"Mysterious bands" in Raman spectra of Kokchetav microdiamonds were photoluminescence - Internal structures of the microdiamond grains by laser excited photoluminescence

In the previous presentations in 2011 (Harada et al., 2011, Igarashi et al., 2011, Ogasawara et al., 2011a, Ogasawara et al., 2011b), we reported "mysterious bands" in the Raman spectra of the Kokchetav diamonds using an Ar laser (514.5 nm) of a Raman spectroscope. We had regarded these bands at 1400-1500 cm^-1 as "Raman shift", and had proposed a model that these bands might be attributed to metastable intermediate carbon phases (like fullerenes) for the diamond formation. Recently, we conducted Raman spectroscopy using a different excitation line, He-Ne laser (632.8 nm); however, no Raman shift was observed at ca. 1450 cm^-1 in the selected diamond grains which showed typical mysterious bands using the Ar laser. Based on new data, we concluded that our interpretation in the previous works was wrong and "fullerene model" was denied. The new data suggested that "mysterious bands" at ca. 1450 cm^-1 with the Ar laser were not Raman shifts but were photoluminescence (PL) bands at ca. 556 nm. Here, we report these strange PL spectra of the Kokchetav diamond using a laser Raman spectroscope. Based on our previous 2D mappings at difference depths using the Ar laser, the bands of PL (ca. 556 nm) were generated from small domains (1-3 μm) of diamond grains. These bands were detected in several grains of diamond in dolomite marble and garnet-biotite gneiss. The bands are usually strong and mainly detected at 554-558 nm (FWHMs: 1.1-1.4 nm) with relatively small FWHM compared to PL bands already obtained in the Kokchetav diamonds. In our new experiments, we used both the Ar and He-Ne lasers for Raman spectroscopy. The spectra with the He-Ne laser have PL bands mainly at 645-676 nm (FWHMs: 1.0-2.0 nm). The 2D mappings of PL spectra using two lasers were also conducted to clarify the distribution of domains of the sources of bands. The distributions of the PL bands in the diamond grains showed similar patterns by both lasers, each other. In the both spectra by the Ar and He-Ne lasers, Raman shift was only observed diamond and garnet bands; no other Raman shift was detected. Many other bands of PL in the spectra were observed from the Kokchetav diamonds. These PL bands are caused from inside diamond grains and restricted to limited domains of a diamond grain. There are two possibilities of source for the PL bands; one is fine inclusions in a diamond grain, and the other is diamond itself. The former possibility is low because almost no Raman shifts were detected excepting diamond and garnet Raman bands. Diamond is highly possible to be the source for such PL bands. These heterogeneous distributions of PL in a diamond grain using a laser Raman spectroscope may characterize a new aspect of the internal structure of microdiamond grains from the Kokchetav UHPM rocks. References Harada, Y., Igarashi, M., Hashiguchi, Y. & Ogasawara, Y., 2011, Abstract of AGU Fall Meeting, V23E-2599. Igarashi, M., Hashiguchi, Y. & Ogasawara, Y., 2011, Abstract of AOGS, SE53-A005. Ogasawara, Y., Igarashi, M. & Hashiguchi, Y., 2011a, Abstract of AOGS, SE53-A004. Ogasawara, Y., Igarashi, M., Hashiguchi, Y. & Harada, Y., 2011b, GSA Abstracts with Programs, 43, No. 5, p. 47.