We present a quantitative model for the infrared emission from dust in the diffuse interstellar medium .
The model consists of a mixture of amorphous silicate grains and carbonaceous grains , each with a wide size distribution ranging from molecules containing tens of atoms to large grains \gtrsim 1 micron in diameter .
We assume that the carbonaceous grains have polycyclic aromatic hydrocarbon ( PAH ) -like properties at very small sizes , and graphitic properties for radii a \gtrsim 50 Å .
On the basis of recent laboratory studies and guided by astronomical observations , we propose “ astronomical ” absorption cross sections for use in modeling neutral and ionized PAHs from the far ultraviolet to the far infrared .
We also propose modifications to the far-infrared emissivity of “ astronomical silicate ” .

We calculate energy distribution functions for small grains undergoing “ temperature spikes ” caused by stochastic absorption of starlight photons , using realistic heat capacities and optical properties .
Using a grain size distribution consistent with the observed interstellar extinction , we are able to reproduce the near-IR to submillimeter emission spectrum of the diffuse interstellar medium , including the PAH emission features at 3.3 , 6.2 , 7.7 , 8.6 , and 11.3 \micron .
The model is compared with the observed emission at high Galactic latitudes as well as in the Galactic plane , as measured by the COBE/DIRBE , COBE/FIRAS , IRTS/MIRS , and IRTS/NIRS instruments .
The model has 60 \times 10 ^ { -6 } of C ( relative to H ) locked up in PAHs , with 45 \times 10 ^ { -6 } of C in a component peaking at \sim 6 Å ( N _ { C } \approx 100 carbon atoms ) to account for the PAH emission features , and with 15 \times 10 ^ { -6 } of C in a component peaking at \sim 50 Å to account for the 60 \mu m flux .
The total infrared emission is in excellent agreement with COBE/DIRBE observations at high galactic latitudes , just as the albedo for our grain model is in accord with observations of the diffuse galactic light .
The aromatic absorption features at 3.3 \mu m and 6.2 \mu m predicted by our dust model are consistent with observations .

We calculate infrared emission spectra for our dust model heated by a range of starlight intensities , from 0.3 to 10 ^ { 4 } times the local interstellar radiation field , and we tabulate the intensities integrated over the SIRTF/IRAC and MIPS bands .
We also provide dust opacities tabulated from the extreme ultraviolet to submillimeter wavelengths .