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This study employs correlated photoelectron and ion imaging alongside few-femtosecond pump-probe measurements and nonadiabatic simulations to elucidate the real-time formation of molecular hydrogen from ionized heavy water (D$_2$O). The researchers reveal that the reaction proceeds through three distinct pathways鈥攄irect, roaming, and delayed鈥攖aking approximately 34 and 72 femtoseconds for the direct and delayed branches, respectively. Notably, the formation of hydrogen requires an initial symmetry breaking in the molecule, highlighting the critical role of random asymmetric motion in facilitating the electronic-state switch necessary for bond formation.
The formation of molecular hydrogen from water occurs through three distinct pathways, with the reaction's timing hinging on an unexpected symmetry-breaking motion.
Removing an electron from a water molecule can drive its two hydrogen atoms to pair up and depart as molecular hydrogen. However, even for this elementary reaction, the route from start to finish has remained hidden because measurements have yet to follow the electronic and nuclear motion simultaneously. Combining correlated photoelectron and ion imaging, few-femtosecond pump--probe measurements, and nonadiabatic simulations, we track the complete pathway in isolated heavy water (D$_2$O) molecules. The reaction takes an indirect route and dissociates along three distinct pathways (direct, roaming, and delayed) with formation times of about 34 and 72 femtoseconds for the direct and delayed branches. Yet bond formation requires the molecule to first break its own symmetry. Only random asymmetric motion enables the electronic-state switch at a conical intersection, joining the two hydrogen atoms before the oxygen--hydrogen bond breaks. These results establish a time-resolved picture of molecular hydrogen formation from water and provide a general strategy for linking electronic excitation to chemical outcomes in settings from radiation damage to hydrogen production.
