Gay, C. D. ; Abel, N. P. ; Porter, R. L. ; Stancil, P. C. ; Ferland, G. J. ; Shaw, G. ; van Hoof, P. A. M. ; Williams, R. J. R.

published in The Astrophysical Journal, 746, pp. 78 (2012)

Abstract: 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 Å 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.

DOI: 10.1088/0004-637X/746/1/78
Links: link