Sub-doppler measurements and terahertz rotational spectrum of C-12(18)-O

The five lowest rotational transitions of (CO)-C-12-O-18 (J = 2 <-- 1 to J= 6 <-- 5) have been measured by saturation dip spectroscopy with an experim ental accuracy of 1 to 1.5 kHz, employing phase-stabilized backward-wave os cillators. The five J rotational transitions cover the frequency range betw een 219 and 658 GHz. In addition, we have measured in the Doppler limited m ode the rotational transitions J = 1 <-- 0, J = 7 <-- 6 and J= 8 <-- 7. The accuracy achieved for the individual frequencies ranges between 5 and 20 k Hz. Moreover the three rotational transitions J = 16 <-- 15 to J = 18 <-- 1 7 in the frequency region 1.7-2.0 THz were measured with an accuracy of 15 to 30 kHz by using the Cologne sideband spectrometer for terahertz applicat ions COSSTA.