The Astrophysical Journal, 568:448–453, 2002 March 20 # 2002. The American Astronomical Society. All rights reserved. Printed in U.S.A.
INFRARED ABSORPTION AND EMISSION SPECTRA OF HYDROGENATED AMORPHOUS CARBON PREPARED IN THE PRESENCE OF OXYGEN, NITROGEN, AMMONIA, AND CARBON MONOXIDE V. I. Grishko and W. W. Duley Department of Physics, University of Waterloo, 200 University Avenue, Waterloo, ON N2L 3G1, Canada;
[email protected] Received 2001 October 12; accepted 2001 November 30
ABSTRACT The formation of carbonaceous condensates in the atmospheres of asymptotic giant branch (AGB) carbon stars and in the nebulae of post-AGB objects occurs in a chemical environment that also contains oxygen and nitrogen. The extent of incorporation of chemical groups containing these elements in carbonaceous solids will depend on the ratios C/O and C/N. The presence of chemical groups containing O and N can have a significant effect on the infrared spectrum of this material through the introduction of new absorption and emission features and by perturbing spectral lines arising from CH and other hydrocarbon groups. To simulate some aspects of this chemistry, we have prepared samples of hydrogenated amorphous carbon (HAC) containing these elements by laser ablation of graphite in mixed H2/N2, H2/O2, and H2/CO gases and in NH3. Absorption and emission spectra are reported for these materials in the 2.5–20 lm region. We find that the inclusion of oxygen has a profound effect on both 3.4 lm and longer wavelength absorption in HAC even at low concentration. Condensation of HAC in the presence of N2, CO, and NH3 produces changes in the profile and relative intensity of components of the 3.4 lm CH2, 3 hydrocarbon band, but some of these effects can be reversed by heating to higher temperature. We find that the condensation of a crystalline diamond component is greatly facilitated in the presence of O2 or NH3. A number of sharp spectral features attributable to diamond have been observed in these spectra. Subject headings: circumstellar matter — infrared: ISM — ISM: lines and bands — ISM: molecules — methods: laboratory Molecule formation and dust condensation in carbonrich circumstellar shells occur in an environment where many other elements are present (Tsuji 1964; Gail & Sedlmayr 1984; Frenklach & Feigelson 1989). The influence that these species have on the primary chemical products such as polycyclic aromatic hydrocarbon (PAH) molecules and hydrogenated amorphous carbon (HAC) has yet to be determined and is the subject of this paper. In previous laboratory experiments (Duley & Grishko 2001) we have demonstrated that the presence of oxygen during the condensation of HAC suppresses the formation of aromatic carbon and facilitates the appearance of particles of crystalline diamond. In this paper we report the results of studies of the condensation of HAC in the presence of NH3, N2, and CO and extend our measurements on O2-HAC mixtures to other H/O ratios.
1. INTRODUCTION
Analysis of the infrared absorption and emission spectra of interstellar material (Chiar et al. 1998, 2000; Allamandola, Tielens, & Barker 1989) has revealed the products of a complex inorganic and organic chemistry. When carbon chemistry dominates, as in asymptotic giant branch (AGB) carbon stars and in the nebulae that evolve from these objects, spectra show the presence of a wide range of aromatic and aliphatic hydrocarbons. In oxygen-rich objects such as M giants, the chemistry is primarily inorganic and the condensates include a variety of silicates and oxides. Some objects, for example, HD 44179 in the Red Rectangle, appear to combine both chemistries with a carbon-rich dust shell offset from a disk containing silicate dust (Waters et al. 1998). Absorption spectra of material in the diffuse interstellar medium (DISM) toward the Galactic center, a sight line which includes both dense and diffuse cloud components, show the presence of both carbonaceous and silicate chemistry, together with other species such as CO, H2O, etc. (Chiar et al. 2000). The separation of oxygen-rich and carbon condensates occurs as a natural consequence of the C/O ratio in AGB winds, but there is considerable evidence that this ratio can influence other spectral characteristics such as the intensity of the 3.28 lm hydrocarbon emission feature (Martin 1987; Roche et al. 1996). Both the 3.28 and 11.3 lm emission features, attributable to aromatic carbon groups, are suppressed in objects with C=O < 1 (Roche & Aitken 1986; Volk & Cohen 1990; Roche et al. 1996). In addition, the observations of Roche et al. (1996) show that the relative strength of the 3.4 lm emission band, compared to that at 3.28 lm, is enhanced in objects with high N/O ratios.
