Ubiquitous argonium (ArH$^+$) in the diffuse interstellar medium -- a
  molecular tracer of almost purely atomic gas
  We describe the assignment of a previously unidentified interstellar
absorption line to ArH$^+$ and discuss its relevance in the context of hydride
absorption in diffuse gas with a low H$_2$ fraction. The column densities along
several lines of sight are determined and discussd in the framework of chemical
models. The column densities of ArH$^+$ are compared to those of other species,
tracing interstellar medium (ISM) components with different H$_2$ abundances.
Chemical models are constructed, taking UV radiation and cosmic ray ionization
into account. Due to the detection of two isotopologues, $^{36}$ArH$^+$ and
$^{38}$ArH$^+$, we are confident about the carrier assignment to ArH$^+$.
NeH$^+$ is not detected with a limit of [NeH$^+$]/[ArH$^+$] $\le$ 0.1. The
derived column densities agree well with the predictions of chemical models.
ArH$^+$ is a unique tracer of gas with a fractional H$_2$ abundance of
$10^{-4}- 10^{-3}$ and shows little correlation with H$_2$O$^+$, which traces
gas with a fractional H$_2$ abundance of $\approx $0.1. A careful analysis of
variations in the ArH$^+$, OH$^+$, H$_2$O$^+$ and HF column densities promises
to be a faithful tracer of the distribution of the H$_2$ fractional abundance,
providing unique information on a poorly known phase in the cycle of
interstellar matter, its transition from atomic diffuse gas to dense molecular
gas traced by CO emission. Abundances of these species put strong observational
constraints upon magnetohydrodynamical (MHD) simulations of the interstellar
medium, and potentially could evolve into a tool to characterize the ISM.
Paradoxically, the ArH$^+$ molecule is a better tracer of \new{almost} purely
atomic hydrogen gas than H{\sc i} itself, since H{\sc i} can also be present in
gas with a significant molecular content, but ArH$^+$ singles out gas that is
$>99.9$\% atomic.
