Scientists recreated the first chemical reaction in the Universe

German researchers from the Max Planck Institute for Nuclear Physics in Heidelberg recreated the first chemical reaction in the Universe, which took place after the Big Bang, about 13.8 billion years ago. 

Immediately after the Big Bang the Universe was a dense, seething space extremely high temperature. However, within a few seconds, the Universe began to cool so rapidly that the first atomic particles were able to combine to form the first lightest elements — hydrogen and helium.

At that time, these elements were fully ionized, meaning that their electrons were not yet bound to their nuclei. It took another 380 thousand years before the Universe finally cooled enough to form neutral atoms. This process is called recombination, allowing electrons to combine with nuclei to form stable atoms for further first chemical interactions.

One of the first such chemical interactions was the formation of the helium hydride ion (HeH+), which is considered to be the first molecule in the Universe. This molecule was formed by combining a neutral helium atom with a positively charged hydrogen nucleus.

Its formation laid the foundation for a series of reactions that eventually led to the formation of molecular hydrogen (H₂). Today, it is the most abundant molecule in the Universe.

After the recombination was completed, the Universe plunged into darkness. Outer space became transparent because free electrons were already bound to atoms, and stars and other light sources had not yet formed. Several hundred million years passed before the first stars illuminated the cosmos.

At this early stage of the Universe, simple molecules like HeH⁺ and H₂ played a key role in the formation of the first stars. In order for the gas cloud of a protostar to shrink to the size of a point and begin the process of nuclear fusion, heat must be dissipated. This happens through collisions that excite atoms and molecules, and they begin to emit energy in the form of photons. However, at temperatures below about 10 thousand °C, this process becomes inefficient for the predominant hydrogen atoms.

Further cooling becomes possible only at the expense of molecules capable of emitting additional energy through rotation or vibration. Thanks to pronounced dipole moment, the HeH⁺ ion is particularly efficient at such low temperatures and has long been considered a potentially important candidate for the role of a cooling agent in the formation of the first stars. Thus, the concentration of helium hydride ions in the Universe can significantly affect the efficiency of early star formation.

At that time, collisions with free hydrogen atoms were the key decay pathway HeH⁺, leading to the formation of a neutral helium atom and an H₂⁺ ion. They then reacted with another hydrogen atom to form a neutral H₂ molecule and a proton, which contributed to the formation of molecular hydrogen.

German researchers have performed this reaction for the first time in conditions that closely resemble the early Universe. They studied the reaction of HeH⁺ with deuterium, an isotope of hydrogen that contains an additional neutron in the nucleus of the atom along with a proton. The reaction of HeH⁺ with deuterium produces the HD⁺ ion instead of H₂⁺, along with a neutral helium atom.

The experiment was conducted on a cryogenic storage device (CSR) at the Center for Plasma and Atomic Physics (MPIK) in Heidelberg. This is a unique device, to study molecular and atomic reactions in space-like conditions. For this purpose, HeH⁺ ions were stored in a 35-meter ion storage device for up to 60 seconds at a temperature of -267 °C and superimposed on a beam of neutral deuterium atoms.

By adjusting the relative velocities of the two particle beams, the researchers studied the dependence of the collision frequency on the collision energy, which is directly related to temperature. The scientists found that, contrary to previous predictions. The rate of this reaction does not slow down with decreasing temperature, but remains almost constant.

This observation is in line with the findings of a team of theoretical physicists led by Johann Scribano, who discovered an error in the calculation of the potential surface used in all previous calculations of this reaction. New calculations using the improved potential surface now agree well with the CSR experiment. Since the concentrations of molecules such as HeH⁺ and molecular hydrogen (H₂ or HD) played an important role in the formation of the first stars, this result brings us closer to solving the mystery of their formation.

The results of the study are published in the journal Astronomy & Astrophysics

Source: SciTechDaily