Infrared spectra (2-25 µm) of obscured sources such as deeply embedded protostars and field stars, clearly show absorption features superposed to the continuum (e.g., 3.1, 4.27, 4.67, 6.0, 6.8, 9.7, 15.2 µm). These features are characteristic of species in solid form and their identification has been mainly based throughout the years on the comparison with laboratory spectra. Apart from the features due to the silicates in interstellar grains the others are attributed to frozen molecules i.e., icy mantles which form on interstellar grains in dense molecular clouds. The comparison with laboratory spectra of thin icy films has often been successful and several molecules have been firmly identified such as H2O (Hagen et al. 1980), CO (Lacy et al. 1984; Tielens et al. 1991; Chiar et al. 1994, 1995; Teixeira et al. 1998), CO2 (de Graauw et al. 1996; Gerakines et al. 1999), OCS (Palumbo et al. 1995, 1997), CH4 (Lacy et al. 1991; Boogert et al. 1996, 1997), CH3OH (Tielens & Allamandola 1987; Allamandola et al. 1992; Dartois et al. 1999). However it is well known that particle size and shape can have very important effects on the profile (shape, width and peak position) of absorption features. For strong transitions in small particles this class of electromagnetic modes is referred to as "surface modes" (van de Hulst 1957; Bohren & Huffman 1983). Because of surface modes absorption features can be shifted with respect to laboratory (bulk) spectra, and subpeaks appear. Mie-calculations have been performed (e.g., H2O (Léger et al. 1983; Greenberg et al. 1983), CO (Tielens et al. 1991; Ehrenfreund et al. 1997), OCS (Palumbo et al. 1995), CO2 (Ehrenfreund et al. 1997)) and it has been shown that when absorption band profiles are affected by "surface modes" laboratory spectra are no longer representative of the small particles extinction spectrum.
For a long time we have studied in the laboratory the infrared spectra of several icy mixtures. Here we present spectra which have been taken with the substrate plane (on which the icy sample is deposited) forming different angles (normal incidence and oblique incidence) with respect to the infrared beam of the spectrometer. Furthermore, spectra have been taken with the electric vector of the beam parallel (P polarized) and perpendicular (S polarized) to the plane of incidence. We have found that some instances exist in which the incidence angle and the polarization strongly affect the absorption band profiles. Using the optical constants measured by Baratta & Palumbo (1998) we have performed Mie scattering calculation for spherical grains. We have found that when band profiles are influenced by surface modes then they are also affected by the incidence angle and the polaritazion in the laboratory spectra. This result provide us with an experimental method to know when laboratory spectra of thin films are representative of small particle extinction spectra and hence can be directly compared with astronomical spectra.
We describe the experimental set-up in Sect. 2; we present the laboratory results in Sect. 3 and discuss their implications in Sect. 4.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000