  We present ISO-PHOT spectra of the regions 2.5-4.9um and 5.8-11.6um for a
sample of 45 disk galaxies from the U.S. ISO Key Project on Normal Galaxies.
The spectra can be decomposed into three spectral components: (1) continuum
emission from stellar photospheres, which dominates the near-infrared (2.5-
4.9um; NIR) spectral region; (2) a weak NIR excess continuum, which has a color
temperature of ~ 1000K, carries a luminosity of a few percent of the total
far-infrared luminosity L(FIR), and most likely arises from the ISM; and (3)
the well-known broad emission features at 6.2, 7.7, 8.6 and 11.3 um, which are
generally attributed to aromatic carbon particles. These aromatic features in
emission (AFEs) dominate the mid-infrared (5.8-11.6 um; MIR) part of the
spectrum, and resemble the so-called Type-A spectra observed in many
non-stellar sources and the diffuse ISM in our own Galaxy. The relative
strengths of the AFEs vary by 15-25% among the galaxies. However, little
correlation is seen between these variations and either IRAS 60um-to-100um flux
density ratio R(60/100) or the FIR-to-blue luminosity ratio L(FIR)/L(B),
suggesting that the observed variations are not a direct consequence of the
radiation field differences among the galaxies. We demonstrate that the NIR
excess continuum and AFE emission are correlated, suggesting that they are
produced by similar mechanisms and similar (or the same) material. On the other
hand, as the current star-formation activity increases, the overall strengths
of the AFEs and the NIR excess continuum drop significantly with respect to
that of the far-infrared emission from large dust grains. This is likely a
consequence of the preferential destruction in intense radiation fields of the
small carriers responsible for the NIR/AFE emission.
