Investigation of unknown quantum states by using a wideband passive spectroscopy at terahertz frequency region

We have applied a laboratory passive spectroscopy at terahertz frequency region to the molecule whose precise energy level structure is hard to obtain. We chose methyl formate for the present study. The molecule is known to have low energy torsional modes and to give complex spectral features. The spectral assignment is underway and plenty of unassigned spectral lines are observed. We measured absolute spectral intensity of the selected lines of the molecule in the ground states at given temperature and pressure. We estimated an 'effective partition function' of the molecule at our observed temperature with this result, and found that the value is consistent with the calculated energy structure derived by a high-level ab initio calculation. The present information will help us to obtain an abundance of the methyl formate in interstellar space by astronomical observation.