  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 > 1 um 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 > 50 A. 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" due to 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 11.3um. The model is compared with the
observed emission at high Galactic latitudes as well as in the Galactic plane,
as measured by COBE and IRTS. We calculate infrared emission spectra for our
dust model heated by a range of starlight intensities, and we provide tabulated
dust opacities (extended tables available at
http://www.astro.princeton.edu/~draine/dust/dustmix.html)
