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Anomalous proton dynamics in ice at low temperatures.
L E Bove1, S Klotz, A Paciaroni
1IMPMC, CNRS-UMR 7590, Université P&M Curie, F-75252 Paris, France.
Physical Review Letters
|November 13, 2009
Summary
Hydrogen atoms in ordinary (ice Ih) and cubic (ice Ic) ice exhibit localized, nonvibrational motion at low temperatures. This dynamics, absent in ordered ice VIII, provides evidence for concerted proton tunneling.
Area of Science:
- Condensed matter physics
- Materials science
- Physical chemistry
Background:
- Ice Ih and Ic exhibit complex hydrogen dynamics influenced by proton disorder.
- Understanding hydrogen motion is crucial for explaining ice properties and phase transitions.
- Previous studies suggested vibrational motion, but nonharmonic dynamics remained elusive at low temperatures.
Purpose of the Study:
- To investigate the nature of hydrogen motion in ice Ih and Ic at cryogenic temperatures.
- To determine if nonharmonic dynamics exist and characterize their properties.
- To provide evidence for quantum mechanical effects like proton tunneling in these ice phases.
Main Methods:
- Incoherent quasielastic neutron scattering (IQNS) was employed to probe hydrogen dynamics.
- Measurements were conducted on polycrystalline samples of ice Ih and Ic.
- Data analysis focused on identifying localized, nonvibrational motions and their characteristic parameters.
Main Results:
- Nonharmonic motion of hydrogen atoms was observed in ice Ih and Ic down to 5 K.
- This dynamics was found to be localized, nonvibrational, and dependent on hydrogen disorder.
- A characteristic jump distance of 0.75 Å and a high rate of 2.7x10^11 s^-1 were identified, indicative of concerted proton tunneling.
Conclusions:
- The observed dynamics provide strong evidence for concerted proton tunneling in disordered ice phases.
- The findings challenge previous assumptions of purely vibrational motion at low temperatures.
- This study opens new avenues for understanding quantum effects in hydrogen-rich materials.
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