| Abstract: | This paper reports the first Raman study of a representative set of interplanetary dust particles (IDPs) belonging to different infrared spectral classes. Six different groups of Raman spectra can be discerned among the 20 IDPs studied. All particles, except the four belonging to group 6, exhibit the Raman signature of poorly crystallized carbonaceous material. The degree of “disorder” of this material, as evidenced by the width and relative strength of the two first-order Raman bands at 1360 relative cm−1 and 1600 relative cm−1, varies among particles from different spectral groups (increasing from group 1 to group 5). The most ordered carbonaceous material, with an estimated upper crystallite size limit of 60Å(group 1), is found in particles that tend to show deuterium depletions and that have infrared spectra characteristic of olivines. Raman spectra of particles belonging to groups 2, 3 and 4 (none of which have FTIR spectra characteristic of olivines) show clear evidence for the presence of carbonaceous material, although the “degree of order” is noticeably less than for carbons in group 1 particles. All seven particles with documented deuterium enrichments 1,2] fall into these three intermediate Raman groups. This relationship of the hydrogen isotopic compositions with the Raman data suggests different carbonaceous carrier phases for deuterium depletions and enrichments. Particles from Raman groups 5 and 6, whose spectra show little or no evidence for the presence of carbonaceous material, show deuterium abundances within the range of terrestrial rocks. In general, there is no obvious relationship between Raman groups and infrared classes which are based on the 10 μm absorption band due to silicates. In part, this is due to the fact that no Raman bands for silicates are seen, although silicates are known to be present from infrared and analytical electron microscopic measurements. The lack of silicate bands indicates that the silicate grains are coated with and/or imbedded in a carbonaceous material, which is presumably the reason that the particles look black in the visible. Several particles also show broad visible laser-induced photoluminescence, which is also probably produced by a carbonaceous component. |
