Knowledge Bank

The millimeter-wave spectrum of tricarbon oxide sulfide, $C3OS$, has been recorded continuously between 78.3 GHz and 118.1 GHZ with average detection sensitivity of around $10-7cm-1$. The measurements were made possible with a newly designed millimeter-wave synthesizer provided by the company Kvarz, Nizhny Novgorod, Russia. The instrument features a backward wave oscillator tube as a source of radiation. The output frequency is phase-locked to an external quarz oscillator stabilised on a standard radio signal. All settings of the instrument (frequency increments, number of frequency steps per second etc.) and the data acquisition are controlled by a personal computer. The stability of the output signal in frequency, amplitude and phase allows us to subtract the background spectrum, recorded separately with the absorption cell empty, from the sample spectrum. The difference yields a spectrum with a very flat zero-level. The measured frequency range covers the rotational spectrum of the linear molecule $C3OS$ starting at $J=28\leftarrow 27$ up to $J=41\leftarrow 40$. About 5000 pure rotational transitions have been recorded in this range. The spectrum is dominated by rotational transitions arising from excited states of the low-lying bending vibration $\nu 7$ around $78cm-1$. Rotational satellites of hotbands up to $\nu 7=9$ and many combination bands, where this vibrational mode is involved, can easily be detected. This explains the large number of rotational transitions for this linear molecule. The experimental conditions in recording the spectra and a first analysis of the $C2OS$ data will be presented.