Shock-wave processing of C₆₀ in hydrogen

¹ Institut de Physique de Rennes, UMR CNRS 6251, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France
e-mail: ludovic.biennier@univ-rennes1.fr
² Shock Induced Materials Chemistry Lab, Solid State and Structural Chemistry Unit, Indian Institute of Science, 560012 Bangalore, India
³ Inorganic and Physical Chemistry, Indian Institute of Science, 560012 Bangalore, India
⁴ UJF-Grenoble 1/UMR CNRS 5588, Laboratoire Interdisciplinaire de Physique, 38041 Grenoble, France
⁵ Institut d’Astrophysique Spatiale, UMR CNRS 8617, Université Paris-Sud, Bât. 121, 91405 Orsay, France
⁶ Department of Aerospace Engineering, Indian Institute of Science, 560012 Bangalore, India

Received: 6 June 2016
Accepted: 24 October 2016

Abstract

Context. Interstellar carbonaceous particles and molecules are subject to intense shocks in astrophysical environments. Shocks induce a rapid raise in temperature and density which strongly affects the chemical and physical properties of both the gas and solid phases of the interstellar matter.

Aims. The shock-induced thermal processing of C₆₀ particles in hydrogen has been investigated in the laboratory under controlled conditions up to 3900 K with the help of a material shock-tube.

Methods. The solid residues generated by the exposure of a C₆₀/H₂ mixture to a millisecond shock wave were collected and analyzed using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman micro-spectroscopy, and infrared micro-spectroscopy. The gaseous products were analyzed by Gas Chromatography and Cavity Ring Down Spectroscopy.

Results. Volatile end-products appear above reflected shock gas temperatures of ~2540 K and reveal the substantial presence of small molecules with one or two C atoms. These observations confirm the role played by the C₂ radical as a major product of C₆₀ fragmentation and less expectedly highlight the existence of a single C atom loss channel. Molecules with more than two carbon atoms are not observed in the post-shock gas. The analysis of the solid component shows that C₆₀ particles are rapidly converted into amorphous carbon with a number of aliphatic bridges.

Conclusions. The absence of aromatic CH stretches on the IR spectra indicates that H atoms do not link directly to aromatic cycles. The fast thermal processing of C₆₀ in H₂ over the 800–3400 K temperature range leads to amorphous carbon. The analysis hints at a collapse of the cage with the formation of a few aliphatic connections. A low amount of hydrogen is incorporated into the carbon material. This work extends the range of applications of shock tubes to studies of astrophysical interest.