Phase-of-the-Phase Electron Momentum Spectroscopy on Single Metal Atoms in Helium Nanodroplets

Area of Science:

• Atomic and Molecular Physics
• Quantum Dynamics
• Condensed Matter Physics

Background:

• Helium nanodroplets provide a unique environment for studying atom-matter interactions.
• Laser-induced multiphoton ionization is a key technique for probing electronic structures.
• Understanding electron propagation in dense media is crucial for various physical phenomena.

Purpose of the Study:

• To investigate the influence of a dense, neutral medium (helium nanodroplets) on electron propagation during laser ionization.
• To analyze the differences in photoelectron spectra between atoms in nanodroplets and free atoms.
• To explore the impact of the nanodroplet environment on electron emission processes.

Main Methods:

• Multiphoton above-threshold ionization (ATI) of magnesium atoms embedded in helium nanodroplets using two-color laser pulses.
• Angular-resolved photoelectron spectroscopy to analyze electron emission patterns.
• Phase-of-the-phase spectroscopy to study electron signal phase dependence.
• Monte Carlo simulations incorporating laser-assisted electron scattering.

Main Results:

• Photoelectron spectra from Mg atoms in helium nanodroplets exhibit significant differences compared to free atoms.
• Scattering within the helium environment leads to an almost isotropic angular distribution of photoelectrons.
• Higher-energy electrons are observed, indicating the nanodroplet's influence on concerted electron emission.
• A marked loss in the 2ω-ω phase dependence of the electron signal was detected.

Conclusions:

• The dense, neutral helium environment profoundly affects single electron propagation after ionization.
• Electron scattering within the nanodroplet alters angular distributions and electron energies.
• The study provides insights into strong-field-induced electron emission from disordered systems using simulations that match experimental data.