaneously to a single set of Π2 parameters with fixed isotopic ratios. The isotope relations among the parameters have provided a means of decorrelating the electron spin-rotation constant γ from the fine-structure centrifugal distortion constant, AD, and have allowed the first determination of an effective value for γ. The rotation–vibration constants correspond to the equilibrium molecular properties re(X1)=186.762 pm, re(X2)=188.468 pm, ωe(X1)=681.69 cm−1, and ωe(X2)=645.29 cm−1. These constants have been used to calculate X1 and X2 Rydberg–Klein–Rees potentials encompassing energies up to 40% of the dissociation limit. A complete set of hyperfine coupling constants has been determined for the first time and interpreted using appropriate relativistic atomic radial integrals.