  Context. In the framework of the interstellar PAH hypothesis, far-IR skeletal
bands are expected to be a fingerprint of single species in this class. Aims. A
detailed model of the photophysics of interstellar PAHs is required for such
single-molecule identification of their far-IR features in the presently
available Infrared Space Observatory data and in those of the forthcoming
Herschel Space Observatory mission. Methods. We modelled the detailed
photophysics of a vast sample of species in different radiation fields, using a
compendium of Monte-Carlo techniques and quantum-chemical calculations. This
enabled us to validate the use of purely theoretical data and assess the
expected accuracy and reliability of the resulting synthetic far-IR emission
spectra. Results. We produce positions and intensities of the expected far-IR
features which ought to be emitted by each species in the sample in the
considered radiation fields. A composite emission spectrum for our sample is
computed for one of the most favourable sources for detection, namely the Red
Rectangle nebula. The resulting spectrum is compared with the estimated dust
emission in the same source, to assess the dependence of detectability on key
molecular parameters. Conclusions. Identifying specific PAHs from their far-IR
features is going to be a difficult feat in general, still it may well be
possible under favourable conditions.
