THz measurements of propane

Propane is present in many planetary atmospheres, as confirmed by vibrational spectroscopy in the infrared wavelengths. As the simplest alkane with a permanent dipole moment, propane has a rotational spectrum. The relatively light atom framework causes the strongest rotational transitions to appear in the submillimeter wavelength range. This fact, as well as the small dipole moment, have prevented the molecule from being observed in the interstellar medium. Telescopes with high sensitivity at high altitude such as the Atacama Large Millimeter Array or above the Earth's atmosphere like the Herschel Space Observatory are likely to be sensitive enough for detection of this species in the interstellar medium, planetary nebulae, hot cores and/or planetary atmospheres. We present the rotational spectra of propane in its ground and first two excited vibrational states, measured through 1.6 THz. The ground state submillimeter spectrum is approximated well by semi-rigid rotor. The submillimeter data has been combined with high resolution centimeter wavelength data to enable analysis of the equivalent dual-hindered rotor torsional substates using a simplified version of a symmetric two-top Hamiltonian. The same model has been applied to the torsionally excited states that experience more large-amplitude motion and require additional tunnelling parameters.