H2 Photoexcitation Mechanisms in Reflection Nebulae: Analysis of the 1000-2000 Å Emission Spectrum of IC 63

A detailed examination of the 1000-2000 Å ORFEUS II spectrum of the reflection of nebula IC 63 reveals that most of the H₂ VUV emission bands that are observed in this spectrum cannot satisfactorily be explained on the basis of standard linear photoexcitation theory. A novel H₂ non-linear photoexcitation model is proposed, based upon physical principles governing the interaction of light with atoms and molecules in collisionless media. It is presupposed that various stages (or steps) of H₂ photoexcitation can occur in reflection nebulae cloudlets, which are modelled to contain both H atoms and H₂ molecules. In a necessary first step, the density of photons within the natural linewidth of Ly-a becomes enormously enhanced throughout the cloudlet, the combined result of resonant linear elastic scattering by H atoms and diffusion. In a next stage, the photon densities at a few select "primary" frequencies become enormously enhanced within the cloudlet via the combined effects of non-linear elastic scattering by H₂ molecules and diffusion. The non-linear elastic scattering processes are based upon resonant two-quantum-driven transitions, in which one driving field is the greatly enhanced photon density at Ly-a. It is proposed that H₂ IR vibrational emission, which is ubiquitously observed to be radiated from reflection nebulae, is powered during this stage via such two-quantum-driven transitions. A final stage of photoexcitation can occur when the photon densities at some of the "primary" frequencies become sufficiently enhanced to pump beyond threshold resonantly-enhanced simulated Raman scattering (SRS) processes, leading to coherent generation of light on several transitions occurring both in the IR and in the VUV. The observed ORFEUS II IC 63 emission bands can readily be assigned to the latter.