2. EXPERIMENTAL
For absorption measurements, HAC thin films were deposited on a 5 mm thick KBr window while molybdenum was used as a substrate in emission experiments. Samples were prepared by laser ablation of graphite in the presence of hydrogen and other gases. The deposition chamber was continuously evacuated maintaining a slow flow of gas at a total pressure of 1 torr. Gas mixtures were premixed and included H2/N2 (9 : 1), H2/CO (9 : 1), H2/O2 (2 : 1, 7 : 1, 9 : 1, 19 : 1), and H2/H2O, in addition to pure H2 and NH3. The ablation source was an XeCl excimer laser, operating at 308 nm with average energy of 70 mJ pulse1. Laser radiation was focused on the graphite target 4 cm away from the substrate. The products of laser ablation react with hydrogen 448
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Fig. 1.—Room temperature 3.4 lm absorption of HAC, HAC/N2, and HAC/NH3 and emission spectrum of HAC/NH3 at 500 C.
Fig. 2.—Same as Fig. 1, but covering the range 4000–600 cm1 (2.5–16.6 lm). The decrease in emission near 11 lm in the top spectrum is an interference effect in the emitting film.
and other gases to form HAC, which is deposited as an adherent layer on the surface of the substrate. At a pulse frequency of 10 Hz, a 1 lm thick HAC layer can be deposited in approximately 1 hr. For emission measurements the molybdenum substrate was placed in a mount inside a stainless steel chamber containing an emission port. The substrate was then heated to the desired temperature, and thermal emission from the HAC/Mo sample was recorded using BOMEM MB100 Fourier transform spectrometer with an MCT detector. Absorption spectra of deposits on KBr substrates were collected with the same spectrometer. The spectrometer operated at 1 cm1 resolution, and 500 scans were averaged to obtain spectra.
its. The absorption spectrum of HAC/NH3 shows a decrease in absorbance near 2920 and 2850 cm1 (3.43 and 3.51 lm) that can be attributed to scattering by small diamond particles (Duley & Grishko 2001). The emission spectrum of HAC/NH3 recorded at 500 C (Fig. 1) shows that the CH2, 3 features present in the room temperature absorption spectrum of pure HAC reappear on heating. This suggests that heating to 500 C leads to the suppression of chemical groups containing N. At longer wavelengths (Figs. 2 and 3), emission and absorption spectra of HAC/NH3 films and absorption spectra of HAC/N2 are radically different from that of pure HAC. In absorption spectra, the region near 2200 cm1 (4.55 lm) is characterized by features from isonitrile (”N‰C) and nitrile (”C‰N) structures at 2100 and 2220 cm1 (4.76 and 4.40 lm), respectively. These features are not as evident in the emission spectrum of HAC/NH3 at 500 C. The absorption spectrum of HAC/N2 shows two broad peaks at 1620 and 1250 cm1 (6.17 and 8 lm), but the absorption spectrum of HAC/NH3 is much more complex with many sharp features (Table 2). Most of these sharp lines can be identified with spectral features observed in nanodiamonds, specifically material extracted from the
3. RESULTS
Spectra of HAC prepared in H2/N2 and NH3 are shown in Figures 1 and 2. In the 3.4 lm region (Fig. 1, Table 1), the CH2,3 absorption bands attributable to hydrocarbon groups in HAC/N2 and HAC/NH3 samples are less well defined than in pure HAC and are slightly shifted to shorter wavelength. This effect is likely due to an increase in the relative concentration of fully saturated CH3 groups in these depos-
TABLE 1 Comparison of 3.4 lm Spectral Features Observed in HAC/X Deposits, where X is O2, N2, CO, and NH3
System
CH3, Asymmetric (cm1, lm)
CH2, Asymmetric (cm1, lm)
CH2, Symmetric (cm1, lm)
H2 ..................................... H2/O2, 19 : 1 ...................... H2/O2, 4 : 1, em, 400 C ...... H2/N2, 9 : 1........................ H2/CO, 9 : 1....................... NH3 .................................. NH3, em, 500 C ................
