Fragmentation cross sections of protonated water clusters

Area of Science:

• Physical Chemistry
• Chemical Physics
• Materials Science

Background:

• Protonated water clusters are fundamental systems for understanding solvation and proton transfer.
• Investigating cluster fragmentation provides insights into energy transfer dynamics and reaction mechanisms.

Purpose of the Study:

• To measure fragmentation cross sections of protonated water clusters upon collision with water molecules.
• To determine the energy transfer efficiency and fragmentation pathways for varying cluster sizes and collision energies.
• To compare fragmentation behavior between protonated and deuterated water clusters.

Main Methods:

• Collision-induced fragmentation experiments were conducted on protonated water cluster cations ((H2O)n H+) and deuterated water cluster cations ((D2O)n D+).
• Collision energies were systematically varied from 0.5 to 300 eV.
• Fragmentation cross sections were measured as a function of cluster size (n=30-50 for H2O, n=5-45 for D2O) and collision energy.

Main Results:

• The primary fragmentation mechanism observed is sequential thermal evaporation of molecules.
• The initial transfer of kinetic energy into internal energy of the cluster is unexpectedly low, averaging around 1% of the collision energy.
• A significant probability (over 35%) of no fragmentation was observed for direct collisions, independent of cluster size and collision energy within the studied range.

Conclusions:

• Water cluster fragmentation is dominated by molecular evaporation following inefficient initial energy transfer.
• The observed high probability of non-fragmentation suggests a protective mechanism against dissociation, even at low collision energies.
• These findings challenge conventional understanding of energy transfer in cluster collisions and highlight the stability of water clusters.