Researchers believe that the helium hydride ion, HeH +, was the first molecule to form in the cosmos. But it escaped detection in space. We have just confirmed its discovery in the interstellar medium, in the debris of an end-of-life star.
During the birth of the Universe, the Big Bang released a considerable amount of energy which was transformed into an extremely hot soup of fundamental particles, in particular the electrons, the photons and the elementary constituents of the atomic nuclei, the quarks . This medium evolved quickly but the appearance of atoms and molecules was later. If the history of the formation of atoms is well understood and confirmed today by various observations, that of the first molecules is more uncertain. Which molecule was formed first? For astrochemists, it is very likely that it was helium hydride, composed of one atom of hydrogen and another of helium. But to confirm this hypothesis, it was necessary to find this molecule in the interstellar medium. This quest was only completed very recently.
To understand why helium hydride formed first, you must go back to the primordial soup of the Universe. From its birth, the cosmos was animated by a dynamic of spatial expansion resulting in a rapid drop in its temperature. This cooling allowed the formation of new elements. In less than a microsecond after the Big Bang, quarks have joined through strong interaction to give rise to protons and neutrons. And in the minutes that followed, these protons and neutrons began to combine and form the simplest atomic nuclei: those of hydrogen (a simple proton), deuterium (a proton and a neutron), helium ( two protons and two neutrons) and more rarely lithium (three protons and three or four neutrons).
The first atoms of the Universe are represented here by their orbital models, which exhibit their subatomic constituents. In the following pages, the molecules and ions, made up of at least two atoms, will be represented, in a more traditional way, by balls – representing the atoms – connected by rods, which represent the electrons participating in the chemical bond.
By pairing with as many electrons as they contain protons, the nuclei form atoms, electrically neutral. But because the temperature of the soup was still too high, the very energetic photons instantly snatched the electrons from these associations. The soup has remained a plasma of electrically charged constituents, nuclei, and electrons, for hundreds of thousands of years. Photons, in constant interaction with these charged particles, could not propagate freely over long distances. As a result, the Universe remained opaque for almost 380,000 years. Then, at that time, the temperature dropped enough for the first neutral atoms to be formed durably, which allowed the photons to propagate and escape, constituting a radiation whose vestige, the “Cosmic microwave background”.
Of all the atomic nuclei competing for electrons, helium was the first to form neutral atoms. Why not hydrogen or lithium? Helium, the first inert element, or “noble gas” (or “rare gas”) in the periodic table of the elements, is particularly stable when it has two electrons. In other words, it takes a lot more energy to pull out an electron than it takes, for example, for hydrogen. This stability is explained by the layered structure of the electrons in the atoms. When a layer is filled, the assembly is more stable; however, the first electronic layer contains two places. In the hydrogen atom, which has only one electron, this first layer is not filled, unlike helium.
In the early youth of the Universe, when the helium nuclei began to meet electrons, they therefore filled the first layer of their electronic cloud without the hydrogen nuclei being able to catch up in this process of capturing electrons, and long before the lithium nuclei are even present to collect, each, the three electrons necessary to become neutral atoms.