Polycyclic aromatic hydrocarbons with SPICA
  Thanks to high sensitivity and angular resolution and broad spectral
coverage, SPICA will offer a unique opportunity to better characterize the
nature of polycyclic aromatic hydrocarbons (PAHs) and very small grains (VSGs),
to better use them as probes of astrophysical environments. The angular
resolution will enable to probe the chemical frontiers in the evolution process
from VSGs to neutral PAHs, to ionized PAHs and to "Grand-PAHs" in
photodissotiation regions and HII regions, as a function of G$_0$/n (UV
radiation field / density). High sensitivity will favor the detection of the
far-IR skeletal emission bands of PAHs, which provide specific fingerprints and
could lead to the identification of individual PAHs. This overall
characterization will allow to use PAH and VSG populations as tracers of
physical conditions in spatially resolved protoplanetary disks and nearby
galaxies (using mid-IR instruments), and in high redshift galaxies (using the
far-IR instrument), thanks to the broad spectral coverage SPICA provides. Based
on our previous experience with ISO and Spitzer we discuss how these goals can
be reached.
