rf spectrum and hyperfine structure of formic acid

Abstract

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.

Hyperfine structure in the rotational spectra of trans-formic acid: Lamb-dip measurements and quantum-chemical calculations

“…Due to the accuracy of the Lamb-dip measurements carried out in the millimeter-and submillimeter-wave regions and thanks to the improved THz measurements (i.e., the uncertainties have been reduced from from 5 MHz to 50−75 kHz), we consider our rotational parameters (in particular, the rotational constants) more reliable. Chardon et al (1976) included in the fit. It is evident from the results for hyperfine parameters reported in Table 1 why the results from Chardon et al (1976) predict such overly large hyperfine structures, as seen in Fig.…”

“…Two different fits were carried out: the first one including our measurements together with the radio-frequencies (rf) recorded by means of a molecular-beam spectrometer by Chardon et al (1976), and the second one also including the submillimeter-wave and THz measurements from Winnewisser et al (2002). Both fits have been carried out with Pickett's SPCAT/SPFIT suite of programs (Pickett 1991).…”

“…Their analysis led to all the diagonal terms of the spin-rotation tensor for the two hydrogens, but only the two C aa constants were actually determined to a good accuracy. As explained later in the text, the spin-rotation constants of Chardon et al (1976) provide erroneous predictions for the hyperfine structure in the millimeter-wave frequency range. More precisely, according to the values by Chardon et al (1976), symmetric splittings of tens of kHz were expected in the millimeter-wave region, while smaller asymmetric splittings have actually been observed.…”

“…As explained later in the text, the spin-rotation constants of Chardon et al (1976) provide erroneous predictions for the hyperfine structure in the millimeter-wave frequency range. More precisely, according to the values by Chardon et al (1976), symmetric splittings of tens of kHz were expected in the millimeter-wave region, while smaller asymmetric splittings have actually been observed. To clarify this point, state-of-theart quantum-chemical calculations of the hyperfine parameters involved have been carried out.…”

“…We also refer the reader to Winnewisser et al (2002) for an exhaustive description of previous laboratory A&A 520, A64 (2010) measurements. There is only one study of the hyperfine structure of the rotational spectrum reporting radio-frequencies measured with a molecular-beam spectrometer with an accuracy of 0.15 kHz (Chardon et al 1976). Their analysis led to all the diagonal terms of the spin-rotation tensor for the two hydrogens, but only the two C aa constants were actually determined to a good accuracy.…”