Context :

Aims : 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 confidence of the assignment to ArH ^ { + } is discussed , and the column densities are determined toward several lines of sight .
The results are then discussed in the framework of chemical models , with the aim of explaining the observed column densities .

Methods : We fitted the spectral lines with multiple velocity components , and determined column densities from the line-to-continuum ratio .
The column densities of ArH ^ { + } we compared to those of other species , tracing interstellar medium ( ISM ) components with different H _ { 2 } abundances .
We constructed chemical models that take UV radiation and cosmic ray ionization into account .

Results : Thanks 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 ^ { + } ] \leq 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 to H _ { 2 } O ^ { + } , which traces gas with a fractional H _ { 2 } abundance of \approx 0.1 .

Conclusions : 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 by providing unique information on a poorly known phase in the cycle of interstellar matter and on 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 characterizing the ISM .
Paradoxically , the ArH ^ { + } molecule is a better tracer of almost purely atomic hydrogen gas than H i itself , since H i can also be present in gas with a significant molecular content , but ArH ^ { + } singles out gas that is > 99.9 % atomic .