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Fragmentation cross sections of protonated water clusters

We have measured fragmentation cross sections of protonated water cluster cations (H2O)n= 30−50H+
by collision with water molecules. The clusters have well-defined sizes and internal energies. The
collision energy has been varied from 0.5 to 300 eV. We also performed the same measurements
on deuterated water clusters (D2O)n=5−45D+ colliding with deuterated water molecules. The main
fragmentation channel is shown to be a sequential thermal evaporation of single molecules following
an initial transfer of relative kinetic energy into internal energy of the cluster. Unexpectedly, that
initial transfer is very low on average, of the order of 1% of collision energy. We evaluate that for
direct collisions (i.e., within the hard sphere radius), the probability for observing no fragmentation
at all is more than 35%, independently of cluster size and collision energy, over our range of study.
Such an effect is well known at higher energies, where it is attributed to electronic effects, but has
been reported only in a theoretical study of the collision of helium atoms with sodium clusters in
that energy range, where only vibrational excitation occurs.

Voir en ligne : J. Chem. Phys. 136, 214301 (2012)