  Aims: Archival observations of infrared fine-structure lines of the young
Galactic supernova remnant Cassiopeia A allow us to test existing models and
determine the physical parameters of various regions of the fast-moving knots
(FMKs), which are metal-dominated clouds of material ejected by the supernova
explosion. Methods: The fluxes of the far-infrared [O i] and [O iii] lines are
extracted from the previously unpublished archival ISO data. The archival
Spitzer data are used to determine the fluxes of the O, Ne, Si, S, Ar and Fe
ion fine-structure lines originating in the FMKs. The ratios of these line
fluxes are used for the plasma diagnostics. We also determine the infrared line
flux ratios to the optical [O iii] 5007 A line in the knots having previously
measured reddening. Additionally, we analyze several optical and near-infrared
observations of the FMKs to obtain clearer insight into the post-shock
photoionized region (PIR) structure. Results: We show that the infrared oxygen
line flux predictions of all existing theoretical models are correct only to
within a factor of several. For the models to reproduce the observations it is
essential to include the electron conductivity and effects of the dust.
Detailed analysis of the diagnostic line flux ratios allows us to qualitatively
confirm the general model of the FMK emission and to determine observationally
the physical conditions in the PIR after the shock front. We infer that the
pre-shock cloud densities most probably constitute several hundred particles
per cm^3. We also determine the Cas A luminosities in the infrared continuum
and lines. We show that accounting for the charge exchange processes in the
post-shock PIR allows us to reproduce most of the relevant spectral line ratios
even in the frame of a single-temperature model of this region. We also
estimate its plasma parameters, thickness, and carbon abundance.
