Silicon nanoparticles ( SNPs ) have been proposed as the source of the observed “ extended red emission ” ( ERE ) from interstellar dust .
We calculate the thermal emission expected from such particles , both in a reflection nebula such as NGC 2023 and in the diffuse interstellar medium ( ISM ) .
Pure neutral Si SNPs would emit at 16.4 \mu { m } , while Si/SiO _ { 2 } SNPs ( both neutral and charged ) produce a feature at 20 \mu { m } .
Observational upper limits on the 16.4 \mu { m } and 20 \mu { m } features in NGC 2023 impose upper limits of < 1.5 ppm in pure Si SNPs , and or < 0.2 ppm in Si/SiO _ { 2 } SNPs .
The observed ERE intensity from NGC 2023 then gives a lower bound on the required photoluminescence efficiency \eta _ { PL } .
For foreground extinction A _ { 0.68 \mu { m } } = 1.2 , we find \eta _ { PL } > 5 \% for Si SNPs , or \eta _ { PL } > 24 \% for Si/SiO _ { 2 } SNPs in NGC 2023 .
Measurement of the R band extinction toward the ERE-emitting region could strengthen these lower limits .
The ERE emissivity of the diffuse interstellar medium appears to require \gtrsim 42 % ( \gtrsim 33 % ) of solar Si abundance in Si/SiO _ { 2 } ( Si ) SNPs .
We predict IR emission spectra and show that DIRBE photometry appears to rule out such high abundances of free-flying SNPs in the diffuse ISM .
If the ERE is due to SNPs , they must be either in clusters or attached to larger grains .
Future observations by SIRTF will be even more sensitive to the presence of free-flying SNPs .