Observation and dynamic control of a new pathway of H${}_3^{+}$ formation

We propose and experimentally demonstrate that the trihydrogen cation (H${}_3^{+}$) can be produced via single photoionization of the molecular hydrogen dimer (H${}_4$). Using near-infrared, femtosecond laser pulses and coincidence momentum imaging, we find that the dominant channel after single ionization of the dimer is the ejection of a hydrogen atom within a few hundred femtoseconds, leaving an H3+ cation behind. The formation mechanism is supported and well reproduced by an ab-initio molecular dynamics simulation. This is a new pathway of H${}_3^{+}$ formation from ultracold hydrogen gas that may help explain the unexpected high abundance of H${}_3^{+}$ in the interstellar medium in the universe.