Figure 1.26 schematically shows the arrangement used to acquire Raman spectra. A continuous Ar-ion laser beam (=514 nm) enters a confocal illuminator perpendicularly to its optical axis, into which it is deflected by a microprism. The confocal optical system is arranged in such a way that any parallel beam incident along the optical axis in the opposite direction of the laser is focused onto the entrance slit of the spectrometer (Triplemate SPEX). By means of two flat mirrors, the laser beam is reflected towards the vacuum chamber, where it is focused on a 40 m spot on the sample. The same objective which focuses the laser beam on the sample collects the Raman-scattered light, which reaches the confocal illuminator going back along the same path of the laser beam. This confocal system makes it possible to obtain Raman spectra of samples located several meters far from the spectrometer with negligible performance losses. The substrate holder is mounted at an angle of 45 both with the ion beam and the Ar-ion laser beam. This configuration offers the advantage that spectra can be easily taken in situ, even during irradiation with ions or UV photons, without tilting the sample. The system is equipped with a CCD detector which can be used for reflectance measurements in the 0.4-1 m range as well.