Temperature-dependent, rovibrationally resolved UV photodissociation cross sections of CH including multichannel nonadiabatic interactions

Abstract

The CH radical is ubiquitous across UV-irradiated astronomical environments, and its photodissociation represents one of the principal destruction pathways in regions exposed to ultraviolet radiation. Reliable photodissociation cross sections are therefore required for astrochemical modeling and the interpretation of UV--optical spectra. We present rovibrationally resolved, temperature-dependent photodissociation cross sections of CH obtained using a local coupled-channel framework that explicitly incorporates nonadiabatic interactions among the relevant excited electronic states. High-level multi-reference single- and double-excitation configuration interaction ab initio calculations were performed to obtain the potential energy curves, transition dipole moments, and radial and rotational nonadiabatic coupling matrix elements of CH. The adiabatic electronic-structure data were transformed into a diabatic representation, and local coupled photodissociation cross sections were calculated by including the couplings within the excited-state manifold while neglecting the weaker couplings between the ground and excited electronic states. Assuming local thermodynamic equilibrium, thermally averaged cross sections were obtained by weighting the contributions from the populated rovibrational levels of the ground state 1^2Π over a wide temperature range. The coupled-channel calculations show that nonadiabatic interactions introduce pronounced Feshbach resonance structures into the CH photodissociation spectra, particularly at low and intermediate temperatures. These Feshbach resonance structures are absent from the corresponding uncoupled calculations, demonstrating the importance of channel coupling for a detailed description of the UV cross sections. At high temperatures, thermal averaging over many populated rovibrational levels progressively smooths the narrow resonance structures, reducing the net difference between the uncoupled and locally coupled cross sections. A comparison with previous datasets shows good overall agreement over much of the wavelength range, while also revealing an additional resonance feature associated with the 1^2Σ^- excited state. The resulting dataset complements existing CH photodissociation data by providing rovibrationally resolved, temperature-dependent cross sections for astrochemical modeling and astrophysical spectral simulations.