Spectra of rotational transitions for diatomic and triatomic molecules in the THz region

Using the widely-tunable monochromatic THz source developed by us recently, we have directly measured the transmission spectra of a few diatomic and triatomic molecules in the THz region. In addition, using the FTIR acquired from Bruker Optics, we have also measured the transmission spectra of the same molecules. As a result, we have observed distinct ro-vibrational and rotational transitions in two carbon monoxide isotopic variants, nitrogen dioxide, and carbon dioxide. After carefully analyzing these transmission spectra, we have identified a series of the transition peaks for each chemical species. The frequencies of the transition peaks measured by using two different methods agree with each other well. In order to determine the rotational constants for each chemical species, it is essential for us to accurately measure the transmission spectrum of the vapor of the chemical species in both of the mid-IR and THz regions.