The Photophysics of the Carrier of Extended Red Emission

Interstellar dust contains a component which reveals its presence by emittinga broad, unstructured band of light in the 540 to 950 nm wavelength range,referred to as Extended Red Emission (ERE). The presence of interstellar dustand ultraviolet photons are two necessary conditions for ERE to occur. This isthe basis for suggestions which attribute ERE to an interstellar dust componentcapable of photoluminescence. In this study, we have collected all publishedERE observations with absolute-calibrated spectra for interstellarenvironments, where the density of ultraviolet photons can be estimatedreliably. In each case, we determined the band-integrated ERE intensity, thewavelength of peak emission in the ERE band, and the efficiency with whichabsorbed ultraviolet photons are contributing to the ERE. The data show thatradiation is not only driving the ERE, as expected for a photoluminescenceprocess, but is modifying the ERE carrier as manifested by a systematicincrease in the ERE band's peak wavelength and a general decrease in the photonconversion efficiency with increasing densities of the prevailing excitingradiation. The overall spectral characteristics of the ERE and the observedhigh quantum efficiency of the ERE process are currently best matched by therecently proposed silicon nanoparticle (SNP) model. Using the experimentallyestablished fact that ionization of semiconductor nanoparticles quenches theirphotoluminescence, we proceeded to test the SNP model by developing aquantitative model for the excitation and ionization equilibrium of SNPs underinterstellar conditions for a wide range of radiation field densities.Comment: 42 p., incl. 8 fig. Accepted for publication by Ap