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Astron. Astrophys. 330, 1080-1090 (1998)

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3. Interpretation of the experimental spectra

3.1. The UV/VIS spectra of the diamonds

The UV/VIS spectra of our presolar and CVD diamond samples are very similar to one another, showing strong absorption in the UV, with a distinct band at 217 nm (46 000 cm-1), a flat maximum around 270 nm (37 000 cm-1) and declining absorption towards the visual end of the spectrum (Fig. 2). These absorption characteristics are also typical for terrestrial diamond containing pairs of nitrogen atoms (Davies 1984). These bulk UV/VIS features deviate significantly from the results by Lewis et al. (1989), but correspond well with the measurements obtained by Mutschke et al. (1995) on presolar diamonds from the Murchison meteorite, except that the strength of the flat maximum around 270 nm seem to be much stronger for the Murchison presolar diamonds.

[FIGURE] Fig. 2. The UV/VIS absorbance spectrum of the presolar and the CVD diamonds, respectively.

The position of the band around 217 nm coincides with that of the interstellar extinction bump at 217.5 nm, as it has also been found by Mutschke et al. (1995) for the Murchison meteorite.

3.2. The IR spectrum of the presolar diamonds

The infrared absorbance spectrum of the presolar diamonds shows several peaks (Table 1 and Fig. 3). The strong peaks are mainly due to impurities, meaning the presence of atoms other than C. These impurities are most likely situated at the grain surface, some of which may be due to the chemical processing in the laboratory, other, of which may refer to the astrophysical environment the grains have experienced. The strong peaks at 3420 cm-1 and 1632 cm-1 are most likely due to H2 O (Chrenko et al. 1967). These peaks are strong and sharp, which makes tightly bound water a likely possibility (Nyquist & Kagel 1971). This interpretation of the spectrum is further supported by the peaks at 607 cm-1 and 471 cm-1.

The shoulder peak at 3236 cm-1 can be assigned to N-H stretching.


[TABLE]

Table 1. Wavenumber of our measured IR bands, in cm-1, detected in the spectra of the presolar diamonds from the Allende meteorite and from CVD diamonds, respectively. Also listed are our suggestions for assignments of the bands.



[FIGURE] Fig. 3. Infrared absorbance spectrum of the presolar diamonds from the Allende meteorite.

[FIGURE] Fig. 4. Infrared absorbance spectrum of the CVD diamonds. Due to the compensation for the ethanol in which the diamonds were suspended (see Sect. 2), the spectrum is over compensated around 3640 cm-1 and 1640 cm-1.

The peak frequencies and shapes of the triplet at 2954 cm-1, 2924 cm-1 and 2854 cm-1, can be attributed to long aliphatic hydrocarbon chains C [FORMULA] with n [FORMULA] 8 and indicating the presence of both -CH3 and -CH2 symmetric and antisymmetric deformation modes. This is further supported by part of the broad peak between 1462 cm-1 and 1385 cm-1 and the weak absorption at 721 cm-1 (Trombetta et al. 1991). The broad peak between 1462 cm-1 and 1385 cm-1 can also partly be assigned to interstitial N in diamond (Lewis et al. 1987).

The broad peak at 1122-1054 cm-1 can be attributed to C-O stretching in aliphatic ethers and/or C-N stretching.

The shoulder at 633 cm-1 could be due to a C-Cl stretch, see later in the text.

3.2.1. The O-H bands

The indication of tightly bound water which may be intrinsic, is not found in the other spectra obtained on presolar diamonds from the Allende meteorite that has been published (Lewis et al. 1989; Koike et al. 1995). See Table 2. Normally a diamond cannot be wetted with water, but under certain conditions the water-repelling nature of the surface disappears (Davies 1984.) It has been shown for terrestrial diamonds that if the diamond is heated to a few hundred degrees centigrade in O2, the water-repelling nature of the surface disappears; while heating the diamond in vacuum or in an atmosphere of H restores its water-repelling nature (Sappok & Boehm 1968). This means that, if these water features are intrinsic (as oppose to being an effect of not fully reduced water spectral features from humidity in the KBr pellet), they cannot originate from when the diamonds were formed in a carbon/hydrogen rich atmosphere, but rather is an artifact which most likely originates from the very rough chemical treatment that was used in order to extract the diamonds.


[TABLE]

Table 2. Spectral bands, in cm-1 unless otherwise stated, detected in the obtained spectra of the presolar diamonds from the Allende, Murchison and Orgueil meteorites. The different interpretations by the various authors are indicated by corresponding numbers.


