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

TL;DR

This paper explains the unusual infrared emission intensities of CH$^+$ in a planetary nebula through high-accuracy quantum calculations, revealing interference effects caused by centrifugal distortion.

Contribution

It introduces a theoretical approach combining rovibrational wavefunctions and ab initio dipole moments to explain anomalous emission intensities in CH$^+$.

Findings

Reproduces observed J-dependent intensity variations quantitatively.

Identifies centrifugal distortion-induced interference as the cause.

Provides a simple estimate for similar effects in other molecules.

Abstract

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.