Bling Bling in the Universe
It seems that there are diamond-like links in dust clouds around stars and in the space between the stars. Spectra on wavelengths in the infrared recorded by astronomers strongly indicate to this. An international group of researchers, including scientists at the FOM Institute for Plasma Physics Rijnhuizen in Nieuwegein, The Netherlands, have analysed laboratory spectra of a number of diamond-like molecules and compared them to astronomical observations. The laboratory measurements produce spectra that can be compared very well to the astronomical spectra. The spectra show different shapes and appear to be dependent on the size of the diamond-like molecules in question and on the fact that they can occur as a gas or in ice. An extra indication that there are diamond-like particles in the universe, supports the finding that there are diamond nuclei in micrometeorites. The size of these nuclei roughly correspond to the size that can be deduced on the basis of the molecular spectra. The researchers will have their findings published in The
Astrophysical Journal of 1 June 2007.
Diamond-like molecules or 'diamondoids' consist of a carbon frame in the shape of a small piece of diamond grid that has been cut off with hydrogen on the surface. The smallest member of the family of diamondoids is adamantane, which consists of one unit cell of the diamond grid (see figure 1, all to the left). It is being produced industrially for use in plastics and medicines. Larger diamondoids, to which one unit of adamantane will be added time and again (diamantane, triamante et cetera), are hardly or not at all to synthesize.
Diamondoids in the universe
As the diamondoids are solid and well-balanced, it was put forward in the eighties of the past century that these molecules could be found in interstellar clouds. In these clouds astronomers perceived absorption bands and emission bands around a wavelength of 3.5 micrometers, and around this wavelength also the spectra of diamondoids show up. The absorption bands and emission bands that respectively arise, as particles intercept radiation or emit radiation themselves, have quite a different surface relief. There is no obvious explanation for this. Comparison with interstellar spectra had been mainly founded on spectra of diamond nanoparticles of a known average size. The emission bands show a fine similarity to spectra of diamond nanoparticles of an average size of 50 nanometers or higher that have been put on a surface.
Very recently, Bob Carlson and Jeremy Dahl of Chevron-Texaco have been succeeded in isolating larger diamondoids out of crude oil. Diamondoids are found in crude oil in very small quantities by nature. Last year a group of researchers, including researchers at the FOM Institute for Plasma Physics Rijnhuizen in Nieuwegein, published an article on the first infrared spectra of these molecules. They were recorded by Yuko Ueno of NYY Docomo in Japan at the University of California in Berkely. The involvement of the researchers in Rijnhuizen was caused by the fact that they already worked at a FOM-project (relevantly entitled 'Diamonds in the sky?') concerning spectra of ionised diamondoids and calculations on similar spectra (of neutral as well as ionic diamondoids). The spectra put a different light in the existence of diamondoids in the universe.
Spectra dependent on size and position molecules
The researchers have now compared very detailed laboratory infrared spectra to spectra at two kinds of celestial objects: absorption spectra of the interstellar dust cloud W3 (IRS 5) and emission spectra of very young stars that are being surrounded by dust and gas (HD 97048 and Elias 1). The spectrum that arises by adding up all laboratory molecules, bears a reasonable resemblance to the astronomical absorption spectra (figure 2A). The emission spectra resemble a split band surface relief, which can be reasonably copied by adding only the spectra of the higher symmetrical tetrahedal diamondoid molecules (figure 2B). A possible explanation for the different shapes of the spectra seems to be in the position and the sizes of the molecules. Researchers assume that the emission spectra originate from molecules in the gas phase and that the absorption spectra, on the other hand, originate from molecules in interstellar ice. The balance of gaseous molecules on interstellar positions depends, among other things, on their sizes: the larger, the better they are able to dispose of the energy of absorbed UV photons without a loss, which means that they are the more balanced.
The gaseous molecules that are found in emission, are probably larger on average than the molecules that are found in absorption. As the molecules enlarge, their spectra are resembling more and more strongly to an 'infinite large diamondoid molecule', which would have the pure octahedral symmetry of diamond and would become closest to a molecule with tetrahedal symmetry.
The relative intensity of the two bands in the spectra of the tetrahedal diamondoids is dependent on the size of the molecules, because this is defined by the relation between CH and CH2 groups in the molecule (figure 2C). Calculations indicate that the relative intensity of the two bands for diamondoid molecules consisting of about 150 carbon atoms, approximates that of the interstellar emission spectra. The size of interstellar diamondoid molecules would be then about 2 nanometers. This is much smaller than the size of 50 nanometers or higher, that is distracted from comparisons with spectra of diamond nanoparticles. It is hard to make an estimation of their average sizes, because it is not known as yet how interstellar diamondoids are being shaped chemically. The diamond nuclei are, interestingly enough, being found in micrometeorites (small dust particles coming down out of the universe onto earth in large numbers) and characteristically a diameter of some nanometers.
For more information, please contact Dr. Jos Oomens at the FOM Institute Rijnhuizen, phone: (030) 609 69 99.
The article is entitled: : 'Infrared spectroscopy of diamondoid molecules: new insights into the presence of nanodiamonds in the interstellar medium'; it will be published on 1 June 2007 in The Astrophysical Journal (volume 661, page 919). The authors are Olivier Pirali1, Michel Vervloet2, Jeremy E. Dahl3, Robert M.K. Carlson3, Xander G.G.M. Tielens4 and Jos Oomens1.
1 FOM Institute Rijnhuizen
2 Université Paris Sud
3 Chevron-Texaco Technology Ventures
4 NASA Ames Research Center