2955, 3.384 2988, 3.347 2955, 3.384 2965, 3.373 2960, 3.38 UR 2958, 3.381
2921, 3.423 2946, 3.394 2925, 3.419 2935, 3.407 2930, 3.413 UR 2920, 3.425
2852, 3.506 UR 2850, 3.509 2875, 3.478 2875, 3.478 UR 2850, 3.509
Note.—Some features are unresolved (UR).
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Fig. 3.—Expanded view of HAC/NH3 absorption spectrum showing sharp components.
Fig. 4.—Comparison of absorption spectrum of HAC with HAC/CO (3200–2000 cm1 region).
Orgueil meteorite (Hill, Jones, & d’Hendecourt 1998). These features arise from various chemical groups adsorbed on the diamond surface rather than from diamond itself (Andersen et al. 1998). They appear weakly in the emission spectrum of HAC/NH3 at 500 C. The absorption spectrum of HAC/CO (Figs. 4 and 5) shows that the addition of CO during condensation has little effect on the 3.4 lm feature, other than to broaden individual CH3, 2 components. The CH2 band at 2850 cm1 (3.51 lm) in pure HAC is shifted to 2875 cm1 (3.48 m) in HAC/ CO (Table 1). An absorption associated with the nitrile (”C‰N) group is seen at 2205 cm1 (4.535 lm). The strong band observed at 1700 cm1 (5.88 lm) (Fig. 5) can be attributed to the presence of carbonyl groups. No new features appear in the spectrum of HAC/CO at wavelengths greater than 6 lm. The presence of CO seems to result in a strong
reduction in intensity of aromatic CH wagging modes in the region near 800 cm1 (12.5 lm). When oxygen is present during the preparation of HAC, the chemical equilibrium shifts to favor diamond-like material (Chang et al. 1988) and diamond particles nucleate in the deposited films. Preliminary experiments have shown that these particles can be observed spectroscopically in the 3.4–3.5 lm region (Duley & Grishko 2001). The size and concentration of these particles depend on the H2/O2 ratio during deposition, and this can result in significant changes in the appearance of IR spectral features. This effect can be seen in the absorption spectrum of HAC/O2 in the 3.4–3.5 lm band (Fig. 6) that shows scattering-induced emission replacing absorption in samples produced at low H2/O2 ratios. Emission spectra of HAC/O2 in this region are given in Figure 7 and show sharp-line components attributable to tertiary CH groups on diamond particles at 2920 and 2850 cm1 (3.43 and 3.51 lm) in spectra at 300 C. These broaden and evolve into the usual peaks seen in the spectrum of
TABLE 2 Absorption Features in HAC/NH3 between 1800 and 1000 cm1 (5.55–10 lm) Wavenumber (cm1)
Wavelength (lm)
Assignment
1724 ................ 1610 ................ 1455 ................ 1409 ................ 1375 ................ 1342 ................ 1304 ................ 1266 ................ 1244 ................ 1230 ................ 1168 ................ 1120 ................ 1100 ................ 1020 ................
5.80 6.21 6.87 7.10 7.27 7.45 7.67 7.90 8.04 8.13 8.56 8.93 9.09 9.80
C»O in ”COOH C»C (aromatic) CH2, 3 deformation CH2, 3 deformation CH2, 3 deformation C”N (amine) C”N (amine) C”O stretch N”O stretch C”C NH2 rock NH2 rock ... O”C
Note.—See Fig. 3. These features are not apparent in emission at 500 C. Note that these features arise predominantly from surface groups adsorbed on the surface of crystalline diamond.
