Anomalous intensities in the infrared emission of CH$^+$ explained by quantum nuclear motion and electric dipole calculations

The unusual infrared emission patterns of CH$^+$, recently detected in the planetary nebula NGC 7027, are examined theoretically with high-accuracy rovibrational wavefunctions and $ab$ $initio$ dipole moment curves. The calculated transition dipole moments quantitatively reproduce the observed $J$-dependent intensity variation, which is ascribed to underlying centrifugal distortion-induced interference effects. We discuss the implications of this anomalous behavior for astrochemical modeling of CH$^+$ production and excitation, and provide a simple expression to estimate the magnitude of this effect for other light diatomic molecules with small dipole derivatives.