Interstellar dust contains a component which reveals its presence by emitting a broad , unstructured band of light in the 540 to 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 which 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 ionization cross section of SNPs of average diameter of 3.5 nm for single-photon ionization of \leq 3.4 \cdot 10 ^ { -15 } cm ^ { 2 } .
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 will remove selectively the smaller components of an existing SNP size distribution .