AAS 197, January 2001
Session 7. Gas in the Galactic ISM
Display, Monday, January 8, 2001, 9:30am-7:00pm, Exhibit Hall

## [7.09] Nonlinear Photoexcitation of H{2} in PDRs: Origin of the Unidentified Infrared Bands (UIBs)'

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

The Unidentified Infrared Bands (UIBs)' are a set of ubiquitously appearing emission bands at 3.3, 6.2, 7.6, 7.8, 8.6, 11.2, and 12.7 \mum that originate from just beyond the ionization fronts in strongly irradiated PDRs. In this poster, a plausible explanation of the UIBs based upon nonlinear photoexcitation of H2 is suggested. One first notes that, from a select few vibrationally excited X-state quantum levels [\textit{e.g.} (X13, J\prime\prime=5), (X10, J\prime\prime=5), and (X11, J\prime\prime=3)], there occur strong transitions to B- and C-state levels with frequencies lying within the assumed ~20-cm-1-wide spectral profile of Ly-\alpha radiation entering the neutral region from the H II region (please refer to adjacent poster). These can serve as pumping transitions for Stimulated Raman Scattering (SRS) processes, generating coherent IR light on strong transitions to EF-state quantum levels. The SRS processes are accompanied by parametric oscillation (PO), bringing the molecules back to the `starting levels' (\textit{e.g.} X13, J\prime\prime= 5), and generating coherent light both at additional IR wavelengths and in the VUV. It is shown that the IR wavelengths predicted to be generated in the most probable of these SRS-PO processes match well the UIB wavelengths. It is also shown that each SRS-PO starting level can be optically connected to X0, J\prime\prime=1 (in the case of orthohydrogen) by a simple sequence of strongly allowed, resonant, two-photon transitions. Either Ly-\alpha radiation or VUV light coherently generated in an SRS-PO process can here serve as a driving field. With only one driving field present, Inverse Raman Scattering (IRS) can act to maintain equal populations in two optically linked quantum levels. Thus sufficient population is always maintained in the SRS-PO starting levels.