Spontaneous polarization of thick solid ammonia films.
Roey Sagi1, Michelle Akerman1, Sujith Ramakrishnan1
1Institute of Chemistry, Edmond J. Safra Campus, Givat-Ram, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel.
The Journal of Chemical Physics
|October 2, 2020
Summary
Solid ammonia films spontaneously polarize, forming internal electric fields. Growth temperature influences this polarization and subsequent annealing effects, impacting interstellar chemistry.
Area of Science:
- Astrochemistry
- Surface Science
- Condensed Matter Physics
Background:
- Ammonia is crucial for nitrogen-containing molecule formation in interstellar chemistry.
- Solid ammonia films on interstellar grains are less studied than water ice films.
- Ammonia films, like water, can cage coadsorbed molecules.
Purpose of the Study:
- Investigate the effect of growth temperature on solid ammonia film polarization.
- Analyze the impact of annealing on film depolarization.
- Demonstrate ammonia's ability to cage coadsorbed molecules.
Main Methods:
- Growth of solid ammonia films on a Ru(0001) substrate under ultra-high vacuum.
- Temperature-programmed contact potential difference measurements using a Kelvin probe.
- Analysis of temperature derivative of potential difference to track film changes.
Main Results:
- Spontaneous polarization of ammonia films observed, generating internal electric fields of ~10^5 V/m.
- Growth temperature (30 K-85 K) significantly affects film polarization.
- Kelvin probe measurements effectively track film reorganization and crystallization at low temperatures.
Conclusions:
- Solid ammonia films exhibit spontaneous polarization dependent on growth temperature.
- Ammonia films can cage coadsorbed molecules, similar to water ice.
- Kelvin probe techniques are valuable for studying solid film dynamics in astrochemistry.
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