The lines were taken from
(1) R. D. Brown, P. D. Godfrey, D. McNaughton, A. P. Pierlot, and W. H. Taylor, 1990, J. Mol. Spectrosc. 140, 340.
Additional lines used in
(2) A. Guarnieri and A. Huckauf, 2003, Z. Naturforsch. 58a, 275,
were kindly communicated by A. Huckauf.
Some higher order centrifugal distortion constants were kept fixed to values estimated from the main isotopomer, see e042501.cat. Predictions for higher values of K_a should be viewed with caution.
At low temperatures, it may be necessary to discern between ortho-H₂C₂O and para-H₂C₂O. The ortho states are described by K_a odd, the para states by K_a even. The nuclear spin-weights are 3 and 1 for ortho-H₂C¹³CO and para-H₂C¹³CO, respectively. The J_KaKc = 1₁₁ level is the lowest ortho level. It is 9.7395 cm^–1 above ground.
The dipole moment was assumed to be the same as for the main isotopomer, see e042501.cat.