The detection of argonium (ArH+ or protonated argon) was reported in 2013 by
Barlow et al. toward the
Crab Nebula. On the basis the new experimental data of
Cueto et al., two rotational lines of
ArH+ were identified in spectra collected by the
Herschel space observatory. The argonium detection has already been
confirmed by the observations of Schilke et al.
It has also been observed in the extragalactic source
PKS 1830-211 a quasar, by
Müller et al..
Argonium is the only protonated rare gas molecule to be detected to date in
space: neither protonated helium
nor protonated neon have been
observed. The animation depicts the bond formation of ArH+ via
dative bonding: the bare
proton (with an empty 1s orbital) draws some of the charge of one of the three Ar 3p2
orbitals onto itself at short separations.