The 15-21 um PAH plateau
  We present 14--21 um emission spectra of star forming regions and compare them with the PAH spectra from the Ames PAH database. We show that while the emission in this region can be quite variable, the bulk of these variations can be accommodated by variations in the IS PAH population. 
E. Peeters, A.L. Mattioda, F. Kemper, D.M. Hudgins, L.J. Allamandola