AAS 196th Meeting, June 2000
Session 26. The Galactic ISM: Observations and Modelling
Display, Tuesday, June 6, 2000, 10:00am-6:30pm, Empire Hall South

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[26.11] A Model for Nonlinear Photoexcitation of Molecular Hydrogen in Space

J. H. Glownia, P. P. Sorokin (IBM Research Division)

A model for nonlinear photoexcitation of H2 molecules in tenuous clouds near bright stars (e.g. PDRs or PNe) is presented. In the model, H atoms and H2 molecules coexist in a cold neutral cloud surrounding an H II region represented by a conventional Strömgren sphere. An intense band of Ly-\alpha radiation is produced by H++ e- recombination and frequency redistribution occuring in the H II region. Due to elastic scattering by H atoms, the Ly-\alpha radiation slowly diffuses outward through the neutral cloud, its photon density becoming enormously enhanced in the process. This provides the basic pumping field required for the nonlinear effects to be described.

Via resonant inverse Raman scattering (IRS), the intense Ly-\alpha radiation field induces strong nonlinear absorption of VUV continuum starlight by orthohydrogen molecules in X0, J''=1 around three ''primary'' frequencies (B9-0P1, B6-0P1, and B3-0R1), the primary IRS terminal levels (X5, J''=1), (X4, J''=1), and (X3, J''=1) simultaneously becoming strongly populated. (Parahydrogen absorbs via IRS on B3-0R0.) Via either Ly-\alpha -pumped, spontaneous resonant Raman scattering, or secondary IRS processes, molecules in the primary IRS terminal levels are selectively redistributed into higher-lying X-state levels such as (X10, J''=5), (X13, J''=5), and (X14, J''=1). A thin shell (thickness ~10,000 km) of H2 molecules populating select vibrationally excited X-state levels thus surrounds the Strömgren sphere.

>From a handful of the populated high-lying X-state levels, there occur strong resonances with Ly-\alpha . Intense Ly-\alpha radiation can thus induce broadband stimulated Raman scattering (SRS) to occur on these transitions, generating broadband IR Stokes-wave light on strong transitions to EF-state levels. An SRS process would occur as part of a 2n-wave parametric oscillation (SRS-PO) process, with light at additional frequencies being generated on strong transitions ultimately returning molecules to the X-state level from which the SRS-PO process originated. The Unidentified Infrared (Emission) Bands (UIBs) can be successfully assigned to IR emissions generated in such SRS-PO processes.

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