This is an update of version 1 from Mar. 2000 and of version 2 from Mar. 2003/Aug. 2005. The experimental data were summarized and reevaluated by
(1) H. S. P. Müller and F. Lewen, 2017, J. Mol. Spectrosc. 331, 28.
Besides new data betwen 1.37 and 1.5 THz from that study, transition frequencies were taken mostly from
(2) S. Brünken, H. S. P. Müller, F. Lewen, and G. Winnewisser, 2003, Phys. Chem. Chem. Phys. 5, 1515;
and from
(3) R. Bocquet, J. Demaison, L. Poteau, M. Liedtke, S. Belov, K. M. T. Yamada, G. Winnewisser, C. Gerke, J. Gripp, and Th. Köhler, 1996, J. Mol. Spectrosc. 177, 154.
Additional larger sets of transition frequencies were taken from
(4) R. Cornet and G. Winnewisser, 1980, J. Mol. Spectrosc. 80, 438;
and from
(5) S. Eliet, A. Cuisset, M. Guinet, F. Hindle, G. Mouret, R. Bocquet, J. Demaison, 2012, J. Mol. Spectrosc. 279, 12.
Transition frequencies with K_a = 1 and resolved ¹H hyperfine splitting were taken from
(6) K. D. Tucker, G. R. Tomasevich, and P. Thaddeus, 1971, Astrophys. J. 169, 429;
and from
(7) K. D. Tucker, G. R. Tomasevich, and P. Thaddeus, 1972, Astrophys. J. 174, 463.
Additional hyperfine splitting at higher K_a as well as center frequencies were reported by
(8) J. C. Chardon and D. Guichon, 1973, J. Phys. 34, 791.
Further data were taken from
(9) J. C. Chardon and D. Guichon, 1977, J. Phys. 38, 113;
from
(10) J. C. Chardon and J. J. Miller, 1981, Can. J. Phys. 59, 378;
from
(11) M. Takami, 1968, J. Phys. Soc. Japan 24, 372;
from
(12) D. R. Johnson, F. Lovas, and W. H. Kirchhoff, 1972, J. Phys. Chem. Ref. Data 1, 1011;
and from
(13) D. Dangoisse, E. Willemot, and J. Bellet, 1978, J. Mol. Spectrosc. 71, 414.
Extensive infrared ground state combination differences were also used in the fit; they were reported in
(14) H. S. P. Müller, G. Winnewisser, J. Demaison, A. Perrin, and A. Valentin, 2000, J. Mol. Spectrosc. 200, 143.
Lines with uncertainties of larger than 100 kHz measured below 1000 GHz as well as lines with uncertainties larger than 200 kHz above 1000 GHz have not been merged because of the large number of lines with smaller uncertainties.
The present data set improves in particular the prediction of a-type R-branch transitions with high K_a. The predictions should be sufficiently accurate for all astronomical purposes. Predictions with calculated uncertainties of larger than 200 kHz should be viewed with caution.
The ¹H hyperfine splitting can be resolved for Q-branch transitions with low values of J and K_a = 1. Transitions with K_a = 2 do not show hyperfine splitting. Those with K_a = 3 are likely too high in energy. The entry also includes now R-branch transitions with J up to 5 and K_a = 1. A separate hyperfine calculation is provided. The partition function takes into account the hyperfine splitting.
At low temperatures, it may be necessary to discern between ortho-H₂CO and para-H₂CO. 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-H₂CO and para-H₂CO, respectively. The J_KaKc = 1₁₁ is the lowest ortho state. It is 10.5390 cm^–1 above ground.
Separate para and ortho predictions are available along with separate para and ortho partition function values.
The dipole moment was measured by
(15) B. Fabricant, D. Krieger, and J. S. Muenter, 1977, J. Chem. Phys. 67, 1576.