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5 Summary

We produce QCC films with a hydrocarbon plasma deposition technique. We have obtained emission and absorption spectra of as-deposited and thermally annealed QCC in the 3 $\mu $m region. The relative strength of sp2 hybridized CH (aromatic C-H) bonds increases with the annealing temperature with respect to the intensity due to sp3 hybridized CH (aliphatic C-H) bonds, suggesting graphitization of the QCC films by thermal annealing. Whilst the HRTEM image shows amorphous structure in general (Sakata et al. [1994]), carbon onion-like spherules are conspicuous in the QCC annealed at higher temperature. It was consistent with the spectroscopic transformation in the 3 $\mu $m region, and we think that the graphitization of QCC in the high temperature environment occurs to form carbon onion-like spherules.

We compare the spectral sequence of 3 $\mu $m spectra of the annealed QCC with observed spectra of PPNe categorized in class B by Geballe ([1997]) and Tokunaga ([1997]). Based on the similarity between the laboratory spectra and observed ones particularly in the spectral morphology and the feature to continuum ratio, we propose a scenario that carbonaceous dust is formed as condensates similar to QCC samples in the PPN phase, and suffers spectroscopic transformation when driven outward from the star, transversing a warm region of the dust shell. Further observational and laboratory works would confirm this hypothetical view by utilizing high spatial resolution spectroscopy in observations and UV excited QCC emission spectroscopy in laboratory, for instance.

Acknowledgements
We thank the referee, W.W. Duley for his helpful comments that improved the manuscript. We are also grateful to A.T. Tokunaga and T.R. Geballe who kindly provided the spectra of PPNe. MG is supported by the JSPS fellowship.


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