The microwave spectrum of the DO 2 radical

The a-type N = 1 ← 0,2 ← 1, and 3 ← 2 and the b-type 1 ₁₁ ← 2 ₀₂, 2 ₁₂ ← 3 ₀₃, 3 ₁₃ ← 4 ₀₄, 6 ₀₆ ← 5 ₁₅, and 7 ₀₇ ← 6 ₁₆ transitions, 14 in total, of the DO ₂ free radical in the ground vibronic state were observed by microwave spectroscopy in the frequency region up to 200 GHz. The DO ₂ radical was generated in a free-space absorption cell by a DC discharge in a mixture of CH ₃OD and O ₂. 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 DO ₂ were determined by the present work for the first time. A comparison of the in-plane components of the dipole-dipole interaction tensor of DO ₂ with those of HO ₂ made it possible to determine the off-diagonal component T_ab, and thus the principal values and the principal axes of the tensor.