Fig. 5.—Same as Fig. 4, but for the region 2000–400 cm1
ABSORPTION AND EMISSION SPECTRA OF HAC
3.60
3.55
3.50
3.45
3.35
3.30
3.25
3.54
3.52
3.50
Wavelength, µm 3.48
3.46
3.44
3.42
3.40
3.38
3.36
3.34
Wavelength, µm
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3.40
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HAC (H2/O2, 4:1)
HAC (H2/O2, 19:1)
0.06 0.04
0.04 HAC (H2/O2, 7:1)
0.02
HAC (H2/O2, 2:1)
Emissivity, Arbitrary Units
Absorbance, Arbitrary Units
HAC (H2/O2, 9:1)
500 oC
0.02 400 oC
HAC (H2)
0.00 3000
2950
2900
2850 -1
Wavenumber, cm
Fig. 6.—Comparison between 3.4 lm absorption spectrum of HAC and that of HAC/O2. Note the inversion that appears in spectra with H2 =O2 ¼ 21 and 7 : 1. This arises from scattering in diamond particles.
HAC on heating to 500 C. This change indicates that heating of a mixed diamond-HAC solid results in further hydrogenation reactions. It is uncertain whether or not the tertiary CH features disappear on heating or whether they are obscured by stronger CH2, 3 emission. The absorption spectrum of HAC/O2 at longer wavelengths (Fig. 8) is strongly influenced by the H2/O2 ratio during preparation. This effect is particularly noticeable in the 1800–1300 cm1 (5.6–7.7 lm) region where absorption bands at 1720, 1600, and 1400 cm1 (5.8, 6.25, and 7.1 lm) are suppressed at low H2/O2 ratios. Under these conditions, a number of sharp lines attributable to chemical groups on diamond particles can be observed. In some samples, these features appear with emission or derivative-like structure, as predicted from scattering models (Bohren & Huffman 1983). These features are accompanied by a strong absorption band at 3250 cm1 (3.08 lm) arising from OH groups. Absorption spectra of deposits prepared in an H2/H2O gas are virtually identical to those prepared in low H2/O2 ratio mixtures and show sharp-line features in the 3.4 lm region that can be assigned to small diamond particles. These particles can be seen in microscope images.
300 oC
0.00 3100
3050
3000
2950
2900
2850
2800
2750
-1
Wavenumber, cm
Fig. 7.—Emission spectrum of HAC/O2 (4 : 1) at 300 C, 400 C, and 500 C. The sharp tertiary CH feature at 2920 cm1 (3.43 lm) in the 300 C spectrum is seen to be absent at elevated temperature.
many cases, this results in the creation of diamond particles and a reduction in the proportion of aromatic carbon. This suggests that interstellar nanodiamond material could originate under conditions where circumstellar condensation occurs in shells with C=O 1 or where N is enhanced. The detection of sharp emission lines at 3.43 and 3.53 lm (2915 and 2833 cm1), attributable to tertiary CH groups on dia-
4. DISCUSSION
Laboratory spectra indicate that the composition of HAC can be significantly altered when deposition occurs in the presence of gases such as O2, H2O, CO, N2, and NH3. The primary effect of these gases appears to be to influence the relative abundance of sp2- and sp3-bonded carbon. In
Fig. 8.—Same as Fig. 6, but covering the region 2000–400 cm1 (5–25 lm).
