Cernicharo & Guélin reported the discoveries
of three diatomic chloride speceies, NaCl,
KCl, and AlCl, as well as the tentative detection of aluminum monofluoride (AlF), all in the circumstellar shell
of the carbon star IRC +10216 (also known as CW Leonis).
Although the lines for three successive rotational transitions were observed, the features in question were weak
and not confirmed with isotopic lines, as was the case for AlCl. Confirmation of AlF followed in 1994, when
Ziurys et al. found three additional lines in
the same source using the
Caltech Submillimeter Observatory. AlF was also found in a second
circumstellar shell, CRL 2688 (the Egg Nebula), by
Highberger et al. in 2001.
While aluminium monofluoride
is a closed-shell species (1Σ+ ground state), it is much less stable than trivalent
AlF3. The availability of the 3s2 electrons for bonding is also reflected by the presence
of low-lying 3Π and 1Π excited states of AlF that arise through
recoupled pair bonding.