Researchers further determine the infrared spectrum of iconic ‘football molecule’

HFML-FELIX researchers Jos Oomens, Giel Berden and Laura Finazzi
Years ago, Nobel Prize winner Harry Kroto predicted that the robust molecule buckminsterfullerene (C60) – a molecule discovered by himself, that looks a lot like a football – should occur in space abundantly. Especially also in protonated form, which means: in combination with a positively charged hydrogen particle (H+) that attaches to the football as a tiny antenna.
In 2020 – approximately 35 years after the discovery of C60 – researchers at HFML-FELIX succeeded in determining the infrared fingerprint for this protonated form (C60 H+). Thanks to new measurements they can now add even more detail to this, which should make it a lot easier to detect the molecule in space.
See the molecules
Before you can study how much of a certain molecule occurs somewhere, you have to know how to find it. That means: establish a good fingerprint. This can be done by spectroscopy, so: looking at how much light of every frequency is absorbed by the molecule.
In this study scientists first made C60 H+ in the lab. These lab made molecules are exactly the same as the ones you would find in space. Then, by combining a mass spectrometer with free electron lasers, they recorded its fingerprint.
Not an easy measurement
This combination of techniques is necessary because seeing the spectrum of electrically charged particles (C60 H+ in this case) is pretty hard. The particles that you want to study repel each other. As a result the concentration of your sample is very low and that means you are not able to determine the spectrum in the conventional way.
If you use a very strong free electron laser however, you can push the C60 H+ particles to a higher energy state. At specific wavelengths that will make them break up into fragments. The mass will change, and that is something you can measure really well with a mass spectometer. This is how they succeeded in determining the C60 H+ fingerprint.
Now, thanks to new research, they can add important details to their earlier findings. If you look at the most recent measurements, in the graph of the spectrum you can see a very specific peak that represents the motion of the single proton (H+) relative to the (other wise symmetrical) ‘football’, as a little antenna stretching like a spring. This is a unique peak that is distinct from other molecules with C-H bonds. So knowing where it sits in the fingerprint will be an important diagnostic to find C60 H+ in space.
How much of it
Knowing the spectrum of course does not tell us whether C60 H+ is in fact as abundant as predicted. In an attempt to answer that question the researchers compared their lab measurements to existing astronomical data of the Small Magellanic Cloud. This is also where the neutral C60 molecule was first detected.
What they saw was a strong overlap between the emission spectra of this interstellar object and the peaks in both the C60 and C60 H+ spectrum. This at least suggests that both molecules may occur in this area in space. The spectrum also suggests that they are not alone. Some peaks can not yet be explained. That is why they have already started to look at other interesting candidates that could be in the mix as well.
Planets and stars
The combination of spectroscopic measurements, astronomical observations and theoretical models brings us closer and closer to understanding the chemical composition of interstellar clouds. The presence or absence of certain molecules can also be a good indicator of temperature, density and radiation intensity in these gas clouds.
Thanks to studies such as this one, but of course with the help of new instruments like the James Webb Space Telescope as well, we will further our knowledge to better understand how stars and planets are formed.
The latest paper, with contributions from HFML-FELIX researchers Jos Oomens, Giel Berden and Laura Finazzi (picture), can be found in The Astrophysical Journal:
Experimental Determination of the Unusual CH Stretch Frequency of Protonated Fullerenes