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mond nanoparticles in the oxygen-rich shells associated with the young stars HR 4049 and HD 97048, would be consistent with this chemical scenario (Blades & Whittet 1980; Geballe et al. 1989; Guillois, Ledoux, & Reynaud 1999). Our laboratory data also confirm that the aromatic CH emission band at 3.28 lm is weak or absent when the tertiary CH features appear in these spectra. In general, oxygen at high concentration does not prevent the deposition of HAC, but it does greatly reduce the amplitude of the CH2, 3 absorption bands near 3.4 lm (Fig. 6). These bands do not recover unless the H2/O2 ratio is increased to 19. Emission spectra show, however, that the CH2, 3 features reappear on heating to 500 C (Fig. 7). We attribute this change to the release of hydrogen from weakly bound sites in HAC followed by rehydrogenation of surface carbon atoms. The incorporation of nitrogen in HAC has been found to lead to the formation of imine (C»N), nitrile (”C‰N), and amine (NH2) groups (Kaufman, Metin, & Saperstein 1989; Rodil et al. 2001) and results in the appearance of several new features in IR absorption spectra (Fig. 2). In addition, the CH2, 3 bands near 3.4 lm become weaker and shift to shorter wavelength. A broad feature (not shown in Fig. 1), attributable to the stretching vibration in NH groups, also appears at 3365 cm1 (2.97 lm). In these previous studies of HAC nitride films, the primary effect of nitrogen addition was found to be the activation of two vibrational modes involving sp2 carbon atoms in aromatic rings and olefinic chains (Rodil et al. 2001). This gave rise to additional absorption bands at 1575 and 1350 cm1 (6.35 and 7.41 lm). In our spectra, these new modes are not as obvious (Fig. 2), at least in comparison to the strong narrow lines that can be attributed to vibrational modes in defected diamond crystallites (Table 2). The IR absorption spectra that we observe for HAC/NH3 deposits are very similar to those reported for nanodiamonds extracted from meteoritic material (Lewis, Anders, & Draine 1989; Mutschke et al. 1995; Hill et al. 1998; Andersen et al. 1998; Braatz et al. 2000). As undefected pure diamond material has few intrinsic IR features (Chin et al. 1995), all of the absorption bands at > 4 lm listed in Table 2 can be assigned to various impurities and surface chemical groups. This conclusion is supported by the weakness of these features in the emission spectrum of HAC/NH3 at 500 C (Fig. 2) which would tend to drive off adsorbed impurities. The species producing these bands involve functional groups containing C»O, C”O, C”N, C»C, and C”C in ethers, esters, and other carbonyl compounds. There is also some question as to the origin of the sharp features observed near 2920 and 2850 cm1 (3.42 and 3.51 lm) in the absorption spectrum of meteoritic nanodiamond extracts (Mutschke et al. 1995) as it appears that absorption at these wavelengths can also arise from adsorbed species, particularly those containing CH2, 3 groups. This situation is further complicated by the fact that sharp-line spectra due to tertiary CH groups on the surface of diamond occur at these wavelengths and have been well characterized in a number of careful laboratory experiments (Chang et al. 1995; Chin et al. 1995). Our spectra show narrow regions of decreased absorption at 2920 and 2850 cm1 in HAC/NH3 films containing diamond (Figs. 1 and 2) that replicate those seen in HAC/O2 deposits (Fig. 6; see also Duley & Grishko 2001). These features are also observed in emission from HAC/O2 at low temperature (Fig. 7), but they are replaced by broader emis-
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sion bands at the same wavelengths at higher temperatures. This suggests that the groups responsible for the 2920 and 2850 cm1 features are strongly bonded to the surface of diamond and are able to participate in further hydrogenation reactions at high temperatures. Tertiary CH groups on the surface of diamond are resistant to desorption at temperatures up to at least 900 K (Lin et al. 1996). Despite the fact that the narrow absorption bands in the 1750–1000 cm1 (5.7–10 lm) region arise from these impurities in diamond or on the diamond surface, the wavelengths at which these features appear and their collective presence in laboratory spectra may provide a useful way to determine the abundance of diamond particles in interstellar clouds. The strongest of these bands is that at 