The microwave spectrum of the DO2 radical

The a-type N = 1 ← 0,2 ← 1, and 3 ← 2 and the b-type 1₁₁ ← 2₀₂, 2₁₂ ← 3₀₃, 3₁₃ ← 4₀₄, 6₀₆ ← 5₁₅, and 7₀₇ ← 6₁₆ transitions, 14 in total, of the DO₂ free radical in the ground vibronic state were observed by microwave spectroscopy in the frequency region up to 200 GHz. The DO₂ radical was generated in a free-space absorption cell by a DC discharge in a mixture of CH₃OD and O₂. From the observed spectra the rotational constants, centrifugal distortion constants, spin-rotation coupling constants, and magnetic hyperfine coupling constants were determined with good precision. The hyperfine coupling constants of DO₂ were determined by the present work for the first time. A comparison of the in-plane components of the dipole-dipole interaction tensor of DO₂ with those of HO₂ made it possible to determine the off-diagonal component T_ab, and thus the principal values and the principal axes of the tensor.