Impact of Nuclear Motion on Light-Induced Bimolecular Interaction Dynamics

In chemical reactions, the nuclear motion of the molecules plays a crucial role in determining the reaction rates and outcomes. Employing the cold target recoil ion momentum spectroscopy and femtosecond pump-probe techniques, we perform a molecular-level study into the influence of nuclear vibrations on light-induced bimolecular reactions within ${\mathrm{H}}_2\text{−}{\mathrm{D}}_2$ dimers. The study focuses on the formation dynamics of ${\mathrm{D}}_2{\mathrm{H}}^{+}$ and ${\mathrm{H}}_2{\mathrm{D}}^{+}$ cations, shedding light on the interplay between translational and vibrational motions of the nuclei steering the bimolecular reactions. Our observations reveal a notable yield ratio of $1:1.6$ between ${\mathrm{H}}_2{\mathrm{D}}^{+}$ and ${\mathrm{D}}_2{\mathrm{H}}^{+}$ channels, accompanied with a faster formation of ${\mathrm{D}}_2{\mathrm{H}}^{+}$ compared to ${\mathrm{H}}_2{\mathrm{D}}^{+}$. Molecular dynamics simulations unveil that the faster vibrational motion of ${\mathrm{H}}_2^{+}$ than that of ${\mathrm{D}}_2^{+}$ upon single ionization within the dimer accounts for these differences. Our findings provide new insight into the time-resolved kinetic isotope effect on the bimolecular reactions, highlighting the critical relationship between nuclear vibrational motions and reaction dynamics.