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.

H ₂ –H ₂分子二聚体由于其独特的键合特性以及模拟更复杂系统的能力，在化学相互作用的研究中具有根本重要性。三氢阳离子H ₃ ⁺也是星际环境中一系列化学过程的关键中间体，例如各种有机分子和早期恒星的形成。_{然而，分子云中 H 3} ⁺的意外高丰度仍然难以解释。在这里，使用近红外飞秒激光脉冲和重合动量成像，我们发现氘分子二聚体（D ₂ –D ₂）光电离后的主要通道是在几百飞秒内喷射氘原子，从而导致形成D ₃ ⁺。从头算分子动力学模拟支持并很好地再现了形成机制。这种由超冷 D ₂ –D ₂气体形成D ₃ ⁺的途径可能有助于了解星际介质中H ₃ ^{+的高丰度。}