Laboratory and astronomical IR spectra : an experimental clue for their comparison

It is well known that particle shape and size can have very important effects on the profile of icy absorption features. In fact Mie-scattering calculations show that absorption features can be shifted with respect to laboratory (bulk) spectra of thin films and subpeaks appear. The difference between bulk spectra and small particle extinction spectra depends on the optical constants (n, k) of the sample and in particular it is not negligible for those species with " strong transitions " and at high concentration in the ice mixture. In order to know whether a band profile in a specific ice mixture would be affected by particle shape and size effects it is necessary to have the optical constants of that mixture and perform small particles cross-section calculations. However if in principle optical constants can always be measured in practice this is not straightforward. After several years of experiments and IR spectroscopy of icy mixtures we have found an experimental method to know whether particle shape and size would affect the profile of an absorption feature and hence whether laboratory spectra of thin films are representative of small particles extincion spectra. In particular, we have found that some instances exist in laboratory spectra for which the profile of absorption bands depends on the inclination of the ice film with respect to the infrared beam of the spectrometer. Furthermore when this is the case if the spectrum is taken at oblique incidence the band profile depends on the polarization of the infrared beam as well. Here we show that when a band profile in a laboratory spectrum depends on the inclination of the sample or, equivalently, on the polarization of the electric vector of the IR beam this cannot be directly compared to astronomical observations.