Foreign gas relaxation of the J=0→1 transition of HC15N. A study of the temperature dependence by coherent transients

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Abstract
The temperature dependence of the foreign gas relaxation of hydrogen cyanide, one of the best candidate for radioastronomical observations in planetary atmospheres, is investigated in the 135–300 K range. The measurement process exploits the delayed nutation phenomenon which leads to the determination of the population relaxation. This coherent transient technique avoids any deconvolution of the Doppler effect and a novel pulse sequence which alleviates the difficulties related to the finite saturation actually achieved is proposed. The great sensitivity of the method requires active gas pressures (≲10−4 Torr) much lower than in linewidth techniques, and thus results in an extension by about 60 K of the lower bound of attained temperatures. The millimeter Stark spectrometer used, driven by a microprocessor system, is temperature controlled by a cold gaseous nitrogen flow; its careful design allows a strong reduction of thermal transpiration effects, and thus leads to proper pressure measurements for working pressures in the 10−3–10−2 Torr range. Measurements are made on the J=0→1 transition of HC15N with N2, H2, D2, Ar, and He as buffer gases. The usual T−n temperature dependence of relaxation rates is deduced, but departures are observed at temperatures lower than 160 K for the diatomic perturbers. By comparison with previously published works, neither isotopic nor vibrational dependences are observed, and population and coherence relaxation rates are similar. Experimental results are fairly reproduced by numerical calculations made in the frame of the Anderson–Tsao–Curnutte theory, which allows theoretical predictions for other HCN rotational lines of planetary interest.