2002ApJ...565..304S

Interstellar dust contains a component that reveals its presence by emitting a broad unstructured band of light in the 540-950 nm wavelength range, referred to as extended red emission (ERE). The presence of interstellar dust and ultraviolet photons are two necessary conditions for ERE to occur. This is the basis for suggestions that attribute ERE to an interstellar dust component capable of photoluminescence. In this study, we have collected all published ERE observations with absolute-calibrated spectra for interstellar environments, where the density of ultraviolet photons can be estimated reliably. In each case, we determined the band-integrated ERE intensity, the wavelength of peak emission in the ERE band, and the efficiency with which absorbed ultraviolet photons are contributing to the ERE. The data show that radiation is not only driving the ERE, as expected for a photoluminescence process, but is modifying the ERE carrier, as manifested by a systematic increase in the ERE band's peak wavelength and a general decrease in the photon conversion efficiency with increasing densities of the prevailing exciting radiation. The overall spectral characteristics of the ERE and the observed high quantum efficiency of the ERE process are currently best matched by the recently proposed silicon nanoparticle (SNP) model. Using the experimentally established fact that ionization of semiconductor nanoparticles quenches their photoluminescence, we proceeded to test the SNP model by developing a quantitative model for the excitation and ionization equilibrium of SNPs under interstellar conditions for a wide range of radiation field densities. With a single adjustable parameter, the cross section for photoionization, the model reproduces the observations of ERE intensity and ERE efficiency remarkably well. The assumption that about 50% of the ERE carriers are neutral under radiation conditions encountered in the diffuse interstellar medium leads to a prediction of the single-photon ionization cross section of SNPs with average diameter 3.5 nm of ≤3.4x10^–15 cm². The shift of the ERE band's peak wavelength toward larger values with increasing radiation density requires a change of the size distribution of the actively luminescing ERE carriers through a gradual removal of the smaller particles by size-dependent photofragmentation. We propose that heat-assisted Coulomb decay of metastable, multiply charged SNPs is such a process, which selectively removes the smaller components of an existing SNP size distribution.