1724 cm1 (5.80 lm). A similar band has been observed at 1774 cm1 (5.637 lm) in the diamond extract from the Allende meteorite (Lewis et al. 1989) and at 1746 cm1 (5.727 lm) in material from the Murchison meteorite (Mutschke et al. 1995). Hill et al. (1998) detect a band at 1728 cm1 (5.787 lm) in diamonds from the Orgueil meteorite. Heating of pure HAC to temperatures in the 500 C range has been shown to result in the development of additional broad emission features at 3100 and 3036 cm1 (3.23 and 3.29 lm) (Grishko & Duley 2000). The 3100 cm1 peak can be associated with the CH2 stretching vibration in C»C bonded material, while the 3036 cm1 feature arises as the CH stretch in aromatic ring compounds. Both of these bands are absent in emission spectra of HAC/O2 films at 300 C and 500 C (Fig. 7) and of HAC/NH3 films at 500 C (Fig. 2). This suggests that the addition of these impurities during condensation inhibits the formation of sp2-bonded carbon, in addition to promoting the formation of diamond-like material. This experimental result is consistent with observational data showing the strong correlation between the C/O ratio and the equivalent width of the 3.29 lm aromatic emission feature in planetary nebulae and the relative weakness of this band in nitrogen-rich objects (Roche et al. 1996). In the 3.4 lm region, most of the spectral changes that accompany the addition of O and N are relatively subtle and primarily involve a broadening of CH2, 3 components together with a change in the relative intensity of the CH2 and CH3 bands. This effect can be seen in Figures 1 and 4. As a result, the detection of impure HAC from 3.4 lm spectra may be difficult. The presence of nanodiamond material in the DISM cannot, however, be excluded because the narrow (6 cm1) features from tertiary CH occur at or near 2850 and 2920 cm1 (3.42 and 3.51 lm), where they may be difficult to detect against stronger absorption from CH2, 3 groups. The absorption band observed at 1724 cm1 (5.80 lm; Fig. 3) may be a good diagnostic of the presence of crystalline diamond in interstellar clouds even though the chemical origin of this band is somewhat uncertain. It is perhaps significant that a spectral feature has been detected near this wavelength in several embedded protostars (Keane et al. 2001) including NGC 7538 IRS 9 where other evidence points to the existence of tertiary CH groups (Allamandola et al. 1992; Grishko & Duley 2000). However, spectra of HR 4049 and HD 97048, which both exhibit emission features near 3.4 lm associated with tertiary CH groups on diamond nanoparticles, do not appear to show a corresponding 5.8 lm band (Roche, Aitken, & Smith 1991; Molster et al. 1996). This observation would be consistent
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ABSORPTION AND EMISSION SPECTRA OF HAC
with the 5.8 lm band originating in more weakly bound surface species, rather than those responsible for the 3.4 lm CH features. 5. CONCLUSIONS
Deposition of HAC in the presence of non-hydrocarbon gases such as NH3 and CO has been shown to influence the composition and IR spectrum of the resulting material. We find that both O2 and NH3 can suppress the formation of aromatic carbon compounds in HAC and enhance the proportion of sp3-bonded carbon. Crystalline diamond particles are present in HAC/O2 and HAC/NH3 under a range of deposition conditions and may be detectable in interstellar clouds via IR transitions of certain adsorbed species. The effect of foreign atom/molecule inclusion in HAC on the 3.4 lm CH2, 3 absorption bands ranges from minimal
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(CO) to profound (O2). In particular, the incorporation of O is often found to completely suppress these bands, while NH3 results in the disappearance of structure. More subtle changes involve profile and wavelength shifts within the CH2, 3 3.4 lm bands in HAC prepared in H2/N2 and H2/ CO atmospheres. These variations may be detectable in the spectra of AGB and post-AGB objects with extreme C/O and C/N compositions. Infrared absorption bands associated with ”CN and ”CO groups can be seen in the 5–10 lm (2000–1000 cm1) region in many deposits. Emission spectra of HAC/O2 and HAC/NH3 obtained at 500 C are quite different from the absorption spectrum of room temperature deposits, indicating that heating can have a significant effect on composition. This research was supported by the NSERC of Canada.
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