Measurement of H 2CO hyperfine structure with a two-cavity maser

Hyperfine structure on the 1 ₁₀ → 1 ₁₁ rotational transition in H ₂CO was measured using a two-cavity maser spectrometer with 0.3 kHz resolution. The measured spin-rotation constants are C(1 ₁₀) = -0.80 ± 0.05 kHz and C(1 ₁₁) = -3.07 ± 0.05 kHz. The hydrogen spin-spin interaction strength is D = 17.74 ± 0.10 kHz. The line center is at 4 829 659.89 ± 0.12 kHz. Properties of the beam source and computer line resolving methods are discussed briefly.