A multi-wavelength observation and investigation of six infrared dark
  clouds
  Context. Infrared dark clouds (IRDCs) are ubiquitous in the Milky Way, yet
they play a crucial role in breeding newly-formed stars.
  Aims. With the aim of further understanding the dynamics, chemistry, and
evolution of IRDCs, we carried out multi-wavelength observations on a small
sample.
  Methods. We performed new observations with the IRAM 30 m and CSO 10.4 m
telescopes, with tracers ${\rm HCO^+}$, HCN, ${\rm N_2H^+}$, ${\rm C^{18}O}$,
DCO$^+$, SiO, and DCN toward six IRDCs G031.97+00.07, G033.69-00.01,
G034.43+00.24, G035.39-00.33, G038.95-00.47, and G053.11+00.05.
  Results. We investigated 44 cores including 37 cores reported in previous
work and seven newly-identified cores. Toward the dense cores, we detected 6
DCO$^+$, and 5 DCN lines. Using pixel-by-pixel spectral energy distribution
(SED) fits of the $\textit{Herschel}$ 70 to 500 $\mu$m, we obtained dust
temperature and column density distributions of the IRDCs. We found that ${\rm
N_2H^+}$ emission has a strong correlation with the dust temperature and column
density distributions, while ${\rm C^{18}O}$ showed the weakest correlation. It
is suggested that ${\rm N_2H^+}$ is indeed a good tracer in very dense
conditions, but ${\rm C^{18}O}$ is an unreliable one, as it has a relatively
low critical density and is vulnerable to freezing-out onto the surface of cold
dust grains. The dynamics within IRDCs are active, with infall, outflow, and
collapse; the spectra are abundant especially in deuterium species.
  Conclusions. We observe many blueshifted and redshifted profiles,
respectively, with ${\rm HCO^+}$ and ${\rm C^{18}O}$ toward the same core. This
case can be well explained by model "envelope expansion with core collapse
(EECC)".
