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Photoionization dynamics in pure helium droplets
Darcy S Peterka1, Jeong Hyun Kim, Chia C Wang
1Department of Chemistry, University of California, Berkeley, California 94720, USA.
The Journal of Physical Chemistry. A
|June 19, 2007
Summary
Photoionization of helium (He) droplets reveals fast electrons from direct ionization and slow electrons from indirect mechanisms. The "dimer model" explains the enhanced kinetic energy of fast photoelectrons observed in helium clusters.
Area of Science:
- Atomic and Molecular Physics
- Quantum Fluids and Solids
- Photoionization Spectroscopy
Background:
- Helium (He) droplets are weakly bound quantum systems studied to understand fundamental interactions.
- Photoionization provides insights into electronic structure and dynamics of atomic and cluster systems.
- Previous studies on He droplets focused on their superfluid properties and nucleation phenomena.
Purpose of the Study:
- Investigate the photoionization and photoelectron spectroscopy of pure He droplets.
- Determine the effect of droplet size on photoionization dynamics.
- Explore the mechanisms responsible for fast and slow electron production.
Main Methods:
- Utilized time-of-flight mass spectrometry and photoelectron imaging.
- Performed experiments at photon energies from 24.6 eV to 28.0 eV.
- Varied molecular beam source temperatures and pressures to control droplet size.
Main Results:
- Photoelectron images were dominated by fast electrons from direct ionization at low temperatures (below 16 K).
- A small contribution of very slow electrons (< 1 meV) was observed, attributed to an indirect mechanism.
- Fast photoelectrons from He droplets exhibited up to 0.5 eV higher kinetic energy than those from atomic He.
Conclusions:
- The enhanced kinetic energy of fast photoelectrons is explained by a 'dimer model' involving vertical ionization of nearest-neighbor He atoms.
- Slow electrons may arise from vibrational autoionization of Rydberg states bound to the HeN+ core.
- Droplet size significantly influences photoionization dynamics, with larger droplets favoring direct ionization pathways.
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