Observations and analysis of CH$^+$ vibrational emissions from the
  young, carbon-rich planetary nebula NGC 7027: a textbook example of chemical
  pumping
  We discuss the detection of 14 rovibrational lines of CH$^+$, obtained with
the iSHELL spectrograph on NASA's Infrared Telescope Facility (IRTF) on
Maunakea. Our observations in the 3.49 - 4.13 $\mu$m spectral region, obtained
with a 0.375" slit width that provided a spectral resolving power
$\lambda/\Delta \lambda \sim 80,000$, have resulted in the unequivocal
detection of the $R(0) - R(3)$ and $P(1)-P(10)$ transitions within the $v=1-0$
band of CH$^+$. The $R$-branch transitions are anomalously weak relative to the
$P$-branch transitions, a behavior that is explained accurately by rovibronic
calculations of the transition dipole moment reported in a companion paper
(Changala et al. 2021). Nine infrared transitions of H$_2$ were also detected
in these observations, comprising the $S(8)$, $S(9)$, $S(13)$ and $S(15)$ pure
rotational lines; the $v=1-0$ $O(4) - O(7)$ lines, and the $v=2-1$ $O(5)$ line.
We present a photodissociation region model, constrained by the CH$^+$ and
H$_2$ line fluxes that we measured, that includes a detailed treatment of the
excitation of CH$^+$ by inelastic collisions, optical pumping, and chemical
("formation") pumping. The latter process is found to dominate the excitation
of the observed rovibrational lines of CH$^+$, and the model is remarkably
successful in explaining both the absolute and relative strengths of the CH$^+$
and H$_2$ lines.
