Laboratory spectroscopic study and astronomical detection of vibrationally excited

Context. We performed a spectral line survey called Exploring Molecular Complexity with ALMA (EMoCA) toward Sagittarius B2(N) between 84.1 and 114.4 GHz with the Atacama Large Millimeter/submillimeter Array (ALMA) in its Cycles 0 and 1. We determined line intensities of n-propyl cyanide in the ground vibrational states of its gauche and anti conformers toward the hot molecular core Sagittarius B2(N2) which suggest that we should also be able to detect transitions pertaining to excited vibrational states. Aims. We wanted to determine spectroscopic parameters of low-lying vibrational states of both conformers of n-propyl cyanide to search for them in our ALMA data. Methods. We recorded laboratory rotational spectra of n-propyl cyanide in two spectral windows between 36 and 127 GHz. We searched for emission lines produced by these states in the ALMA spectrum of Sagittarius B2(N2). We modeled their emission and the emission of the ground vibrational states assuming local thermodynamic equilibrium (LTE). Results. We have made extensive assignments of a- and b-type transitions of the four lowest vibrational states of the gauche conformer which reach J and Ka quantum numbers of 65 and 20, respectively. We assigned mostly a-type transitions for the anti conformer with J and Ka quantum numbers up to 48 and 24, respectively. Rotational and Fermi perturbations between two anti states allowed us to determine their energy difference. The resulting spectroscopic parameters enabled us to identify transitions of all four vibrational states of each conformer in our ALMA data. The emission features of all states, including the ground vibrational state, are well-reproduced with the same LTE modeling parameters, which gives us confidence in the reliability of the identifications, even for the states with only one clearly detected line. Conclusions. Emission features pertaining to the highest excited vibrational states of n-propyl cyanide reported in this work have been identified just barely in our present ALMA data. Features of even higher excited vibrational states may become observable in future, more sensitive ALMA spectra to the extent that the confusion limit will not have been reached. The 13C isotopomers of this molecule are expected to be near the noise floor of our present ALMA data. We estimate that transitions of vibrationally excited iso-propyl cyanide or aminoacetonitrile, for example, are near the noise floor of our current data as well