Infrared signatures of C60 and its derivatives in the gas phase – FULLDIBS
Infrared signatures of C60 and its derivatives in the gas phase
The recent detection in the interstellar medium of the C60 fullerene in its neutral form has tremendously re-launched the involvement of fullerenes as potential carriers of the mysterious diffuse interstellar bands (DIBs). In 2015, four near-infrared DIBs were attributed to the C60+ cation, confirming the role of fullerenes as DIBs carriers. The “FULLDIBS” project aims to produce laboratory infrared spectra of C60 and its derivatives in the gas phase and to compare them to the DIBs.
Fullerenes and diffuse interstellar bands
The carriers of the diffuse interstellar bands (DIBs) raise many questions since their discovery by the astronomer Marie Hea Heger in 1922. The DIBs include about 500 absorption bands observed in the visible but also in the near infrared and in the ultraviolet. Their presence has been confirmed in the entire interstellar medium of the Milky Way but also in other galaxies. The DIBs are too structured to be the signatures of solid particles, they are molecular carriers which constitute the most plausible hypothesis. Polycyclic aromatic hydrocarbons and fullerenes are among the most serious candidates, which was confirmed in 2015 by the unambiguous attribution of four DIBs to the C60+ cation. However, there is no reason to exclude the neutral C60 from the list of potential candidates, as there are still no reference infrared spectra of this molecule in the gas phase, at temperatures close to the interstellar medium, i.e. typically between a few kelvins and a few tens of kelvins. The ambition of this project is to produce such reference spectra in the laboratory. This undertaking must overcome a number of experimental difficulties. One of these difficulties is related to the controlled production of C60 vapor which is in the form of a powder. The vapor produced at 1000 K must then be cooled to a few tens of kelvins and probed by infrared spectroscopy before it recondenses.
Supersonic expansion of a gas is a proven technique for lowering its temperature to a few kelvins. In general, the gas escapes from a slit-shaped orifice of a high-pressure (and high-temperature in the case of the C60) reservoir into an expansion chamber maintained at low pressure. This particular slit shape increases the absorption path length of the infrared laser through the cold gas jet and thus improves the sensitivity of the detection. However, this approach is inoperative for a molecule as large as C60 because the number of molecular collisions in the gas jet is too low to effectively relax the vibration of the molecule. The heating phase of the C60 sample is indeed responsible for the distribution of the population on a myriad of vibrational levels that spread the intensity of the spectrum over a huge number of absorption bands, making any detection impossible. We have developed a new approach based on the use of a Laval nozzle which produces a cold, high density and non divergent supersonic flow. This expansion process allows about 40 million collisions to each C60 molecule compared to a few thousand in a slit-jet. We hope to efficiently relax the vibration of the molecule and significantly populate its fundamental vibrational state. It becomes then possible to detect the absorption bands that start from this fundamental vibrational state (cold bands). An ultrasensitive spectrometer of the cavity ringdown spectroscopy (CRDS) type based on a high finesse optical cavity allows to probe the gas jet in the near infrared. Finally, a planar Laval nozzle is being developed to further increase the experimental sensitivity by increasing the interaction length between the laser beam and the gas jet.
A planar hypersonic Laval nozzle is about to be built. This will be the first time that a nozzle of this type will be used for spectroscopic data production (patent possibility).
A CRDS-type ultrasensitive spectrometer has been built to detect one of the fundamental modes of C60 in the mid-infrared (8.47 µm). This spectrometer is based on the use of a quantum cascade laser. To our knowledge, it is the first coupling of a laser of this type with a high finesse cavity.
The spectroscopic data produced from the C60 fullerene in the near infrared will be systematically compared to the DIBs identified in this spectral range, which will allow to refute or confirm the role of C60 as a potential carrier of some DIBs.
Other fullerenes could also be studied like C70, also detected in the mid-infrared in several astrophysical environments.
Low Fidelity Approach for Contoured Nozzle Design
A. Jraisheh, E. Dudás, N. Suas-David, R. Georges, V. Kulkarni, Journal of Spacecrafts and Rockets AIAA, published online 18 Dec 2022.
The recent detection in the interstellar medium of the C60 fullerene in its neutral form has tremendously re-launched the involvement of fullerenes as potential carriers of the mysterious diffuse interstellar bands (DIBs). In 2015, four near-infrared DIBs were attributed to the C60+ cation, confirming the role of fullerenes as DIBs carriers. The “FULLDIBS” project aims at producing laboratory spectra of C60 and C70 in the gas phase, from the mid infrared to the visible range. This project relies on high-density and low-temperarture uniform supersonic flows to relax the molecules on their fundamental vibrational state. These low temperature flows will be probed using the most up-to-date ultra-sensitive spectroscopic techniques based on high-finesse optical cavities. The produced spectral signatures will be systematically compared to the DIBs of the interstellar medium.
Monsieur Robert Georges (INSTITUT DE PHYSIQUE DE RENNES)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
IPR INSTITUT DE PHYSIQUE DE RENNES
LIPHY Laboratoire Interdisciplinaire de Physique
Help of the ANR 459,969 euros
Beginning and duration of the scientific project: - 48 Months