D₃⁺ formation through photoionization of the molecular D₂-D₂ dimer

The H₂-H₂ molecular dimer is of fundamental importance in the study of chemical interactions because of its unique bonding properties and its ability to model more complex systems. The trihydrogen cation H₃⁺ is also a key intermediate in a range of chemical processes in interstellar environments, such as the formation of various organic molecules and early stars. However, the unexpected high abundance of H₃⁺ in molecular clouds remains challenging to explain. Here using near-infrared, femtosecond laser pulses and coincidence momentum imaging, we find that the dominant channel after photoionization of a deuterium molecular dimer (D₂-D₂) is the ejection of a deuterium atom within a few hundred femtoseconds, leading to the formation of D₃⁺. The formation mechanism is supported and well-reproduced by ab initio molecular dynamics simulations. This pathway of D₃⁺ formation from ultracold D₂-D₂ gas may provide insights into the high abundance of H₃⁺ in the interstellar medium.