PN physical and spectr. param. of RRLs

We compare these observational data sets with synthetic models produced with the radiation transfer code Co3RaL. Results. Our observations towards the target PNe reveal the presence of several H and He i RRLs at mm wavelengths in the spectra of IC 418 and NGC 7027 and also of He ii RRLs in the spectrum of NGC 7027.

Many of these lines had remained undetected until now due to their weakness and the lack of high-sensitivity observations at these frequencies. The data also confirm the absence of molecular emission towards IC 418, above a detection level of ~3mK [Tmb]. Conclusions.

These mm observations represent the most extended RRL line survey of two C-rich PNe carried out so far, with most of the lines never reported before. These extremely complete catalogs evidence the importance of high-sensitivity observations and are expected to be very helpful in the line identification process in mm observations, in particular for still unknown or poorly characterized molecular species existing in the vicinity of ionized environments.