2025ApJ...981...36Z

First identified in 1964, inverse Raman scattering (IRS) is a nonlinear stimulated phenomenon that induces Raman-scattered absorptions where Raman emissions would be expected. This paper highlights the significance of IRS in analyzing the spectra of stars located in the distant background of H I interstellar clouds. Specifically, ultraviolet emission lines Raman scattered by atomic hydrogen, typically observed in emission at wide scattering angles in the optical spectra of symbiotic stars and nebulae, should appear as IRS absorptions in the optical spectra of the background stars. I show that all known interstellar Raman-scattered emission lines in the Hα wavelength region are detected in absorption as diffuse interstellar bands (DIBs) in the spectra of reddened stars, and conclude that IRS by atomic hydrogen resolves the long-standing DIB puzzle. This sheds new light on the perplexing relationship between DIBs and the Red Rectangle nebula emission bands (RRBs). The conditions under which either DIBs or RRBs are detected emphasize the importance of considering the physical relationship between the observer, the H I medium, and the direction of the illuminating radiation field (i.e., the geometry of the observation) in observations of H I interstellar matter. Observing in the direction of the radiation field or on its side determines whether IRS, yielding DIBs and the 2200 Å bump, or spontaneous Raman scattering at wide scattering angles, resulting in extended red emission, Raman-scattered emission lines (including RRBs), and unidentified infrared bands, will be observed.