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We present one of the first Shanghai Tian Ma Radio Telescope (TMRT) K Bandobservations towards a sample of 26 infrared dark clouds (IRDCs). We observedthe (1,1), (2,2), (3,3), and (4,4) transitions of NH$_{3}$ together with CCS(2$_{1}$-1$_{0}$) and HC$_{3}$N $J\,$=2-1, simultaneously. The surveydramatically increases the existing CCS-detected IRDC sample from 8 to 23,enabling a better statistical study of the ratios of carbon-chain molecules(CCM) to N-bearing molecules in IRDCs. With the newly developed hyperfine groupratio (HFGR) method of fitting NH$_{3}$ inversion lines, we found the gastemperature to be between 10 and 18 K. The column density ratios of CCS toNH$_{3}$ for most of the IRDCs are less than 10$^{-2}$, distinguishing IRDCsfrom low-mass star-forming regions. We carried out chemical evolutionsimulations based on a three-phase chemical model NAUTILUS. Our measurements ofthe column density ratios between CCM and NH$_{3}$ are consistent with chemicalevolutionary ages of $\lesssim$10$^{5}$ yr in the models. Comparisons of thedata and chemical models suggest that CCS, HC$_{3}$N, and NH$_{3}$ aresensitive to the chemical evolutionary stages of the sources.

The TMRT K Band Observations towards 26 Infrared Dark Clouds: NH$_{3}$, CCS, and HC$_{3}$N