Using ab initio potential curves and dipole transition moments, cross-section calculations were performed for the direct continuum photodissociation of H2 through the B1Σ+uX1Σ+g (Lyman) and C1ΠuX1Σ+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.

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Published in The Astrophysical Journal, v. 746, no. 1, 78, p. 1-8.

© 2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

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We acknowledge support from NASA grant NNG06GJ11G from the Astrophysics Theory Program (C.D.G., P.C.S., G.J.F.) and program number HST-AR-11776.01-A (P.C.S., N.P.A.), which was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. P.A.M.v.H. acknowledges support from the Belgian Federal Science Policy Office via the PRODEX Programme of ESA.