CESYRA@Eufele
The possibility to perform a set of experiments on free carbon clusters excited with UV light is mostly of interest for fundamental research and particularly to astrophysics research. The prominent hump centred at 217.5 nm (4.6 µm-1, 5.7 eV) is the dominant feature in the interstellar extinction curve and also the most controversial and a long-standing problem in astrophysics. Actual models of dust astrophysics lack of experimental data about carbon dust in gas phase, their optical proprieties, mass distribution, reactivity and of course their absorption of the UV light in the 217 nm region.
In a series of recent in situ experiments on cluster assembled ns-C films, it has been shown how a relatively high abundance of sp carbon can be synthesized by SCBD [1, 2]. The cold aggregation conditions in the PMCS cluster source favour the growth of highly metastable structures rich in sp hybridized carbon. These structures, though very unstable, can survive the deposition process and be embedded in a ns-C matrix. Under UHV conditions sp containing ns-C films can be investigated for several hours; the presence of these structures in deposited films has been studied by both vis-Raman and NEXAFS spectroscopy [3, 1]. The possibility to perform this kind of experiments on free particles would bring important insight into the process of nanostructured film formation under low energy molecular beam deposition conditions.
Resonant Raman scattering of free carbon particles will be investigated tuning the FEL wavelength across the region of 217 nm where the UV absorption hump in astrophysical data is observed and where a number of electronic transitions exist for variable size linear carbon chains. A second region of observation will be around 532 nm where previous Raman spectroscopy data were acquired and where interesting DIBs (Diffuse Interstellar Bands) are observed. Two photon ionization experiments could also be performed at this wavelength, adopting the same PEPICO scheme available for the metal cluster experiment (CESYRA@GASPHASE).
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[1] L. Ravagnan, F. Siviero, C. Lenardi, P. Piseri, E. Barborini, P. Milani, C. Casari, A. Li Bassi and C.E. Bottani,
Cluster Beam Deposition and in situ Characterization of Carbyne-rich Carbon Films. Phys Rev Lett 89, 285506 (2002).
[2] C.S. Casari, A. Li Bassi, L. Ravagnan, F. Siviero, C. Lenardi, P. Piseri, G. Bongiorno, C.E. Bottani and P. Milani,
Chemical and thermal stability of carbyne-like structures in cluster-assembled carbon films. Phys. Rev. B 69, 5422 (2004).
[3] L. Ravagnan, G. Bongiorno, D. Bandiera, E. Salis, P. Piseri, P. Milani, C. Lenardi, M. Coreno, M. de Simone and K.C. Prince,
Quantitative evaluation of sp/sp2 hybridization ratio in cluster-assembled carbon films by in situ near edge X-ray absorption fine structure spectroscopy. Carbon 44, 1518 (2006).