  We present results on mid-IR (5--16micron) spectral imaging of a sequence of
interacting galaxies, observed by ISOCAM. The galaxies are part of the well
known Toomre's ``merger sequence'' which was defined as a sample of galaxies
depicting progressive snapshots in the time evolution of a merging event. To
trace the intensity of the radiation field in a starburst, we use the ratio of
the 15 to 7 micron flux. Our analysis indicates that this ratio increases as
galaxies move from the pre-starburst to the starburst phase and goes again down
to ~1 in the post-starburst phase, a value typical of normal star forming
regions in galactic disks. Moreover, we find that the variation of this ratio
is well correlated with the one of the IRAS 25micron/12micron and
60micron/100micron flux ratios.
