Anion and cation emission from water molecules after collisions with 6.6-keV $^{16}$O$^{+}$ ions
Résumé
Anion and cation emission following water dissociation was studied for 6.6-keV $^{16}$O$^{+}$ + H$_{2}$O collisions. Absolute cross sections for the emission of all positively and negatively charged fragments, differential in both energy and observation angle, were measured. The fragments formed in hard, binary collisions appearing in peaks were distinguishable from those created in soft collisions with many-body dynamics that result in a broad energy spectrum. A striking feature is that anions and cations are emitted with similar energy and angular distributions, with a nearly constant ratio of about 1:100 for H$^{-}$ to H$^{+}$. Model calculations were performed at different levels of complexity. Four-body scattering simulations reproduce the measured fragment distributions if adequate kinetic-energy release of the target is taken into account. Providing even further insight into the underlying processes, predictions of a thermodynamic model indicate that transfer ionization at small impact parameters is the dominant mechanism for H$^{+}$ creation. The present findings confirm our earlier observation that in molecular fragmentation induced by slow, singly charged ions, the charge states of the emitted hydrogen fragments follow a simple statistical distribution independent of the way they are formed.
Mots clés
Ion
electron
cation
anion
negative ion
H- ion
H$^{-}$ ion
hydride ion
hydrogen
anion emission
anion formation
fragment emission
water molecule
H2O molecule
H$_{2}$O molecule
gas-phase
collision
atomic collisions
molecular collisions
molecular fragmentation
molecular dissociation
scattering
recoil
impact parameter
momentum transfer
charge transfer
electron capture
cross section
differential cross section
keV energies
energy distribution
angular distribution
two-body processes
binary processes
binary encounter
many-body processes
excitation
electronic dissociative excitation
ionization
transfer ionization
vibration
kinetic energy release
KER
Coulomb explosion
post-collision interaction
model calculations MOLPRO
potential energies
statistical model
thermodynamic model
experiment
electrostatic spectroscopy
time of flight
TOF
mass spectroscopy
mass spectrometry