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Astron. Astrophys. 343, 933-938 (1999) 3. Spectroscopy3.1. Measurement proceduresSeveral groups have carried out experimental work on the
spectroscopic properties of commercially available and laboratory
produced SiC particles. Stephens (1980) has studied laser-produced
All the groups have done their measurements by embedding the sub-micrometer particles in a solid matrix either of KBr or CsI. In the KBr/CsI pellet technique, small quantities of the sample are mixed thoroughly with powdered KBr/CsI. Due to the softness of the matrix material and its bulk transparency in the mid-IR, the material can be pressed into a clear pellet. According to scattering calculations, embedding the sample in a matrix will influence the wavelength at which the frequency-dependent extinction falls as well as the intensity, in a way which depends on the sample considered and the matrix in which it is included (Bohren & Huffman 1983; Papoular et al. 1998; Mutschke et al. 1999). Friedemann et al. (1981) and Borghesi et al. (1985) tried to
correct the influence of the matrix by blue-shifting the whole feature
by an amount of Our transmission measurements were performed by placing the presolar SiC grains on a polished Si substrate. This means that the grains were mainly but not fully surrounded by air, resulting in much less matrix effects than if the sample had been embedded in a solid medium. Samples tend to cluster both in the KBr pellets and on a substrate and may do this with different cluster morphologies. At the moment theory is not able to determine what the optical effect of clustering is for SiC grains. We have not been able to find any systematic change of band profiles as a result of clustering. We have also not found systematic changes of band profiles related to whether we used a Si, NaCl or a KBr substrate (refractive index: KBr and NaCl (n=1.5), Si (n=3.4)). The samples were mounted on the Si substrates by dispersing them in a droplet of chloroform. All the spectroscopic measurements were made with an infrared microscope attached to a Bruker 113v Fourier Transform Infrared Spectrometer. The detector is a liquid-nitrogen-cooled mercury cadmium telluride detector with a spectral range of 7000-600 cm-1. The sampling diameter of the microscope can be as small as 30 µm. Spectra were obtained on different grain clusters of the samples which were 10 to 80 µm in size. The microscope aperture used for the measurements was always 80 µm since this gave a sufficient signal-to-noise ratio with a reasonable number of scans (64). The measurements were performed with a resolution of 1 cm-1. The reference spectra were obtained on a blank part of the substrate. 3.2. ResultsOne of the drawbacks with using the infrared microscope is that a reliable mass estimate of the fraction responsible for the spectral feature cannot be obtained. With the microscope one measures different parts of the sample and depending on how good the sample was mounted the mass can be more or less evenly distributed on the substrate. Different densities will result in different depths of the features. The results shown in Fig. 1 for sample I and in Fig. 2 for sample II are the raw data (y is shifted for better comparison). There is a remarkable difference between the spectral appearance of the SiC grains in the two samples, despite the fact that they were prepared by using almost identical extraction procedures and came from the same larger piece of meteorite. This difference can be explained by different grain sizes (see later).
Fig. 1 (sample I ) shows the expected broad
absorption feature of small SiC grains between the positions of the
longitudinal and transverse lattice vibration modes at
As an attempt to try to get rid of the Teflon-related contaminate,
we considered a density separation of the Teflon and the SiC grains by
use of e.g sodium-polywolframate ( In Fig. 2 (sample II ) there is apparently no similar SiC band. Instead one observes a maximum of the transmission spectrum more or less at the place of the longitudinal lattice vibration mode. At longer wavelengths the spectrum is flat. In the next section we will explain that this does not mean the absence of SiC grains. 3.3. Size effectsDue to collective processes very small solid particles (small
compared to the wavelength) exhibit strong resonances in absorption in
the spectral regions where the real part of the dielectric function
( For the surface modes of very small SiC particles in the infrared,
Gilra (1973) performed Mie calculations for different shapes and found
that for a thin disc the resonances are at 10.3 µm
( If the grain size is larger than or comparable to the wavelength,
scattering becomes more important and diminishes the transmitted light
also at frequencies outside the absorption band. At a special (larger)
frequency outside but close to the absorption band,
In order to investigate these size effects we did infrared
microscope measurements on a commercially available
different size fractions were obtained. Here t is the
settling time, In Fig. 3 it is shown that the spectral appearance of SiC grains
strongly depends on the grain size and that the Christiansen effect
dominates for the larger grains. This means that the absorption
feature between
Fig. 4 shows sample II of the meteoritic SiC
compared to the
It was not possible to obtain a reliable grain size estimate of the
samples by the use of a scanning electron microscope (SEM) due to
clustering of the grains. Sample I looks like
small grains ( The different Christiansen frequencies observed for the meteoritic and the commercial SiC grains (Fig. 4) hint to different optical constants for the two samples. However, so far we have not been able to correlate this to a certain polytype of the meteoritic SiC grains. The measured spectra of sample II (Fig. 4) can
therefore be understood as being a spectrum of large meteoritic SiC
grains, where instead of an extinction maximum around
11.3 µm we see an minimum around 10 µm. The
measured spectra of sample I (Fig. 5) corresponds
to a spectrum of mainly small ( 3.4. Comparison with
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Fig. 6. Presolar SiC plotted with different commercially available ![]() ![]() |
The five spectra presented in Fig. 6 all show the SiC infrared band
of small grains between
10.3 µm and
12.6 µm. Obviously, there are significant differences in
band shape and peak position between the spectra. However, these
differences are not related to the polytypes. The meteoritic spectrum
resembles most the
-SiC (AJ) band
profile but this definitely cannot be taken as a indication of the
polytype of the meteoritic grains as one can see by comparing with the
spectrum of
-SiC (Klotz) in
Fig. 6.
This is consistent with the findings of Papoular et al. (1998) and Mutschke et al. (1999) that the band profile and consequently the peak wavelength of the SiC infrared band depends on the distribution of shapes and grain sizes rather than crystal type. Therefore using the IR spectral feature of different polytypes to determine whether one of the other crystal type of SiC dominates in circumstellar outflow, cannot be recommended.
The fact that commercially available SiC samples vary so much in spectral appearance strengthens the importance of studying the spectral feature of presolar SiC. However, so far it seems that from the band profiles we will learn rather about grain shape and size than about the polytype of extra-solar grains. In any case further intensive laboratory studies are needed. A step towards a better understanding will be presented in Mutschke et al. (1999).
© European Southern Observatory (ESO) 1999
Online publication: March 1, 1999
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