We use the absorption features from the resonance transitions of neutral argon at 1048 and 1066 Å to determine interstellar argon abundances or their lower limits toward nine early-type stars .
These features were observed with the Interstellar Medium Absorption Profile Spectrograph ( IMAPS ) along sight lines with low reddening and low fractional abundances of molecular hydrogen .
Four of the sight lines in our sample have reliably measured interstellar Ar abundances , of which three also have good determinations of N ( { H~ { } I } ) .
We find that the interstellar Ar is below its solar and B-star abundance with respect to hydrogen toward \zeta Pup , \gamma ^ { 2 } Vel and \beta Cen A with ( logarithmic ) reduction factors D = -0.37 \pm 0.09 { dex } , D = -0.18 \pm 0.10 { dex } , and D = -0.61 \pm 0.12 { dex } , respectively .

While Ar can condense onto the surfaces of dust grains in the interiors of dense clouds , it is unlikely that argon atoms are depleted by this process in the low-density lines of sight considered in this study .
Instead , we propose that the relatively large photoionization cross section of Ar makes it much easier to hide in its ionized form than H. In regions that are about half ionized , this effect can lower Ar I/H I by -0.11 to -0.96 { dex } , depending on the energy of the photoionizing radiation and its intensity divided by the local electron density .
We apply this interpretation for the condition of the gas in front of \beta Cen A , which shows the largest deficiency of Ar .
Also , we determine the expected magnitudes of the differential ionizations for He , N , O , Ne and Ar in the partly ionized , warm gas in the local cloud around our solar system .
For the local cloud and others that can be probed by UV studies , the observed Ar I to H I ratio may be a good discriminant between two possible alternatives , collisional ionization or photoionization , for explaining the existence of partly ionized regions .