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In this work we present the results of a systematic search for diffuse bands (DBs, hereafter) in the circumstellar envelopes of a carefully selected sample of post-AGB stars. We concentrated on analyzing 9 of the DBs most commonly found in the interstellar medium. The strength of these features is determined using high-resolution optical spectroscopy, and the results obtained are compared with literature data on field stars affected only by interstellar reddening. Based on the weak features observed in the subsample of post-AGB stars dominated by circumstellar reddening, we conclude that the carrier(s) of these DBs must not be present in the circumstellar environment of these sources, or at least not under the excitation conditions in which DBs are formed. This conclusion is applicable to all the post-AGB stars studied, irrespective of the dominant chemistry or the spectral type of the star considered. A detailed radial velocity analysis of the features observed in individual sources confirms this result, as the Doppler shifts measured are found to be consistent with an interstellar origin.

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Despite its relevance in astrophysical scenarios, optical properties and density of ammonia have been scarcely studied. This work presents new data on the real part of the refractive index of ammonia at 632.8 nm and density at different temperatures of deposition from 13 K up to its desorption temperature around 110 K. The results show a significant variability for both parameters versus temperature, representing an increase of 50 % for density and a 10 % for refractive index as temperature increases in the range 13 -60 K, and at temperatures higher than 60 K a constant value is reached for both parameters. This initial variation and the further plateau reflects structural differences in its form at low and high temperatures that can play an important role in its interaction with other molecules of astrophysical interest.

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