UV irradiation of HAC
Interstellar (IS) dust produces, among other spectral features, a pronounced absorption maximum in the IS extinction curve at a wavelength of 217.5 nm, which is stable in its spectral position, somewhat variable in width (around 100 nm FWHM), and the origin of which is not yet finally clarified. Hydrogenated amorphous carbon (HAC) dust produces absorption bands in the 180-280 nm spectral range due to transitions of the carbon π-electrons. However, usually this band is too broad and rarely at exactly 217.5 nm. Mennella et al. (1996) had found that the UV irradiation of HAC materials with a dose comparable to IS irradiation doses (within the average lifetime of IS clouds) creates a strong absorption band at the right wavelengths.
Within a Ph.D. work supported by a stipendium of the Egyptian government, we have irradiated HAC materials produced by laser ablation in H/He atmospheres with UV doses of up to 33% of the average IS dose. Structural investigation by electron microscopy reveals that these materials become more graphitic with UV irradiation. UV spectroscopy of the HACs shows that a stable UV bump appears at around 217 nm, which becomes stronger and narrower with increasing UV dose , confirming Mennella et al.'s results.
Within a new Master's thesis project, we aim at confirming these results under in-situ irradiation (excluding the exposure of the samples to the laboratory atmosphere), and at studying the dependence of the structural variations on the photon energy of the incident radiation.
Project team: Hagen Walter, Cornelia Jäger (MPIA), Harald Mutschke
 V. Mennella et al., 1996, Astrophys. J. Lett., 464, L191
 K.A.K. Gadallah, H. Mutschke, and C. Jäger, 2011, Astron. Astrophys., 528, A56