Recent Searches for the Radio Lines of NH_3 in Comets
  Radio observations in the ammonia inversion lines of four comets, C/2001 A2 (LINEAR), 153P/Ikeya-Zhang, C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR), were performed at the Effelsberg 100-m Radio Telescope during their respective close approaches to Earth. None of the four lowest energy metastable lines (J,K=J),J=1--4, could be detected in these comets. We derive the following 3 sigma upper bounds on the NH_3 production rate, and c omparing to the corresponding water production rates, percentage NH_3 abundances relative to H_2O:   Q(NH_3) < 1.9x10^26 s^-1 (0.63%) for C/2001 A2 (LINEAR),   Q(NH_3) < 2.7x10^26 s^-1 (0.13%) for C/2001 Q4 (NEAT),   Q(NH_3) < 2.3x10^27 s^-1 (0.74%) for C/2002 T7 (LINEAR) and Q(NH_3) <= 6.3x10^26 s^-1 (0.63%) for Comet 153P/Ikeya-Zhang.   At 0.74% or less, the ammonia-to-water ratios are factors of 2 below the value for C/1995 O1 (Hale-Bopp) and 1P/Halley, suggesting chemical diversity between comets. The 18-cm lines of OH were clearly detected in the two comets observed during the 2004 campaign, thereby validating the cometary ephemerides. 
J. Hatchell, M.K. Bird, F.F.S. van der Tak, W.A. Sherwood