Differentiation of CO isotopic variants by frequency tuning a terahertz source

The transmission spectra of two CO isotopic variants, i.e., (12)CO and (13)CO, are measured with a recently developed widely tunable THz source. The pure rotational transition lines of J=6 --> 7, 10 --> 11, 11 --> 12, 12 --> 13, and 13 --> 14 have been identified in the spectra. The transition frequencies and the rotational constant of each CO isotopic variant are also presented. Experimental results indicate that the difference of the rotational constants between the two isotopic variants can be used to reliably differentiate (12)CO and (13)CO. Compared with the measurements made by using a Fourier transform infrared spectrometer, our tunable THz source has produced approximately the same accuracies for measuring transition frequencies or determining rotational constants.