Low‐Temperature Dependence of the Foreign Gas Relaxation of HC3N with Microwave Coherent Transients Induced by Frequency Switching

Abstract The temperature dependence of the rotational foreign gas relaxation of cyanoacetylene HC 3 N is investigated in the 150-300 K range on static gas samples. The experiment exploits the great sensitivity of the coherent transient technique, and a frequency-switched spectrometer has been developed at a 3-mm wavelength: the frequency control setup is described and analyzed from the point of view of coherent transient phenomena. For the J = 11 ← 10 line of HC 3 N, experimental results on the relaxation induced by N 2 , H 2 , and He vs the temperature, and by O 2 and Ar at room temperature, are reported. Using the Anderson-Tsao-Curnutte formalism, the N 2 -induced relaxation of HC 3 N is theoretically predicted for other rotational lines of interest for planetary atmosphere studies.