Lamb-dip spectrum of methylacetylene and methyldiacetylene: precise rotational transition frequencies and parameters of the main isotopic species

Context. Methylacetylene and methyldiacetylene are the first members of the methylpolyynes series, CH3C2nH (n = 1, 2). Their astrophysical relevance has motivated this spectroscopic study. Aims. The aim of this investigation is to provide very accurate rest frequencies, as well as to improve the spectroscopic parameters available in the literature for these molecules. Methods. The Lamb-dip technique was exploited in order to record the rotational spectra of CH3CCH and CH3CCCCH at sub-Doppler resolution in the millimeter- and submillimeter-wave frequency ranges. In addition, for CH3CCH rotational transitions in the THz region were recorded at Doppler resolution. Results. We report the most accurate ground-state rotational parameters known at the moment for the main isotopic species of methylacetylene, as well as of methyldiacetylene. Conclusions. Since both molecules are known to be of astrophysical relevance, we are confident that the improvement in the accuracy of the ground-state rotational parameters as well as of the rest frequencies of rotational transitions could be useful for future observations purposes.