CO/H2, C/CO, OH/CO, and OH/O2 in Dense Interstellar Gas: From High
  Ionization to Low Metallicity
  We present numerical computations and analytic scaling relations for
interstellar ion-molecule gas phase chemistry down to very low metallicities ($
10^{-3} \times$ solar), and/or up to high driving ionization rates. Relevant
environments include the cool interstellar medium (ISM) in low-metallicity
dwarf galaxies, early enriched clouds at the reionization and Pop-II star
formation era, and in dense cold gas exposed to intense X-ray or cosmic-ray
sources. We focus on the behavior for H$_2$, CO, CH, OH, H$_2$O and O$_2$, at
gas temperatures $\sim 100$ K, characteristic of a cooled ISM at low
metallicities. We consider shielded or partially shielded one-zone gas parcels,
and solve the gas phase chemical rate equations for the steady-state
"metal-molecule" abundances for a wide range of ionization parameters,
$\zeta/n$, and metallicties, $Z'$. We find that the OH abundances are always
maximal near the H-to-H$_2$ conversion points, and that large OH abundances
persist at very low metallicities even when the hydrogen is predominantly
atomic. We study the OH/O$_2$, C/CO and OH/CO abundance ratios, from large to
small, as functions of $\zeta/n$ and $Z'$. Much of the cold dense ISM for the
Pop-II generation may have been OH-dominated and atomic rather than
CO-dominated and molecular.
