The TMRT K Band Observations towards 26 Infrared Dark Clouds: NH$_{3}$,
  CCS, and HC$_{3}$N
  We present one of the first Shanghai Tian Ma Radio Telescope (TMRT) K Band
observations towards a sample of 26 infrared dark clouds (IRDCs). We observed
the (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 survey
dramatically 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 group
ratio (HFGR) method of fitting NH$_{3}$ inversion lines, we found the gas
temperature to be between 10 and 18 K. The column density ratios of CCS to
NH$_{3}$ for most of the IRDCs are less than 10$^{-2}$, distinguishing IRDCs
from low-mass star-forming regions. We carried out chemical evolution
simulations based on a three-phase chemical model NAUTILUS. Our measurements of
the column density ratios between CCM and NH$_{3}$ are consistent with chemical
evolutionary ages of $\lesssim$10$^{5}$ yr in the models. Comparisons of the
data and chemical models suggest that CCS, HC$_{3}$N, and NH$_{3}$ are
sensitive to the chemical evolutionary stages of the sources.
