  Astronomical observations toward Sagittarius B2(M) as well as other sources
with APEX have recently suggested that the rest frequency of the J = 1 - 0
transitions of 13CH+ is too low by about 80 MHz. Improved rest frequencies of
isotopologs of methylidynium should be derived to support analyses of spectral
recording obtained with the ongoing Herschel mission or the upcoming SOFIA.
Laboratory electronic spectra of four isotopologs of CH+ have been subjected to
one global least-squares fit. Laboratory data for the J = 1 - 0 ground state
rotational transitions of CH+, 13CH+, and CD+, which became available during
the refereeing process, have been included in the fit as well. An accurate set
of spectroscopic parameters has been obtained together with equilibrium bond
lengths and accurate rest frequencies for six CH+ isotopologs: CH+, 13CH+,
13CD+, CD+, 14CH+, and CT+. The present data will be useful for the analyses of
$Herschel$ or SOFIA observations of methylidynium isotopic species.
