Spectral properties of gas-phase condensed fullerene-like carbon nanoparticles from far-ultraviolet to infrared wavelengths

Carbon solids are ubiquitous material in the interstellar space. However, the formation pathway of carbonaceous matter in astrophysical environments as well as in terrestrial gas-phase condensation reactions is not yet understood. Laser ablation of graphite in different quenching gas atmospheres such as pure He, He/H$_2$, and He/H$_2$O at varying pressures is used to synthesize very small, fullerene-like carbon nanoparticles. The particles are characterized by very small diameters between 1 and 4 nm and a disturbed onion-like structure. The soot particles extracted from the condensation zone obviously represent a very early stage of particle condensation. The spectral properties have been measured from the far-ultraviolet (FUV) ($\lambda$=120 nm) to the mid-infrared (MIR) ($\lambda$=15 ~$\mu$m). The seed-like soot particles show strong absorption bands in the 3.4 ~$\mu$m range. The profile and the intensity pattern of the 3.4 ~$\mu$m band of the diffuse interstellar medium can be well reproduced by the measured 3.4 ~$\mu$m profile of the condensed particles, however, all the carbon which is left to form solids is needed to fit the intensity of the interstellar bands. In contrast to the assumption that onion-like soot particles could be the carrier of the interstellar ultraviolet (UV) bump, our very small onion-like carbon nanoparticles do not show distinct UV bands due to ($\pi-\pi$*) transitions.