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Session 117 - Galactic ISM.
Oral session, Thursday, January 16
Harbour B,
Nonlinear optical processes occurring in cold, thin H_2 clouds (assumed densities \sim10^4 cm^-3) located 1-10 pc from O-type stars are considered. For blackbody VUV light from the nearby hot star that is nearly resonant with the Lyman (B \rightarrow X) and Werner (C \leftarrow X) H_2 absorption lines, elastic scattering is assumed to greatly dominate over inelastic scattering, resulting in the near-resonant VUV flux permeating the entire cloud, with greatly enhanced steady-state flux densities. While one-photon VUV absorptive transitions in the collisionless medium represented by the cloud are highly improbable - essentially because nonconservation of energy would result - the trapped near-resonant VUV light can act to "prime" the cold H_2 molecules so that they can absorb light at visible frequencies - or emit light at infrared frequencies - in simultaneous, energy conserving, two-photon (or Raman) transitions, if visible or IR continuum light is present. In such transitions, the cold H_2 molecules become excited to various singlet-gerade-state quantum levels. We believe that it is from such two-photon absorption processes that the diffuse interstellar bands (DIBs) originate. Two-photon Raman processes are shown to provide enough gain for coherent IR Raman generation (and four-wave parametric oscillation (FWPO)) to occur in the "plane" of the H_2 cloud. This coherent light generation strongly modifies individual intensities of bands in the DIBs spectrum.
