2. Experimental set-up
The infrared spectra here shown have been taken in the 4400-500 cm-1 (2.27-20 µm) range with a Bruker Equinox 55 FTIR spectrometer. A stainless steel vacuum chamber (shown in Fig. 1) faces the spectrometer through KBr windows. Gases are accreted by a gas inlet on a silicon (or KBr) substrate which usually is inclined of 45 degrees with respect to the infrared (IR) beam of the spectrometer. For the purposes of this paper we have also mounted a substrate inclined of 90 degrees with respect to the IR beam. In this case, the incidence angle (i.e., the angle between the IR beam and the normal to the substrate plane) is zero degrees. The substrate is put in thermal contact with a closed-cycle helium cryostat whose temperature can be varied in the 10-300 K range. In this experimental set-up (with the substrate inclined of 45 degrees) it is possible to monitor the thickness of the film during accretion, by looking at the interference pattern (intensity versus time) given by a He-Ne laser beam reflected at an angle of 45 degrees both by the vacuum-film and film-substrate interfaces. For each sample, at all the examined temperatures, two spectra with the electric vector of the infrared beam parallel (P polarized) and perpendicular (S polarized) to the plane of incidence have been taken where the polarization of the infrared radiation is changed by rotating a polarizer placed in the path of the infrared beam (Fig. 1). This polarizer can be removed in order to take unpolarized spectra. All the spectra for a given polarization are ratioed to the corresponding spectrum of the background acquired before deposition. Unless when differently specified, spectra have been taken at a resolution of 1 cm-1 and sampling of 0.25 cm-1. For a more detailed description of the experimental procedure the reader is referred to Baratta & Palumbo (1998).
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000