Several transitions of rf spectrum of K-type doubling of HCOOH were observed, using a molecular beam spectrometer with 1 kHz resolution. The hyperfine structure for about ten transitions is resolved. The spin–rotation coupling constants of the two protons can be deduced. We find Maa1=−7.50±0.2 kHz, Mbb1=−7.2±4 kHz, Mccu1= 7.5±4 kHz, Maau2=−6.55±0.2 kHz, Mbbu2=8.2±4 kHz, Mccu2=−8.6±4kHz. The constants labeled 1 refer to the proton bounded to the oxygen atom.

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Résumé. 2014 Nous présentons d'abord une méthode expérimentale qui permet d'observer les transitions rotationnelles 0394J = ± n (n ~ 1) induites Abstract. 2014 Using a H2CO molecular beam, a method is first described which permits the observation of rotational transitions 0394J = ± n (n ~ 1), induced by collisions with the molecules of a target gas. In the case H2CO H 2CO, selection rules are defined and rotational resonance effects are observed on the differential cross sections for some 0394J = 2014 1 transitions. It appears that a direct electric field E considerably modifies the H2CO cross sections 03C3J. Using the pair H2CO ~ NH3, the study of 03C3J variations with E shows that the 0394J = 0 transitions become forbidden, when E is intense. The experimental results are compared with the theoretical transition probabilities variations, due to E and calculated in the Born approximation. An experimental method is deduced which permits, by application of an electric field, to select AJ = 0 transitions on a molecular beam.

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