Virtual Sequential Picture for Nonsequential Two-Photon Double Ionization of Helium.

Area of Science:

• Atomic Physics
• Quantum Mechanics
• Laser-Matter Interactions

Background:

• Two-photon double ionization of atoms is a complex quantum phenomenon.
• Understanding the mechanisms, especially nonsequential ionization, is crucial.
• Existing models often struggle to capture the nuances across different laser regimes.

Purpose of the Study:

• To develop a simplified model for understanding nonsequential two-photon double ionization in Helium.
• To investigate the role of intermediate states and electron correlation in the ionization process.
• To bridge the gap between sequential and nonsequential ionization descriptions.

Main Methods:

• Utilized a model based on second-order time-dependent perturbation theory.
• Introduced a 'virtual sequential picture' focusing on dominant intermediate single continuum states.
• Calculated total ionization cross sections and compared with sophisticated numerical solutions of the time-dependent Schrödinger equation.

Main Results:

• The virtual sequential picture accurately describes nonsequential two-photon double ionization of Helium.
• Total ionization cross sections calculated by the model show perfect agreement with full-dimensional Schrödinger equation solutions.
• Electron correlation in the final double continuum state is found to be unimportant for total cross sections but crucial for triply differential cross sections.

Conclusions:

• The virtual sequential picture provides a robust framework for understanding two-photon double ionization across various laser parameters.
• The study simplifies the complex process by highlighting dominant intermediate states.
• This work clarifies the distinct roles of electron correlation in different ionization cross-section calculations.