Nonlinear H² Photoexcitation Mechanisms in Reflection Nebulae: Analysis of the 1000-1200 Å Emission Spectrum of IC 63

A detailed examination of the 1000-1200 Å ORFEUS II spectrum of the reflection 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, totally resonant, H₂ nonlinear photoexcitation model is proposed, based upon physical principles governing the interaction of light with atoms and molecules in collisionless media. It is postulated that various states (or steps) of H₂ photoexcitation can occur in reflection nebulae, which are here modelled as 'photon-dominated regions' (PDRs, containing both H atoms and H₂ molecules. In the first stage, it is assumed that very large densities of photons at wavelengths near Ly-a become established in the region of the PDR. Via the combined processes of linear elastic scattering by H atoms and diffusion, the densities of Ly-a photons within the PDR cloudlet become greatly enhanced. In a next stage, nonlinear (inverse Raman) absorption of starlight by ground state H₂ molecules at the primary frequency transitions B9-0P1, B6-0P1, B3-0R1, and B3-0R0 is induced to occur in the PDR cloudlet by the presence of the large Ly-a photon densities that are created in the first step. A calculated estimated shows that the cross-section for each of these nonlinear inverse Raman absorptions should exceed the corresponding linear absorption (i.e. resonant Raman scattering) cross-section by many orders of magnitude, at least for illuminating stars which are main sequence B0 or hotter. Since the inverse Raman absorption transition rates for ground state H₂ molecules also greatly exceed the (~10^-6 s^-1) radiative decay rates of vibrationally excited X-state quantum levels, significant populations should be produced in the terminal levels for the inverse Raman absorption processes, i.e., the quantum levels (X5, J" = 1), (X4, J" = 1), X3, J" = 1), and (X3, J" =0). The presence of significant populations in these specific vibrational quantum levels should allow, in turn, a powerful nonlinear elastic scattering mechanism to be launched, since the Ly-a radiation will (via spontaneous Raman scattering) tend to drive the vibrationally excited molecules back to the H₂ ground levels, thus restoring primary frequency photons. The Ly-a and primary frequency fiels will thus be effectively coupled together via nonlinear elastic scattering by H₂ molecules. Again, via the effect of random-walk-based diffusion, both photon fields will become greatly enhanced within the PDR. Enhancement of light then spreads to a set of "secondary frequencies" through the same nonlinear mechanism. It is proposed that H₂ IR vibrational emission, which is ubiquitously observed to be radiated from PDRs, is powered during this stage via such two-quantum-driven transitions. A final stage of photoexcitation in the PDR can occur when the photon densities at some of the primary and secondary frequencies become sufficiently enhanced to pump beyond threshold resonantly-enhanced stimulated 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 VUV emission bands can be assigned to the latter. In addition, the nonlinear photoexcitation mechanisms which are here discussed provide realistic scenarios for explaining the formation of diffuse interstellar bands (DIBs) assigned to H₂ transitions in previous publications of the authors.