The far-infrared spectrum of hydrogenated amorphous carbon and the 21, 27, and 33 micron features in carbon-rich proto-planetary nebulae

We report laboratory emission spectra of hydrogenated amorphous carbon (HAC) in the wavelength range 19 mum < <lambda> < 120 <mu>m (525-83 cm(-1)) at temperatures between 300 degreesC and 500 degreesC. These spectra show emission features at 21, 27, and approximate to 33 mum (475, 370, and 303 cm(-1)) that can be identified with similar bands in the far-infrared spectra of extreme carbon stars, proto-planetary nebulae, and planetary nebulae. It is suggested that HAC is present in these objects but that the chemical composition of this material depends on the evolutionary state. We also detect a number of broad features at wavelengths longer than 40 mum, attributable to chemical groups in HAC. A strong feature is observed at approximate to 60 mum (165 cm(-1)) in laboratory spectra and may be present in spectra of the carbon-rich planetary nebula CPD -56 degrees 8032. We briefly discuss the nature of the chemical groups responsible for the appearance of these features.