Rotational spectroscopy of urea up to 500 GHz: The ground and eight excited vibrational states

Urea, ((NH2)2CO) is an important industrial and biological molecule. The new generation of the submillimeter telescopes, in particular ALMA, allows detection of extraterrestrial molecular spectra at unprecedented spatial and spectral resolutions. Some of the data presented here facilitated the recent definitive identification of urea in the interstellar medium (Belloche et al. Astron. & Astrophys. 628 (2019) A10:1-62). Its detection yields valuable insight into the mechanisms governing formation of complex prebiotic interstellar molecules. This paper reports spectroscopic analysis of the broadband rotational spectra of urea in the 210–500 GHz spectral range for the vibrational ground state and eight lowest excited vibrational states. The lowest excited vibrational state is only 61 cm−1 above the ground state and could be a useful temperature marker for astrophysical urea. The next two lowest pairs of excited states show significant interstate coupling, which has been successfully fitted with a Coriolis coupling model. Higher vibrational states also show evidence of significant coupling. Intensities of some rotational transitions of urea display nuclear spin statistical weights arising from the equivalence of the two ammonia groups, which enabled the assignment of symmetric and antisymmetric vibrational symmetry to the six lowest studied excited states. The vibrational energies of the new states have been determined from extensive relative intensity measurements and, together with vibrational symmetry, give insight into the highly anharmonic vibrational energy level structure.