Context : Argonium has recently been detected as a ubiquitous molecule in our Galaxy .
Model calculations indicate that its abundance peaks at molecular fractions in the range of 10 ^ { -4 } to 10 ^ { -3 } and that the observed column densities require high values of the cosmic ray ionization rate .
Therefore , this molecular cation may serve as an excellent tracer of the very diffuse interstellar medium ( ISM ) , as well as an indicator of the cosmic ray ionization rate .

Aims : We attempted to detect ArH ^ { + } in extragalactic sources to evaluate its diagnostic power as a tracer of the almost purely atomic ISM in distant galaxies .

Methods : We obtained ALMA observations of a foreground galaxy at z = 0.89 in the direction of the lensed blazar PKS 1830 - 211 .

Results : Two isotopologs of argonium , ^ { 36 } ArH ^ { + } and ^ { 38 } ArH ^ { + } , were detected in absorption along two different lines of sight toward PKS 1830 - 211 , known as the SW and NE images of the background blazar .
The argonium absorption is clearly enhanced on the more diffuse line of sight ( NE ) compared to other molecular species .
The isotopic ratio ^ { 36 } Ar/ ^ { 38 } Ar is 3.46 \pm 0.16 toward the SW image , i.e. , significantly lower than the solar value of 5.5 .

Conclusions : Our results demonstrate the suitability of argonium as a tracer of the almost purely atomic , diffuse ISM in high-redshift sources .
The evolution of the isotopic ratio with redshift may help to constrain nucleosynthetic scenarios in the early Universe .