Accurate rotational rest-frequencies of CH2NH at submillimetre wavelengths

Context. Methanimine (CH₂NH) has been detected in different astronomical sources, both galactic (as in several "hot cores", the circumstellar enevolope IRC+10216, and the L183 pre-stellar core) and extragalactic, and is considered a pre-biotic interstellar molecule. Its ground-state rotational spectrum has been studied in the laboratory up to 172 GHz, well below the spectral ranges covered by Herschel/HIFI and the ALMA bands 9 and 10.
Aims: In this laboratory study, we extend into the submillimetre-wave region the detection of the rotational spectrum of CH₂NH in its vibrational ground state.
Methods: The investigation was carried out using a source-modulation microwave spectrometer equipped with a cell coupled to a pyrolysis apparatus working at 1150 °C. The spectrum was recorded in the frequency range 329-629 GHz, with the detection of 58 transitions.
Results: The newly measured transition frequencies, along with those available from previous microwave studies, allow us to determine fairly accurate rotational constants of CH₂NH and the complete sets of quartic and sextic centrifugal distortion constants, in addition to two octic constants. Several transitions have an hyperfine structure due to the ¹⁴N nucleus, which was accounted for in the analysis.
Conclusions: The determined spectroscopic constants make it possible to build a list of very accurate rest-frequencies for astrophysical purposes in the THz region with 1σ uncertainties lower than 0.01 km s^-1 in radial equivalent velocity.

Table 1 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/544/A19