OH Evolution in Molecular Clouds
  We have conducted OH 18 cm survey toward 141 molecular clouds in various
environments, including 33 optical dark clouds, 98 Planck Galactic cold clumps
(PGCCs) and 10 Spitzer dark clouds with the Arecibo telescope. The deviations
from local thermal equilibrium are common for intensity ratios of both OH main
lines and satellite lines. Line intensity of OH 1667 MHz is found to correlate
linearly with visual extinction $A\rm_V$ when $A\rm_V$ is less than 3 mag. It
was converted into OH column density by adopting excitation temperature derived
from Monte Carlo simulations with one sigma uncertainty. The relationship
between OH abundance $X$(OH) relative to H$_2$ and $A\rm_V$ is found to follow
an empirical formula,
  \begin{equation} \nonumber \frac{X(\textrm{OH})}{10^{-7}} = 1.3^{+0.4}_{-0.4}
+ 6.3^{+0.5}_{-0.5}\times
\textrm{exp}(-\frac{A_\textrm{V}}{2.9^{+0.6}_{-0.6}}). \end{equation} Linear
correlation is found between OH and $^{13}$CO intensity. Besides, nonthermal
velocity dispersions of OH and $^{13}$CO are closely correlated. These results
imply tight chemical evolution and spatial occupation between OH and $^{13}$CO.
No obvious correlation is found between column density and nonthermal velocity
dispersion of OH and HI Narrow Self-Absorption (HINSA), indicating different
chemical evolution and spatial volume occupation between OH and HINSA. Using
the age information of HINSA analysis, OH abundance $X$(OH) is found to
increase linearly with cloud age, which is consistent with previous
simulations. Fourteen OH components without corresponding CO emission were
detected, implying the effectiveness of OH in tracing the `CO-dark' molecular
gas.
