One of the more exotic components that have been detected in interstellar dense molecular clouds is microdiamonds. In 1989, members of the Astrochemistry Laboratory were working with Dr. T. Herbst on NASA's Infrared Telescope Facility on Mauna Kea in Hawaii on a project to detect the infrared signature of methanol-bearing ices in dense interstellar clouds. We intended to find the presence of the methanol (CH₃OH) by searching for its C-H stretching features in the 3.3-3.6 µm region, particularly the characteristic C-H stretching band near 2825 cm^-1 (3.54 µm) (for the infrared spectrum of a methanol ice, click here). To detect the features, we used infrared light from protostars forming within the clouds to probe the intervening dense cloud medium (for more details on how this is done, click here).

Fortunately, we were successful in detecting this feature along the lines-of-sight to a number of protostars embedded in dense clouds. In the figure below we show the telescopic spectra (points) of two of these protostars, NGC 7538 IRS 9 and W33A, compared to the laboratory spectrum of solid CH₃OH at a temperature of 10 K.

Two points are immediately obvious from this comparison. First, the methanol does a good job of explaining the observed feature near 2825 cm^-1 and is presumably also responsible for a portion of the observed absorption between 3000 and 2900 cm^-1. Second, there is addition absorption centered near 2880 cm^-1 that is not explained by the presence of methanol. From a full examination of the spectra from all of the protostars we studied, we determined that the relative strengths of the methanol band and the 2880 cm^-1 band varied considerably from object to object, implying that the bands where due to two independent carriers. Furthermore, the absolute abundance of the methanol varied from object to object, but the carrier of the 2880 cm^-1 band seemed to be more uniformly distributed, suggesting that it was a less thermally or chemically volatile substance than the methanol. The identity of the material responsible for this additional absorption mystified us from some time.

As with all spectral identifications, the key to the identification of the carrier of the 2880 cm^-1 band lies in the band's position and profile. Infrared transitions in this spectral region are usually associated with molecular C-H stretching vibrations. The fact that the bands lie a frequencies below 3000 cm^-1 indicates that 'aliphatic' compounds, i.e., hydrocarbon compounds where the carbons are all linked by single bonds, are responsible. In contrast, the C-H stretching vibrations of polycyclic aromatic hydrocarbons (PAHs) largely fall at frequencies above 3000 cm^-1.

As shown graphically in the figure below, there are three 'classes' of carbon atoms bonded to hydrogen in aliphatic hydrocarbons - primary (1^o, methyl or -CH₃ groups), secondary (2^o, methylene or -CH₂- groups), and tertiary (3^o, C-H groups). These classes correspond to carbon atoms that share single bonds with 1, 2, and 3 other carbon atoms, respectively.

Fortunately, these different configurations produce C-H vibrations having slightly different frequencies. Primary carbons in purely aliphatic compounds produce C-H stretching bands near 2960 and 2870 cm^-1 while secondary carbons produce C-H stretching bands near 2925 and 2860cm^-1. Tertiary carbons produce a single band near 2880 cm^-1. Careful measurements of the strength of the absorption at all of these frequencies demonstrated that the non-methanol carrier was dominated by the presence of tertiary carbons. In our best spectrum (NGC 7538 IRS9, see above), the absorption profile indicates a carrier having mixture of carbon types in the approximate relative abundance of -CH₃ : -CH₂- : C-H = 1 : 3 : 10.

Building an aliphatic hydrocarbon with these ratios is not easy! However, the figure below shows a structure that satisfies the constraints (click on the picture to obtain an expanded image). The rsulting structure is basically a small lump of singly bonded carbon atoms whose surface is 'decorated' with hydrogen atoms. Carbons along the faces of the carbon lump are bonded to one hydrogen, those along edges are bonded to two hydrogens, and carbons sticking up from the surface are bonded to three hydrogens. This is essentially the structure of a microscopically small diamond!

The idea that interstellar space contains microdiamonds strike you as patently crazy, but fortunately for us, there was already good evidence that the microdiamonds should be there! After long years of hard work, meteoriticists had recently managed to prove that a significant fraction (on the order of a few percent) of the carbon in primitive meteorites was in the form of microscopic diamonds. Furthermore, there was strong evidence that at least a portion of these microdiamonds had an interstellar origin because they were associated with isotopic anomalies (see the Anders & Zinner 1993reference below).

To date, the 2880 cm^-1 microdiamond feature has been detected along virtually every line-of-sight through dense molecular clouds for which it has been searched and its strength suggests that on the order of 10% of the carbon in the dense medium is in the form of microdiamonds!

The presence of abundant microdiamonds in the interstellar medium has a number of significant implications beyond their importance as a major reservoir of interstellar carbon. In particular, it has been suggested that they may also be the material responsible for the ubiquitous 2200 Å extinction feature seen in the ultraviolet (see the Sandford 1996 reference below). Also, it is interesting to note that the position and profile of the 2880 cm^-1 band are significantly different from those produced by the aliphatic organics in the Diffuse Interstellar Medium (DISM). To date, the microdiamond feature has not been identified in the spectra of lines-of-sight that pass through the diffuse ISM, although their detection will be difficult since the 2880 cm^-1 band is expected to be overwhelmed by the stronger features produced by the diffuse medium's more H-rich aliphatics.

In a first for the Astrochemistry Laboratory, we have discovered that the work described above has inspired some original poetry! For more about this, click here.

For more detailed information and reviews on our laboratory and telescopic work on interstellar microdiamonds, as well as some information on the interstellar microdiamonds found in primitive meteorites, see:

Bernatowicz, T. J., & Zinner, E. (eds.) (1997). Astrophysical Implication of the Laboratory Study of Presolar Materials. AIP Conf. Proc. 402, (AIP: Woodbury, NY).

Sandford, S. A. (1996). The Inventory of Interstellar Materials Available for the Formation of the Solar System. Meteoritics and Planetary Science 31, 449-476.

Allamandola, L. J., Sandford, S. A., Tielens, A. G. G. M., & Herbst, T. M. (1993). "Diamonds" in Dense Molecular Clouds: A Challenge to the Standard Interstellar Paradigm. Science 260, 64-66.

Anders, E., & Zinner, E. (1993). Interstellar Grains in Primitive Meteorites: Diamond, Silicon Carbide, and Graphite. Meteoritics 28, 490-514.

Allamandola, L. J., Sandford, S. A., Tielens, A. G. G. M., & Herbst, T. M. (1992). Spectroscopy of Dense Clouds in the C-H Stretching Region: Methanol and "Diamonds". Astrophys. J. 399, 134-146.

Bernatowicz, T. J., Gibbons, P. C., & Lewis, R. S. (1990). Electron energy loss spectrometry of interstellar diamonds. Astrophys. J. 359, 246-255.