  We investigate the properties of dust and dust-to-gas ratios in different
starburst regions of luminous infrared galaxies (LIGs). We refer to the sample
of seven LIGs recently observed in the mid-infrared by Soifer et al. (2001)
using the Keck telescopes with spatial resolution approaching the diffraction
limit. These seven objects are among the closest LIGs and have been classified
as starburst galaxies from optical spectroscopy. Our goal consists in modelling
the continuum spectral energy distribution (SED) of each galaxy, particularly
in the infrared range. Models are further constrained by observed emission-line
ratios in the optical range. The multi-cloud models consistently account for
the coupled effect of shock, photoionization by hot stars, and diffuse
secondary radiation from the shock-heated gas. Emission from clouds in the
neighbourhood of evolved starbursts and with high shock velocities (~ 500 km/s)
explains both the bremsstrahlung and reradiation from dust in the mid-infrared.
Clouds with lower velocity (~ 100 km/s) and corresponding to younger starbursts
also contribute to both line and continuum spectra. Both low- and high-velocity
clouds are thus present in nearly all the sample galaxies. For all the
galaxies, an old stellar population is revealed by black body emission in the
optical-NIR range. Dust-to-gas ratios vary in different regions of individual
galaxies.
