Rovibrationally resolved direct photodissociation through the lyman and werner transitions of H₂ for FUV/X-ray-irradiated environments

Using ab initio potential curves and dipole transition moments, cross-section calculations were performed for the direct continuum photodissociation of H₂ through the B ¹Σ⁺ _u ← X ¹Σ⁺ _g (Lyman) and C ¹Π_u ← X ¹Σ⁺ _g (Werner) transitions. Partial cross-sections were obtained for wavelengths from 100 to the dissociation threshold between the upper electronic state and each of the 301 bound rovibrational levels v″J″ within the ground electronic state. The resulting cross-sections are incorporated into three representative classes of interstellar gas models: diffuse clouds, photon-dominated regions, and X-ray-dominated regions (XDRs). The models, which used the CLOUDY plasma/molecular spectra simulation code, demonstrate that direct photodissociation is comparable to fluorescent dissociation (or spontaneous radiative dissociation, the Solomon process) as an H₂ destruction mechanism in intense far-ultraviolet or X-ray-irradiated gas. In particular, changes in H₂ rotational column densities are found to be as large as 20% in the XDR model with the inclusion of direct photodissociation. The photodestruction rate from some high-lying rovibrational levels can be enhanced by pumping from H Lyβ due to a wavelength coincidence with cross-section resonances resulting from quasi-bound levels of the upper electronic states. Given the relatively large size of the photodissociation data set, a strategy is described to create truncated, but reliable, cross-section data consistent with the wavelength resolving power of typical observations.