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Astron. Astrophys. 343, 933-938 (1999)

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3. Spectroscopy

3.1. Measurement procedures

Several groups have carried out experimental work on the spectroscopic properties of commercially available and laboratory produced SiC particles. Stephens (1980) has studied laser-produced [FORMULA]-SiC condensates, Friedemann et al. (1981) measured spectra of commercially available [FORMULA]-SiC, Borghesi et al. (1985) studied commercially produced [FORMULA]-SiC and [FORMULA]-SiC, Kaito et al. (1995) have studied [FORMULA]-SiC and [FORMULA]-SiC produced by simultaneous evaporation of silicon and carbon and Papoular et al. (1998) investigated two samples of [FORMULA]-SiC powders, one commercially available and one produced by laser pyrolysis.

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 [FORMULA] = -0.4 µm and [FORMULA] = -0.1 µm, respectively. They also corrected the intensity by a factor of 0.7 and 0.9, respectively. These procedures have been argued by Papoular et al. (1998) to be incorrect, since independent of the matrix material, absorption should not fall outside the longitudinal and transverse optical phonon frequencies. Instead Papoular et al. (1998) proposed a new method for computing the expected spectrum for the particles in vacuum, which works if the dielectric function of the grains can be described by a single Lorentzian oscillator. This certainly is the case for SiC (Mutschke et al. 1999).

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. Results

One 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).

[FIGURE] Fig. 1. Infrared spectra of meteoritic SiC grains (sample I ) from the Murchison meteorite obtained with the IR microscope at an aperture of 80 µm. The SiC feature is located at about 11.3 µm. The features around 8.2 and 8.6 µm are due to Teflon (see text).

[FIGURE] Fig. 2. Infrared spectra of meteoritic SiC grains (sample II ) from the Murchison meteorite obtained with the IR microscope at an aperture of 80 µm. The appearance of the feature is influenced by the Christiansen effect (see text).

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 [FORMULA] 10.3 µm and [FORMULA] 12.6 µm (Mutschke et al. 1999). The center of the band is located at about 11.3 µm. The peaks around 8.2 and 8.6 µm are due to -CF and -CF2 groups originating from the treatment in Teflon containers with hot concentrated sulphuric acid (H2SO4). The sulphuric acid did not only dissolve spinel grains but also attacked the walls of the container.

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 ([FORMULA]) (Amari et al. 1994). However, all possible solvents, that we could think of, tended to have IR features in exactly the same infrared spectral region as the SiC feature we were interested in. Therefore, we did not follow this approach.

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 effects

Due 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 ([FORMULA]) is negative. The precise positions of these resonances, called surface modes, depend on the particle shape, size, and on the nature and amount of coatings or matrixes surrounding the grain (Bohren & Huffman 1983). There are two distinct energy ranges in which resonances occur. One is in the infrared in the region of strong lattice bands between the transverse optical phonon frequency ([FORMULA]) and the longitudinal optical phonon frequency ([FORMULA]). The other is in the ultraviolet and is due to the transitions of bound electrons.

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 ([FORMULA]) and 12.6 µm ([FORMULA]). As the oblateness decreases, the resonances move towards each other and finally for the spherical case there is only one resonance at 10.73 µm ([FORMULA]). As the particles become prolate, the resonances move away from each other and finally for the case of a needle they are at 12.6 µm ([FORMULA]) and 10.55 µm ([FORMULA]). Gilra (1973) concludes that if the particles are highly irregular there will be a broad feature between about 10.3 µm and 12.6 µm, which agrees very well with the later findings of Bohren & Huffman (1983) resulting in the continuous distribution of ellipsoids (CDE) approximation.

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, [FORMULA] and [FORMULA]. Since these values are close to those of the surrounding air (or free space) then at this spectral point the particles are nearly invisible. In other words, scattering is greatly reduced and a maximum in the transmittens spectrum is observed. This is known as the Christiansen effect (Bohren & Huffman 1983).

In order to investigate these size effects we did infrared microscope measurements on a commercially available [FORMULA]-SiC grain sample (Duisburg) with large grain size of up to 40 µm. This sample was sedimented in acetone and based on settling rates calculated from Stokes law

[EQUATION]

different size fractions were obtained. Here t is the settling time, [FORMULA] is the dynamic viscosity, h the setteling height, g the gravitational constant, d the grain diameter, [FORMULA] the density of the grains and [FORMULA] the density of the liquid. The size fractions that were obtained were 40-20 µm, 20-5 µm, 5-2 µm and [FORMULA] 2 µm. These samples have been mounted on Si substrates in the same way as for the meteoritic samples.

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 [FORMULA] and [FORMULA] is transformed into a flat spectrum with a transmission maximum at about [FORMULA] (see above).

[FIGURE] Fig. 3. Spectral appearance of commercially available [FORMULA]-SiC (Duisburg) sedimented to obtain different grain size fractions. The variation of the spectral appearance as a result of different grain size is apparent. As the grain size increases the influence of the Christiansen effect becomes important (see text).

Fig. 4 shows sample II of the meteoritic SiC compared to the [FORMULA]-SiC (Duisburg) sample with grain sizes 5-20 µm. Fig. 5 shows sample I of the meteoritic SiC grains compared to the same material but with grain sizes [FORMULA] 2 µm. These comparisons show that the meteoritic SiC grain spectra look like the profiles expected from SiC grains of different sizes. It may seem odd that an almost identical extraction procedure on two parts of originally one piece of the Murchison meteorite should result in two samples with different size distributions. However, already the results of Amari et al. (1994) indicate an unusual grain size distribution of extracted Murchison SiC when compared with (less processed) SiC residues from other meteorites (see discussion in Russel et al. 1997). In addition, we cannot exclude that we lost (presumably mostly the finer) grains during our own extensive extraction procedure. A hint that this may have happened comes from the noble gas analysis of one of the diamond fractions, in which we found small, but easily detectable amounts of Ne-E(H), indicating the presence of small amounts of presolar SiC in the nominal diamond fractions.

[FIGURE] Fig. 4. Spectra of meteoritic SiC grains (sample II ) and commercially available [FORMULA]-SiC (Duisburg) with grain sizes 5-20 µm.

[FIGURE] Fig. 5. Spectra of meteoritic SiC grains (sample I ) and commercially available [FORMULA]-SiC (Duisburg) with grain sizes less than 2 µm.

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 ([FORMULA] 1 µm) mixed with Teflon and some of the more resisting meteoritic grains such as spinel, chromite, hibonite and corundum, while sample II looks like larger grains in a cleaner environment.

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 ([FORMULA] 2 µm) meteoritic grains.

3.4. Comparison with [FORMULA]- and [FORMULA]-SiC samples

In the last years, there have been quite a number of papers addressing the structural differences of SiC particles of different polytype as the main important factor influencing the band profile and the peak position of the 11.3 µm feature observed in carbon stars (e.g. Blanco et al. 1994, 1998; Groenewegen 1995; Speck et al. 1997). In another paper (Mutschke et al. 1999) we study these spectral differences due to the polytype in detail. Here, we present infrared microscope measurements on four commercially available SiC powders - two [FORMULA]-SiC and two [FORMULA]-SiC - and compare them to the spectrum of sample I (Fig. 6).

[FIGURE] Fig. 6. Presolar SiC plotted with different commercially available [FORMULA]- and [FORMULA]-SiC samples with the grain sizes indicated. More information about the samples is given in Mutschke et al. (1999).

The five spectra presented in Fig. 6 all show the SiC infrared band of small grains between [FORMULA] 10.3 µm and [FORMULA] 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 [FORMULA]-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 [FORMULA]-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).

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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