For cosmology, the formation of atomic hydrogen in the recombination epoch of the evolution of the early Universe (redshift ) is of special importance because that allows us to treat the thermal history of the Universe. For observations a crucial fact is that the spectrum of the cosmic microwave background (CMB) experienced a unique distortion due to the release of photons during the recombination epoch. These additional photons formed, together with the thermal spectrum, a cosmological recombination spectrum.
The pioneering works of Peebles [1] and of Zel’dovich, Kurt & Syunyaev [2] revealed that any direct combination of an electron and a proton, and thereby the formation of atomic hydrogen in the ground state, was immediately followed by the ionization of an adjacent atom due to re-absorption of the newly released photon. An electron and a proton combined efficiently into the hydrogen atom only in an excited state, from which a rapid cascade occurred into a state with a principal quantum number. A radiative decay from state involving one photon or from state involving two photons then yields the hydrogen atom in its ground state.
In the pre-recombination stage of the evolution of the Universe the temperature and density of matter and radiation were higher than subsequently. Estimation shows that at redshift the average distance between protons was cm. This magnitude is comparable with a radius of the hydrogen atom in an excited state with . In the pre-recombination stage of evolution the combination of an electron and a proton thus occurred in the presence of the nearest neighbouring proton, which participated in the process [3].
According to the non-standard, quasi-molecular mechanism of recombination (QMR), an electron collides with two protons situated one far from another, emits a photon and creates quasi-molecule in a highly excited state, which then dissociates into a hydrogen atom and a proton or descends into a low-lying state [4]. The QMR transforms into the standard mechanism – the recombination of an electron on an isolated proton when redshift decreases.
The presence of another proton reduces the symmetry of a field experienced by an electron involved in the recombination from spherical to axial and leads to a Stark splitting of the hydrogen energy levels. These two effects lead in turn to radiative transitions that are forbidden in the recombination of an electron with an isolated proton. The participation of the nearest neighbouring proton in the process thus opens quasi-molecular channels, and hence has an impact on the recombination history [5]. More details of the QMR will be reported at the presentation.
