  We have performed IRS low resolution 5-12 micron spectroscopy on a sample of
galaxies selected to be at 3 distinct post-starburst evolutionary stages based
on their optical spectral indices. The resulting IRS spectra show distinctive
PAH emission line structures at 6.2, 7.7, 8.6 and 11.3 micron and little
silicate absorption, indicative of ongoing star formation. However the PAH
inter-line ratios, in particular the 11.3/6.2 micron and 7.7/6.2 micron ratio,
show large variations. These variations are found to correlate with both time
since the most recent starburst and AGN activity. We speculate that the
evolution observed in these PAH ratios is related to an increase in AGN
activity with time since star burst.
