Earlier theories had forecast an appreciable drop in the reaction probability at low temperatures, but we could not confirm this either in the experiment or in new calculations by our colleagues on the basis of theory, explains Dr. Holger Kreckel from the Max-Planck-Institut für Kernphysik. The helium hydride ion, HeH⁺, long thought to be the universe’s “primordial molecule,” has again upended expectations for the chemistry of the cosmos this time, in the controlled cold of a cryogenic laboratory.
The saga of HeH⁺ commences in the aftermath of the Big Bang, when the universe’s scorching temperatures and densities yielded, a few seconds later, to the creation of the lightest elements: hydrogen and helium. These elements were fully ionized for almost 380,000 years, their electrons free-floating. Only after the period called recombination did neutral atoms coalesce, making way for the first chemical bonds. The first of these, HeH⁺, was created when a helium atom encountered a proton, ushering in the era of molecular chemistry and triggering a series of events that eventually produced molecular hydrogen (H₂), the most abundant molecule in the universe Researchers have uncovered new insights into the reaction pathways of the universe’s first molecule.
During the so-called “dark age” of the universe when the cosmos was transparent but starless there were simple molecules like HeH⁺ and H₂, which were crucial. In order for a protostar to collapse and start nuclear fusion, it needed to lose heat. Atom-molecule collisions cooled things down, releasing photons and enabling further collapse. But below around 10,000 degrees Celsius, hydrogen atoms alone proved to be ineffective coolants. Only those molecules which could emit energy by rotation and vibration could be able to carry on the process of cooling. In this case, HeH⁺’s strong dipole moment rendered it a particularly effective agent for radiative cooling at low temperatures, possibly dictating the very efficiency of early star formation simple molecules such as HeH⁺ and H₂ were essential to the formation of the first stars.
Although important in theory, HeH⁺ remained elusive in space for decades. It was only in recent times that astronomers, employing the GREAT spectrometer on the SOFIA airborne observatory, identified its trace in the planetary nebula NGC 7027. “SOFIA has provided us with proof that this ion really can form in planetary nebulae. At present, there is no other telescope capable of observing at these wavelengths, so this observation platform will remain unique for many years to come,” explains Anke Pagels-Kerp from the DLR Space Administration SOFIA uncovers ones of the building blocks of the early Universe.
But observing HeH⁺ in nature is half the battle. It takes laboratory simulation of the early universe conditions in order to study its behavior. At the Max-Planck-Institut für Kernphysik in Heidelberg, scientists looked to the Cryogenic Storage Ring (CSR), a special 35-meter-circumference electrostatic storage ring that can cool ions to a few kelvins conditions that are very similar to those of cold interstellar clouds. The CSR’s ultra-high vacuum and cryogenic environment allow molecular ions to be stored for a minute, giving their internal states time to relax to the values found in space the CSR vacuum system is potentially the largest laboratory on Earth that succeeds in closely replicating the environment in a cold molecular cloud.
Here, HeH⁺ ions were combined with a beam of neutral deuterium atoms. By precisely controlling the relative velocities, the group was able to probe the variation of reaction rate with temperature a critical test of years-old theoretical predictions. The expectation, based on decades of quantum chemistry, was for a drastic drop in reaction rate as the temperature decreased. But the data instead showed a remarkable constancy: the reaction rate remained nearly flat across the low-temperature regime. This finding overturns old models and indicates that HeH⁺ was much more chemically reactive in the early universe than they previously believed they discovered that, contrary to earlier estimates, the rate at which this reaction occurs does not decrease with decreasing temperature, but is nearly constant.
Theoretical justification of this unexpected finding followed in short order. Researchers led by Yohann Scribano found a defect in the potential energy surface calculations that formed the basis of previous predictions. With the new surface, theoretical rates now align well with the CSR experiment, solving a 13-billion-year-old mystery regarding the end of the first molecule in the universe the new calculation employing the new improved potential surface now agree well with the CSR experiment.
This coming together of experiment and theory both reveals the role of HeH⁺ in the early universe and highlights the concept of the advanced laboratory instrumentation. The CSR’s capability to prepare and trap cold, state-selected ions for extended periods is a breakthrough for astrochemistry, allowing for measurements that were previously inaccessible. As Dr. Kreckel explains, “The reactions of HeH⁺ with neutral hydrogen and deuterium therefore appear to have been far more important for chemistry in the early universe than previously assumed.”
For scientists investigating the chemical origins of the universe, these results provide a new basis for modeling star formation and molecular evolution during the earliest periods of the universe.
