Atomic photoionization cross sections beyond the electric dipole approximation.

Area of Science:

• Atomic and Molecular Physics
• Quantum Chemistry
• Computational Physics

Background:

• The electric dipole approximation is commonly used for photoionization cross-section calculations.
• Accurate calculations require considering higher-order approximations, especially at higher photon energies.
• Understanding photoionization is crucial for various applications, including plasma physics and astrophysics.

Purpose of the Study:

• To develop and apply a methodology for computing photoionization cross sections beyond the electric dipole approximation.
• To assess the validity and limitations of the plane wave approximation for photoelectrons.
• To quantify the corrections introduced by going beyond the electric dipole approximation for elements in the first four rows of the periodic table.

Main Methods:

• A response theory framework is employed for calculations.
• Gaussian type orbitals are used for initial atomic/ionic states.
• Plane waves represent the final state of the photoelectron.

Main Results:

• Photoionization cross sections were computed for atoms and ions across the first four rows of the periodic table.
• Kinetic and photon energy thresholds were identified above which the plane wave approximation for photoelectrons becomes applicable.
• Corrections beyond the electric dipole approximation generally remain below 10% for photon energies up to 12 keV.

Conclusions:

• The developed methodology provides accurate photoionization cross sections beyond the electric dipole approximation.
• The plane wave approximation for photoelectrons is valid at sufficiently high kinetic energies.
• Higher-order multipole effects are significant at higher photon energies but remain manageable for many elements.