Precise Measurements of CH Maser Emission and Its Abundance in
  Translucent Clouds
  We present high-sensitivity CH 9 cm ON/OFF observations toward 18
extra-galactic continuum sources that have been detected with OH 18 cm
absorption in the Millennium survey with the Arecibo telescope. CH emission was
detected toward six of eighteen sources. The excitation temperature of CH has
been derived directly through analyzing all detected ON and OFF velocity
components. The excitation temperature of CH 3335 MHz transition ranges from
$-54.5$ to $-0.4$ K and roughly follows a log-normal distribution peaking
within [$-$5, 0] K, which implies overestimation by 20% to more than ten times
during calculating CH column density by assuming the conventional value of
$-60$ or $-10$ K. Furthermore, the column density of CH would be underestimated
by a factor of $1.32\pm 0.03$ when adopting local thermal equilibrium (LTE)
assumption instead of using the CH three hyperfine transitions. We found a
correlation between the column density of CH and OH following log$N$(CH) =
(1.80$\pm$ 0.49) log$N$(OH) $-11.59 \pm 6.87$. The linear correlation between
the column density of CH and H$_2$ is consistent with that derived from visible
wavelengths studies, confirming that CH is one of the best tracers of H$_2$
component in diffuse molecular gas.
