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P. P. Sorokin (IBM Research Division), J. H. Glownia (IBM Research Division)
It is shown that, under sufficiently intense OB-star illumination of a stationary photoexcitation front (PDR), the transition rates of nonlinear H 2 photoexcitation processes can far exceed the rates of H 2 linear photoexcitation. A one-dimensional PDR, irradiated by light from a 0.1-pc-distant B0 III star (T ~q 31,500\circK, R ~q 16R \odot), is considered. Ionizing radiation from the star creates a thin H II region on the PDR surface. Within this H II region, roughly two-thirds of the incident ionizing photons are converted into Ly-\alpha photons, with frequencies spread out to an estimated 20-cm-1 width via elastic scattering by H atoms occurring in the ionized region. It is assumed that half of these photons enter the neutral region and are thus able to drive two Ly-\alpha-resonant, Inverse Raman Scattering (IRS) processes which result in light being nonlinearly absorbed around the transitions B9-0P1 and B3-0R1. The total rate of nonlinear photoexcitation of an H2 molecule in (X0, J\prime\prime=1) via the above two IRS processes is calculated to be about 2400 times greater than the total rate of linear photoexcitation of an H2 molecule in the same (X0, J\prime\prime =1) quantum level. No additional photonic mechanisms (\textit{e.g.} trapping of photons via the combined effects of elastic scattering and diffusion) are invoked to enhance the Ly-\alpha photon density in the model.