Unravelling the chemistry of interstellar space on Earth

Despite all appearances, the space between stars, known as the interstellar medium, is anything but empty. Atoms, ions, and molecules reside in this freezing, low-pressure environment.

Scientists across disciplines are working to determine the types of chemical reactions that occur in this unique environment, and which produce more than 200 unique molecules known to form there…

Most recently researchers have successfully emulated interstellar medium (ISM) reaction conditions on Earth and have been able to study a type of chemical reaction called ion-neutral reactions, using “Coulomb crystals”.  

Their techniques are described in a new paper published in the Journal of Physical Chemistry A.

“The field has long been thinking about which chemical reactions are going to be the most important to tell us about the makeup of the interstellar medium,” says physicist Olivia Krohn, the paper’s first author who undertook the research as a PhD student in the Lewandowski laboratory at JILA in the US.

“A really important group of those is the ion-neutral molecule reactions. That’s exactly what this experimental apparatus is suited for, to study not just ion-neutral chemical reactions but also at relatively cold temperatures.” 

The authors write that such an environment allows them to have “precise control over the interactions and to elucidate a new level of understanding of ion−molecule reactions.”

To begin, the researchers loaded various ions and neutral molecules into an ion trap – equipment that uses dynamic electric fields to capture charged particles – in an ultra-high vacuum chamber.

They then cooled the ions using a process known as Doppler cooling, which uses laser light to reduce the motion of atoms or ions by preferentially slowing particles moving toward the laser. 

This cooled the particles’ temperatures to millikelvin levels – typically any temperature below 0.3 Kelvin, very close to absolute zero. At this temperature the ions arranged themselves into the Coulomb crystal.

The Coulomb crystals can remain trapped for hours, allowing researchers to identify and monitor the reaction in real time.

The researchers also used time-of-flight spectrometry, a technique which uses a high-voltage pulse to accelerate ions down a flight tube to collide with a detector. They could determine which particles were present based on the time it took them to hit the plate.

Calculating the mass of the potential products was especially important as the team could then switch out their initial reactants with isotopologues – the same molecules differing only in that at least 1 atom is an isotope (the same element with a different number of neutrons). 

“That allows us to play cool tricks like substituting hydrogens with deuterium atoms or substituting different atoms with heavier isotopes,” says Krohn. Deuterium is a stable isotope of hydrogen that contains a proton, electron, and a neutron.

“When we do that, we can see from the time-of-flight mass spectrometry how our products have changed, which gives us more confidence in our knowledge of how to assign what those products are.” 

Swapping isotopes in experiments like this allows researchers to get one step closer to determining why there are more deuterium-containing molecules in the ISM than is expected from the atomic deuterium-to-hydrogen ratio in the universe.

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