The UV resonance transitions of neutral argon Ar i \lambda 1066 Å and of singly ionized phosphorus P ii \lambda 963 Å originated in the damped Ly \alpha system ( DLA ) at z _ { abs } = 3.3901 towards QSO 0000–2620 have been detected by means of the UVES spectrograph at the 8.2m ESO KUEYEN telescope .
So far , this is the first measurement of Ar i and the second of P ii ever performed in damped galaxies and in high redshift objects .
This DLA is well known for having one of the lowest metal abundances and dust content , and the lowest fractional abundance of molecular hydrogen H _ { 2 } .
The measured Ar abundance is [ Ar/H ] = -1.91 \pm 0.09 which is equal to the abundances of the other \alpha -chain elements ( O , S and Si ) .
The similarity of the Ar abundance with the other \alpha -chain elements implies the absence of significant photoionization by either UV background or stellar sources along the sightline throughout the damped Ly \alpha system .

Both \log ( Ar/O ) and \log ( Ar/S ) ratios are found close to those measured in the extragalactic H ii regions and in blue compact galaxies where O is more abundant by at least one order of magnitude .
This strengthens the universality of the Ar/O and Ar/S ratios and lends support to the existence of a universal IMF .

The abundance of the non-refractory element phosphorus [ P/H ] = -2.31 \pm 0.10 confirms the low amount of chemical evolution in the DLA .
This is the measurement of P in the most metal-poor material and shows a subsolar [ P/Fe ] = –0.27 value .
The measured ratios [ P/Si ] = -0.40 \pm 0.13 and [ P/S ] = -0.33 \pm 0.13 provide evidence for a mild odd-even effect .

Finally , a stringent upper limit to the population of the ^ { 3 } P _ { 1 } level in the ground state of O i is derived , which provides a lower limit to the physical dimensions of the z _ { abs } = 3.3901 system of L > 7 pc .