Nonlinear Optics in Space

To explain the 70 year old astrophysical mystery regarding the carriers of the complex - but universally appearing - visible absorption spectrum known as the Diffuse Interstellar Bands (DIBs) [Fig. 1], the authors have proposed a model based upon nonlinear photoexcitation of H₂ molecules in thin, tenuous clouds located at distances ~0.1 - 1 pc from bright stars. Astrophysicists term such clouds "Photon-Dominated Regions (PDRs)". In earlier descriptions [2, 3] of the H₂ nonlinear DIBs model, primary emphasis was placed on assigning individual DIBs to specific H₂ excited-state transitions. In these studies, comparatively little effort was spent attempting to elucidate the nonlinear photoexcitation channels that result in specific DIB absorptions. By contrast, in a manuscript [4] recently submitted for publication, primary attention was focused on both (1) outlining a plausible general scenario through which nonlinear H₂ photoexcitation could occur in PDRs and (2) specifying pathways for excitation of H₂ molecules to both lower and upper quantum levels of excited-state transitions to which DIBs were previously assigned. The present paper summarizes the dynamical aspects of the model described in [4].