A theory attributing optical diffuse interstellar absorption bands, "unidentified infrared" emission bands, and cloud reddening to H₂ nonlinear absorption

Abstract

A theory is described that attributes the diffuse interstellar absorption bands (DIBs) to coherently driven two-photon absorption by H2 molecules present in sheetlike clouds located near hot stars. Results from three-level nonlinear spectroscopy are used to argue that vacuum ultraviolet (VUV) photons from the star impinging on the H2-containing cloud will be primarily absorbed in simultaneous two-photon (Raman-type) transitions, resulting in the build-up of coherent VUV Stokes-wave radiation in the "plane" of the H2-containing cloud on the transitions [CnX0; P(2) and P(3); n = 0-5] and on some of the transitions [BnX0, P(2) and P(3)]. Via this in-plane VUV Stokes-wave radiation, parahydrogen molecules in the (XO, J″ = 2) level and ortho-hydrogen molecules in the (XO, J″ = 3) level are coherently excited to levels [Cn(∏u+); J′ = 1 and 2; n = 0-5] and [Bn, J′ = 1 and 2], which thus serve as the intermediate states in resonantly enhanced, coherently driven, two-photon absorption. The DIBs result from visible light from the star being simultaneously absorbed in "second step" transitions from the coherently excited (C, B)-state levels to various singlet gerade state levels. In-plane coherent VUV Stokes-wave radiation at [CnX0; P(2) and P(3); n = 1-5] can also act to produce in-plane coherent IR radiation via stimulated Raman scattering processes terminating on various EF singlet gerade levels. Striking coincidences are shown to exist between frequencies of IR Stokes-wave transitions expected to be strong and those of the so-called unidentified infrared emission bands (UIBs). Assuming a common identity, assignments are offered for most of the known UIBs. More than 70 sharp DIBs are assigned to transitions from B- and C-state levels to levels of singlet gerade states. On the basis of in-plane four-wave mixing of VUV and IR-Stokes-waves radiation (i.e., four-wave parametric generation), the most prominent features seen in the emission spectrum of the Red Rectangle are also explained. It is furthermore indicated that cloud reddening should be an additional manifestation of the presence of intense, in-plane, VUV Stokes-wave radiation. © 1996. The American Astronomical Society. All rights reserved.