Diagnostics for specific PAHs in the far-IR:: searching neutral naphthalene and anthracene in the Red Rectangle

Context. In the framework of the interstellar polycyclic aromatic hydrocarbons (PAHs) hypothesis, far-IR skeletal bands are expected to be the fingerprints of single species in this class. Aims. We address the question of detectability of low energy PAH vibrational bands, with respect to spectral contrast and intensity ratio with "classical" Aromatic Infrared Bands (AIBs). Methods. We extend our established Monte Carlo model of the photophysics of specific PAHs in astronomical environments to include rotational and anharmonic band structure. The required molecular parameters were calculated in the framework of the Density Functional Theory. Results. We calculate the detailed spectral profiles of three low-energy vibrational bands of neutral naphthalene and four low-energy vibrational bands of neutral anthracene. They are used to establish detectability constraints based on intensity ratios with "classical" AIBs. A general procedure is suggested to select promising diagnostics and tested on available Infrared Space Observatory data for the Red Rectangle nebula. Conclusions. The search for single, specific PAHs in the far-IR is a challenging, but promising task, especially in view of the forthcoming launch of the Herschel Space Observatory.