Astron. Astrophys. 333, 1025-1033 (1998)

2. Experimental setup

A detailed description of the experimental apparatus used can be found elsewhere (Spinella et al., 1991). A stainless steel vacuum chamber, in which pressure is kept lower than mbar, is faced through KBr windows to an FTIR spectrometer (4400 to 400 cm^-1 = 2.27 to 25 µm). Inside the chamber, a silicon crystal substrate is placed in thermal contact with a cold finger that can be cooled down to 10 K. The gas or gas mixture to be investigated is injected into the chamber through a needle valve and immediately freezes on the substrate. All the spectra in this work have been ratioed to a background spectrum that includes the silicon wafer and were taken at a resolution of 2 cm^-1, and a sampling of 1 cm^-1. The samples can be bombarded during or after condensation by ions of energies ranging from 1.5 to 60 keV. Bombarding the samples during condensation makes it possible to prepare rather thick (several µm), uniformly irradiated films, while samples thinner than the penetration depth of the ions can be bombarded after deposition, enabling us to study the effects of irradiation at increasing doses. The ions are obtained from an ion gun (3 kV) or an ion implanter (30 kV). From this latter also doubly ionized Ar beams (60 keV Ar ions) can be obtained. We used current densities ranging from several to not more than a few µA cm^-2 to avoid macroscopic heating of the target. The substrate plane forms an angle of about with the ion beam and the IR beam. In this way, spectra can be taken at any time, before, during and after irradiation, with no need to tilt the sample. The energy released to the sample by impinging ions (dose) is given in eV per small molecule (16 a. m. u.) because this is a convenient scale to characterize the evolution of the samples upon irradiation and to compare results obtained irradiating different samples.

In some of the experiments, an He-Ne laser beam was used to monitor the thickness of the sample, by watching in real time the interference pattern between the reflections on the interfaces vacuum-sample and sample-substrate. In some experiments, moreover, the spectra were taken in polarized light, allowing us to minimize the effect of the variation of the real part of the refractive index across the band. In particular, the transmittance for light polarized perpendicularly to the plane of incidence depends strongly on the real part of the refractive index. This, for strong absorption can lead to the appearance of spurious features not due to absorption. By choosing a polarization in the plane of incidence, it is possible to eliminate almost completely this effect.

© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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