Laboratory spectroscopic study and astronomical detection of
  vibrationally excited n-propyl cyanide
  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. 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. 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).
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.
