The Importance of Nonlinear H2 Photoexcitiation in Strongly Irradiated PDRs

It is shown that, under sufficiently intense OB-star illumination of a stationary photoexcitation front (PDR), nonlinear H2 photoexcitation processes comprising driven resonant two-photon transitions between X-state quantum levels, with VUV continuum light from the star supplying both driving fields, largely determine the photonic pathways of H2 molecules in the PDR close to the ionization front. Specifically, for a PDR irradiated by a 0.1-pc-distant B0 III star, the total rate at which an H2 molecule is nonlinearly photoexcited out of any X-state quantum level is calculated to be roughly 100 times greater than the total rate at which it is linearly photoexcited out of the same level. In strongly excited PDRs, the populations in almost all of the ~300 bound quantum levels of the X state will be maintained approximately equal via a few myriads of interconnecting two-photon steps. The remarkable importance of two-photon transitions in H2 photoexcitation in strongly irradiated PDRs derives from the exceptionally narrow Raman linewidth that characterizes all two-photon transitions between bound H2 X-state quantum levels.