tokens: of spectra for 10 K, 50 K, and 300 K, while series F[001]a lacks a 10 K spectrum. Figure 1 shows the measured transmission spectra ( T meas , solid and dashed lines) plotted over the wavenumber ( in cm −1 ); the corresponding wavelengths (30 μ m–500 μ m) can be inferred from the upper axes. Spectra for a-polarization are given in panel (a), b-polarization in (b), c-polarization in (c). The two incomplete series F[001]a and F[010]c are plotted as dashed lines. The former has been scaled by a factor of 1.8 in order to demonstrate the good agreement to series F[010]a after multiplicative scaling (see Sect. 3.2 ). The strong temperature dependence of the transmission of the forsterite crystal is easily recognizable in all three polarization directions. While measurable transmission is detected only below 50 cm −1 at room temperature, the crystal becomes transparent already below 250 cm −1 at 10 K. For each temperature, the transmission increases towards smaller wavenumbers, apart from absorption bands positioned at 202 cm −1 (particularly strong in a-polarization, but weaker also in c-polarization) and 145 cm −1 (in b-polarization, very weak in a- and c-polarization). These bands are the already mentioned 49 μ m and 69 μ m single-phonon bands of forsterite, respectively. They are excited exclusively by a- and b-polarized radiation, respectively, the weaker appearances in other polarization directions are due to the small error of the polarizer alignment and to the beam divergence of the FTIR spectrometer (±7.5 deg). Apart from these bands, a relatively weak minimum at ∼90 cm −1 also appears in several of the spectra for a-polarization. As there is no such long-wavelength infrared band predicted by theory ( Iishi 1978 ) and the band is not present at 10 K, we think that it is an artifact of the measurement setup. We note that the position of the band is one of the intensity minima created by multiple reflection in the inner cryostat windows. These minima occurring with a period of 18 cm −1 are sometimes not well compensated by the reference measurements and lead either to periodic structures in the spectrum (e.g., in the 10 K and 50 K spectra in Fig. 1 b), or coincide for unknown reasons with erratic peaks (mainly maxima) in the enstatite spectra (see Sect. 3.3 ). Fig. 1. Temperature-dependent transmission spectra of the forsterite crystal for orientation of the electric field parallel to the a-axis ( panel a ), b-axis ( panel b ), and c-axis ( panel c ). Complete temperature series (F[010]a, F[001]b, F[100]c) are plotted as solid lines, the two incomplete series as dashed lines. The F[001]a curves have been scaled by a factor of 1.8; they approximately match the F[010]a curves for the same temperatures. Dotted lines: calculated maximum transmission curves taking into account the intensity reflected by the crystal surfaces. 3.2. Maximum transmission curves and measurement errors The dotted lines in Fig. 1 indicate the theoretical maximum
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