摘要：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.
