Laser-induced three-body fragmentation dynamics of hydrogen-bonded ( H 2 O ) 2 and ( NH 3 ) 2 dimers

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We investigate the three-body fragmentation dynamics of hydrogen-bonded molecular dimers ${({\mathrm{H}}_{2}\mathrm{O})}_{2}$ and ${({\mathrm{NH}}_{3})}_{2}$ under intense femtosecond laser irradiation. Using a reaction microscope for coincident detection of all ionic fragments, we observe the fragmentation channels ${\mathrm{H}}_{2}{\mathrm{O}}^{+}+\mathrm{O}{\mathrm{H}}^{+}+{\mathrm{H}}^{+}$ and ${\mathrm{NH}}_{3}^{+}+{\mathrm{NH}}_{2}^{+}+{\mathrm{H}}^{+}$. By diagnosing the Newton diagrams, Dalitz plots, and kinetic-energy correlation maps, one concerted pathway and two sequential dissociation pathways are identified in both channels. Native frame analysis enables reconstruction of the branching ratios for these pathways. While the behaviors of the concerted pathway and the sequential pathway involving initial ${\mathrm{H}}^{+}$ emission are comparable in both systems, the sequential pathway with ${\mathrm{H}}^{+}$ emission in the second step exhibits distinct dynamics, with a higher relative branching ratio for ${({\mathrm{NH}}_{3})}_{2}$. These differences arise from the stronger hydrogen bonds in ${({\mathrm{H}}_{2}\mathrm{O})}_{2}$ promoting proton transfer and the weaker hydrogen bonds in ${({\mathrm{NH}}_{3})}_{2}$ favoring hydrogen bond breaking. Our findings underscore the critical role of hydrogen bond strength in shaping the fragmentation dynamics of hydrogen-bonded molecular dimers under laser-induced dissociation.

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