The earliest detections of interstellar and cometary species were made via electronic
spectroscopy in the
region rather than rotational spectroscopy in the microwave and sub-mm regions. The
brief report in 1941 of the
identification of CH+ by Alex Douglas and
Gerhard Herzberg alludes to
an informal gathering at Yerkes Observatory
at which Pol Swings reported that several lines
near 4000 Å could not be matched with known spectra.
The first two lines, at 4233 and 3958Å, had been observed by
Dunham toward three stars: ξ Persei,
χ2 Orionsis, and 55 Cygni. Subsequently,
Adams observed both of these lines
and a third, fainter one at 3579 Å in ζ Ophiuchi (see p. 23). All of these bands lie in the
violet region of visible light.
At the Yerkes meeting, Herzberg and Edward Teller
noted the similarity of these lines to the isoelectronic
species BH and suggested that
CH+ could be the source of the lines. New spectra taken by Douglas and Herzberg at the
University of Saskatchewan confirmed the identification.
The lines are associated with excitations between the ground vibration level of the 1Σ+
ground state of CH+ and the first three vibrational levels the 1Π excited state, the
(0,0), (1,0), and (2,0) transitions. Ledoux
described parts of this story in an article in Popular Astronomy.
Later, Swings used the same spectra
to determine that CH+ was also present in comets. In 1987,
Magain & Gillet reported the detection of
CH+ in the Large Magellanic Cloud.
CH+ has also been detected with rotational spectroscopy by
Cernicharo et al.