The peak around 3400 cm-1 obtained by Lewis et al. (1989), for diamonds from the Allende meteorite, was interpreted by them as coming from -COOH. We consider this peak to be too high in wavenumber and too narrow to be assigned OH in -COOH, while we find a good fit with known spectral bands of water. Also the band at 1774 cm-1 is somewhat high in wavenumber for a -COOH interpretation. The -COOH interpretation has also been questioned by Mutschke et al. (1995) and Hill et al. (1996). Variations in the intensity and in the presence of a peak around 1740 cm-1 have also been observed in the spectra of CVD diamonds, where the peak varied considerably for spectra obtained at different times and with different IR spectrometers (Janssen 1991).

3.2.2. The C-H bands

The triplet around 2900 cm-1 and the peaks around 1400 cm-1 can all be ascribed to the presence of saturated hydrocarbons (aliphatic compounds), but the peaks around 1400 cm-1 are also in the right range for N in diamond (Lewis et al. 1989).

3.2.3. The nitrogen bands

The N-H stretch at 3236 cm-1 and the broad band around 1100 cm-1 present in most of the published spectra of meteoritic diamonds can be ascribed to various forms of nitrogen in and on the diamonds. Presolar diamonds have been shown by Russel et al. (1991) to be nitrogen rich.

Part of the 1100 cm-1 peak can also be attributed to C-O stretching in aliphatic ethers but the feature around 1084 cm-1 could just as well be due to C-N stretching and is in the right range for single N in terrestrial diamonds (Clark et al. 1979).

3.2.4. The C-Cl bands

We attribute the weak shoulder at 633 cm-1 to C-Cl stretch based on a comparison with the spectra obtained by Koike et al. (1995), of diamond-like residues from the Allende meteorite and on the fact that terrestrial diamonds do not show absorption in this wavelength region (Davies 1977). In the spectra obtained by Koike et al. (1995) they have a very sharp peak at 630 cm-1 which fits the pattern of a CCl stretch and the small splitting of the feature in their spectra could be due to isotopic effects (35 Cl, 37 Cl). Since they have used part of the same treatment as we did, it seems very likely that both spectra could contain an impurity which originates from HCl.

3.3. The IR spectrum of the CVD diamonds

The infrared absorbance spectrum of the CVD diamonds also show several peaks, many of which correspond to the peaks found for the presolar diamonds. The primary differences between the spectral properties of the presolar diamonds and the CVD diamonds, are that the CVD diamonds have spectral characteristics closer related to soot than have the presolar diamonds.

The nitrogen which is present in the CVD diamonds is due to nitrogen impurities in the CH4 /H2 gas mixture that was used during the deposition (Locher et al. 1994).

The peaks we attribute to O-H at 3418 cm-1, 1640 cm-1 and 579 cm-1 are broader for the CVD diamonds than for the presolar diamonds, indicating that the water present here is not tightly bound.

3.4. Discussion

The IR spectra of pure diamond have very few absorption features. For terrestrial diamonds the intrinsic absorption present in all types of diamonds are at wavenumber greater than 1400 cm-1 ([FORMULA] m), with a peak around 2000 cm-1 (5 µm) for pure diamond, while that below 1400 cm-1 ([FORMULA] m) is specimen dependent both in strength and shape (Davies 1977) and is caused by impurities, typically nitrogen.

The features that are present in the spectra of presolar diamonds are results of O-H, N-H, C-H, C=O, C=C, C-O, C-N and C-C interactions. It is not possible by IR-spectroscopy to distinguish whether the impurities in or on the diamonds are due to the chemical processing in the laboratory, in the interstellar space, during the Solar System formation or whether the "impurities" are original features which refer to the astrophysical environment the grains have experienced. It has been shown by Russel et al. (1991) that the presolar diamonds carry isotopic anomalous nitrogen with 14 N/15 N = 406 (terrestrial = 272) and by Virag et al. (1989) that the presolar diamonds also carry anomalous hydrogen with 1 H/2 D = 5193 (terrestrial = 6667) implying that at least some of the H and N must be presolar. From a spectroscopic point of view it does not really matter whether the hydrogen bonds on the surface of the grains are "original" from the stellar environment or have been replaced at a later stage, as long as the presence of hydrogen is to be expected on the grain surface in the stellar environment. Of course, then a replacement by a different element will make a difference.

This makes the whole discussion about facts and artifacts in the spectra of presolar diamonds complicated, since if the presolar diamonds are exposed to a very rough chemical treatment to clean them as much as possible, it might be possible to get a clean surface of the diamonds, but the obtained spectra might not resemble what we can expect to observe if the nano-diamonds, at their place of origin, are not clean but characterised by C-N, N-H and C-H bonds on the surface.

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© European Southern Observatory (ESO) 1998

Online publication: January 27, 1998
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